Fungal adaptations to mutualistic life with ants - ku Kooij.pdf · Fungal adaptations to...

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Fungal adaptations to mutualistic life with ants

This thesis dissertation has been submitted in accordance with the requirements for the degree of PhD, at the PhD School of The Faculty of Science, University of Copenhagen, Denmark to be defended publicly before a panel of examiners.

byPepijn W. Kooij

December 2013

Academic advisorsProf. Jacobus J. Boomsma

Dr. Morten Schiøtt

CoverFront: Atta sexdens workers carrying leaves down a tree. Picture: P. W. Kooij,Location: Santa Cruz, Gamboa, Panama

Back: incipient colony of Atta colombica with queen on top. Picture: P. W. Kooij,Location: Gamboa, Panama

This thesis is the result of a three-year PhD project carried out at the Cen-tre for Social Evolution (CSE), Department of Biology, University of Copenhagen in Denmark under the supervision of Professor Jacobus J. Boomsma and Dr. Morten Schiøtt. During the project I spent five weeks at the Laboratory of Genetics, Wagenin-gen University and Research Center, The Netherlands, hosted by Dr. Duur K. Aanen. I additionally carried out a total of seven weeks of fieldwork at the Smithsonian Tropical Research Institute in Panama. I was funded by the Danish National Research Founda-tion and the Department of Biology.

The thesis is comprised of a general introduction on the current understand-ing of life-time mutualistic interactions and how this can be applied to my model system where the fungal symbiont adapted to a life-time commitment with ants. I also present the objectives of the thesis research, and conclude with a summary of the main results in a broader conceptual framework. This is followed by five chapters of origi-nal emperical work, one of which is in press in a peer-reviewed journal, one has been submitted to a peer-reviewed journal and three that are in preparation for submission.

Preface

Pepijn W. KooijCopenhagen, December, 2013

Summaries.....................................................................................................................................................7

General introduction...................................................................................................................................13

Mutualism, commitment and co-adaptation.............................................................................................14

The fungus-growing ants as model system................................................................................................15

Open questions and objectives of this thesis.............................................................................................17The influence of domestication on fungal genetics.......................................................................................17Incompatibility between fungal crops of different ant genera.......................................................................18Decomposition enzymes in the fungus-growing ant symbiosis....................................................................19Special Acromyrmex echinatior fungal symbiont enzymes...........................................................................20

Summary and future perspectives.............................................................................................................21

References....................................................................................................................................................27

Chapter 1:....................................................................................................................................................33Advanced farming ants rear polyploidy crop fungiChapter 2:....................................................................................................................................................61Somatic incompatibility of fungal crops in sympatric Atta and Acromyrmex leaf-cutting antsChapter 3:....................................................................................................................................................85Differences in forage-acquisition and fungal enzyme activity contribute to niche segregation in Panamanian leaf-cutting antsChapter 4:...................................................................................................................................................109Leucoagaricus gongylophorus uses leaf-cutting ants to vector proteolytic enzymes towards new plant sub-strateChapter 5:...................................................................................................................................................133Cellulose degradation patterns in Acromyrmex echinatior colonies

Acknowledgements...................................................................................................................................153

contents

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english, Dansk, esPañol, neDerlanDs

summaries

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Fungus-growing ants (Attini) feed off a fungus they cultivate in a mutualistic symbiosis in under-ground chambers by providing it substrate they collect outside the colony. The tribe of Attine ants ranges from small colonies of the paleo- and basal Attine species with a few hundred workers that forage on crude substrates such as insect frass and dry plant material, to large colonies of the leaf-cutting ants with several thousands to several million workers that provide live plant material to their fungus gardens. Leaf-cutting ants are the dominant herbivores of the Neo-tropics, and have a major contribution to cycling of nitrogen and phosphorus in their direct environment and are, furthermore, considered pest species as they have a large impact on human agriculture. These factors make leaf-cutting ants an ideal study subject to better understand the mechanisms that make this mutualistic symbiosis so successful. To understand the evolutionary development of domestication of the fungus over the phylogeny of the Attine ants, I compared the average number of nuclei per cell for the fungal symbionts, for each of the different groups of fungus-growing ants. I found that the fungal symbionts of the paleo- and basal Attine ants, which have a relative low level of domestication, have two nuclei per cell, the standard for Basidiomy-cete fungi, but that the average number increased to 7-17 nuclei per cell in the highly domesticated fungi of the higher Attine ants and leaf-cutting ants. Furthermore, I was able to estimate that approximately half of these nuclei were represented by different genomes, giving the fungus a ploidy level of 5n-6n. In mutualistic symbioses it is important the partners stay true to each other. In fungus-growing ants, new founding queens bring a piece of fungus to build up their new colony. However, in rare occa-sions fungal symbionts might come into contact with symbionts from other colonies. I showed that in both leaf-cutting ant genera incompatibility reactions between fungal strains can avoid intermixing of different strains, and that these reactions strengthen when genetic distance is increased. This pattern, however, be-comes distorted when fungal symbionts are contested across ant genera. The most important mechanism in the succession of this mutualism of leaf-cutting ants is the con-trolled degradation of plant material. I show that in the area of Gamboa, Panama, the two leaf-cutting ant genera forage for rather different plant material, with Atta species specializing on tree-leaves and Acromyr-mex focusing more on flower material and herbal plant material. This difference is reflected in the overall enzyme activity patterns in the fungus gardens, with Atta specializing more on specific enzyme groups and Acromyrmex having an overall high enzyme activity. Finally, I show that the fungal symbiont of the leaf-cutting ant Acromyrmex echinatior produces large amounts of biodegrading enzymes in special structures called gongylidia. The ants eat these structures, but enzymes pass the ant gut without being digested, and are excreted by the ants in their fecal fluid which they mix with freshly foraged plant material placed on the top of the fungus garden. The enzymes are still active and have therefore an important role in the biodegradation of the plant material. With this I show that the fungus evolved some incredible adaptations to a mutualistic life with the ants.

summary

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Svampedyrkende myrer lever af en svamp de dyrker i en mutualistisk symbiose i underjordiske kamre ved at forsyne den med substrat som de henter udefra. Underfamilien af svampedyrkende myrer (At-tini) spænder over paleo- og basale arter med få hundrede arbejdere der fouragerer på tørret plantemateriale og insekt ekskrementer, til store kolonier af bladskærermyrer med flere tusinde til millioner af arbejdere der indsamler friskt plantemateriale til deres svampehaver. Bladskærermyrer er den dominante herbivor i neotroperne hvor de bidrager væsentligt til nitrogen og fosfor kredsløbet i deres nærmiljø og bliver yder-mere betragtet som et skadedyr idet de har stor effekt på landbruget. Disse faktorer gør det ideelt at benytte bladskærermyrer til bedre at forstå mekanismerne der har gjort denne mutualistiske symbiose så succesfuld. For at forstå udviklingen af domesticeringen af svampen over hele den Attine fylogeni, sam-menlignede jeg det gennemsnitlige antal kerner per celle i svampesymbionten hos forskellige grupper af svampedyrkende myrer. Jeg fandt at svampesymbionten hos myrerne fra paleo- og basale arter, som har relativ lavt niveau af domesticering, havde to kerner per celle som er standard for Basidimyceter, hvorimod gennemsnittet steg til 7-17 kerner per celle hos de meget domesticerede svampesymbionter fra de højere udviklede svampedyrkende myrer og bladskærermyrerne. Ydermere kunne jeg estimere at ca. halvdelen af disse kerner havde forskellige genomer hvilket gav svampen et ploidi niveau på 5-6n. I en mutualistisk symbiose er det vigtigt at parterne er tro mod hinanden. Hos svampedyrkende myrer medbringer de nye dronninger et stykke svamp til at grundlægge nye kolonier, men i sjældne tilfælde kan denne svampesymbiont komme i kontakt med symbionten fra en anden koloni. Jeg påviste at i begge bladskærermyrerslægter sørger inkompatibilitets reaktioner imellem varianter af svampe for, at symbionter ikke bliver blandet og at dette styrkes når den genetiske variation forøges. Dette mønster bliver dog for-vrænget når svampesymbionter fra forskellige slægter af myrer testes imod hinanden. Den vigtigste mekanisme i successionen af denne mutualisme hos bladskærermyrer er den kontrol-lerede nedbrydning af plantemateriale. Jeg påviser at i området omkring Gamboa, Panama, fouragerer de to bladskærermyrerslægter på forskelligt plantemateriale, Atta er specialiseret på træblade og Acromyrmex mere på blomster og urter. Denne forskel reflekteres i enzymaktiviteten i svampehaverne, hvor Atta er spe-cialiseret indenfor specifikke grupper af enzymer og Acromyrmex overordnet har en høj enzymaktivitet. Endelig viser jeg, at svampesymbionten hos bladskærermyren Acromyrmex echinatior producer-er store mængder bionedbrydende enzymer i specielle strukturer kaldet gongylidia. Myrerne spiser disse strukturer, men enzymerne passerer ufordøjet igennem tarmsystemet og ender i myrernes fækalievæske som de blander med friskt indsamlet plantemateriale og placerer øverst i svampehaven. Enzymerne er sta-dig aktive og spiller derfor en vigtig rolle i den biologiske nedbrydning af plante materialet. Dette viser at svampen har udviklet nogle utrolige tilpasninger til en mutualistisk eksistens med myrerne.

resumé

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Las hormigas atinas se alimentan de un hongo que cultivan en simbiosis mutualista en cámaras subterráneas al que

proveen con sustrato colectado fuera de la colonia. La tribu Attini presenta colonias que oscilan en tamaño desde las

pequeñas colonias de las especies basales, con unos pocos cientos de trabajadores que forrajean substratos crudos como

heces de insectos y material vegetal seco, hasta las grandes colonias de las hormigas cortadoras de hojas con varios

miles a millones de trabajadores que forrajean material vegetal vivo. Las hormigas cortadoras de hojas son los herbívo-

ros dominantes del Neotrópico, y contribuyen extensamente a los ciclos de nitrógeno y fósforo en su entorno directo.

Por otra parte, estas hormigas son consideradas una especie peste debido a su fuerte impacto en la agricultura humana.

Estos factores hacen de las hormigas cortadoras de hojas un grupo de estudio ideal para intentar comprender mejor los

mecanismos que hacen tan exitosa esta simbiosis mutualista.

Para entender la sucesión de la domesticación a través de la filogenia de las hormigas Attini, se comparó el

número promedio de núcleos por célula para los simbiontes fúngicos de cada uno de los diferentes grupos de hormigas

cortadoras de hojas. En este estudio encontramos que el hongo simbionte de las hormigas atinas basales, las cuales

presentan un nivel relativamente bajo de domesticación, tienen dos núcleos por célula; lo estándar para hongos basidio-

micetos. Sin embargo, este número incrementa a un rango de 7 a 17 núcleos por célula en los hongos altamente domes-

ticados de los grupos más recientes de hormigas atine y cortadoras de hojas. Asimismo se estimó que aproximadamente

mitad de estos núcleos son representados por diferentes genomas, dándole al hongo un nivel de ploidía de 5n-6n.

En simbiosis mutualistas, es importante que las partes sean fieles entre sí. En hormigas cultivadoras de hon-

gos, nuevas reinas fundadoras traen un pedazo de hongo para construir su nueva colonia. No obstante, en raras oca-

siones, los simbiontes fúngicos puede entrar en contacto con simbiontes de tras colonias. Aquí demostramos que en

ambos géneros de hormigas cortadoras de hojas, reacciones de incompatibilidad entre cepas de hongos pueden prevenir

combinaciones de diferentes cepas, y que estas reacciones se fortalecen con el incremento de la distancia genética. Sin

embargo, este patrón se distorsiona cuando los simbiontes fúngicos se oponen a través de los géneros de hormigas.

El mecanismo más importante en la sucesión de este mutualismo de las hormigas cortadoras de hojas es la de-

gradación controlada del material vegetal. Se encontró que en el área de Gamboa, Panamá, los dos géneros de hormigas

cortadoras de hojas forrajean material vegetal bastante diferente; las especies Atta se especializan en hojas de árboles

y las Acromyrmex en material de flores y hierbas. Esta diferencia se refleja en los patrones generales de la actividad

enzimática en los cultivos de hongos, donde Atta se enfoca más en grupos enzimáticos especializados y Acromyrmex

posee una alta actividad enzimática generalizada.

Finalmente, en este estudio demostramos que el simbionte fúngico de las hormigas cortadoras de hojas Ac-

romyrmex echinatior produce grandes cantidades de enzimas biodegradadoras en estructuras especiales denominadas

gongilidios. Las hormigas comen estas estructuras y las enzimas pasan a través del sistema digestivo de la hormiga,

aunque sin ser digeridas. Luego, estas son excretadas en el líquido fecal, el cual las hormigas mezclan con el material

vegetal fresco que ha sido recientemente colectado y ubicado en la parte superior del jardín de hongo. Las enzimas

permanecen activas y por lo tanto juegan un papel importante en la biodegradación del material vegetal. Con esto dem-

uestro que el hongo evolucionó adaptaciones extraordinarias para una vida mutualista con las hormigas.

resumen

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Mieren van de geslachtengroep Attini eten een schimmel die ze in ondergrondse kamers kweken in een mu-

tualisme. Deze schimmel kweken ze door groeisubstraat te geven dat ze buiten het nest verzamelen. De Attine-mieren

zijn divers, met aan de ene kant kleine kolonies van een paar honderd werkers - bij de basale soorten - die ruw materiaal

verzamelen zoals insecten-faeces en dood plantenmateriaal, en aan de andere kant grote kolonies - bij de bladsnijmieren

- met enkele duizenden en soms miljoenen werkers die levend plantenmateriaal verzamelen. Deze bladsnijmieren zijn

de belangrijkste herbivoren in de Neotropen en hebben een grote rol in het recyclen van stikstof en fosfor. Daarnaast

worden ze gezien als ongedierte vanwege hun impact op de landbouw. Dit alles tezamen maakt bladsnijmieren ideaal

model om te onderzoeken wat de mechanismen zijn die de mutualisme tussen mieren en schimmel zo succesvol maakt.

Om te begrijpen hoe de domesticatie van de schimmel evolueerde over de fylogenie van de Attine-mieren,

heb ik van de schimmel het gemiddelde aantal celkernen per cel vergeleken voor de verschillende groepen van deze

mieren. Uit de resultaten kan ik concluderen dat de minder gedomesticeerde schimmels, die gecultiveerd worden door

de basale groep van Attine-mieren, altijd twee celkernen per cel hebben, de standaard voor Basidiomyceten. Maar bij

de meer gedomesticeerde schimmels van de hogere Attine-mieren en bladsnijmieren is dit aantal toegenomen tot een

gemiddelde van 7-17 celkernen per cel. Daarnaast ben ik in staat geweest om aan te tonen dat ongeveer de helft van deze

celkernen een verschillende genoom hebben, wat betekent dat de schimmel een ploïdie-niveau heeft van 5n-6n.

Voor een mutualisme is het belangrijk dat beide partners trouw aan elkaar blijven. Bij de Attine-mieren neemt

de nieuwe koningin een stukje schimmel mee om de nieuwe kolonie te beginnen. Het kan echter gebeuren dat de

schimmel van de ene kolonie in contact komt met de schimmel van een andere. Ik toon hier aan dat bij de schimmels

van twee soorten, van twee verschillende genera, van bladsnijmieren incompatibiliteitsreacties kunnen voorkomen dat

verschillende lijnen van schimmels met elkaar mengen, en dat deze reacties toenemen naarmate de genetische afstand

tussen deze lijnen toeneemt. Dit patroon verdwijnt echter wanneer schimmellijnen van mieren van verschillende genera

met elkaar in contact komen.

De belangrijkste reden waarom bladsnijmieren zo succesvol zijn is het gecontroleerd afbreken van planten-

materiaal. Ik laat hier zien dat in de omgeving van Gamboa, Panama, de twee bladsnijmierengenera verschillend plant-

enmateriaal verzamelen. De Atta-soorten zijn gespecialiseerd in boombladeren en de Acromyrmex-soorten richten zich

op bloem- en kruidachtig plantenmateriaal. Dit verschil is ook te zien bij de enzymactiviteiten in de schimmeltuinen,

waarbij tuinen van Atta-soorten zich specialiseren op specifieke enzymen, maar de tuinen van Acromyrmex-soorten in

het algemeen een hoge enzymactiviteit hebben.

Tot slot laat ik zien dat de schimmel van bladsnijmierensoort Acromyrmex echinatior grote hoeveelheden

enzymen, gespecialiseerd in het afbreken van plantenmateriaal, produceert in speciale organen die gongylidia worden

genoemd. De mieren eten deze organen, maar de enzymen daarin worden niet verteerd door de mier en komen in de

faeces van de mier terecht. De mieren mengen hun faeces met vers plantenmateriaal en plaatsen dit bovenop de schim-

meltuin. De enzymen in de faeces zijn daar nog steeds actief en hebben daardoor een belangrijke rol in het afbreken

van het plantenmateriaal. Hiermee laat ik zien dat de schimmel belangrijke adaptaties aan een mutualistisch leven met

mieren heeft geëvolueerd.

samenvatting

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general introDuction

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Fungus-growing ants of the tribe Attini live in a tight relationship with crop fungi. This relation-ship is so tight that the one partner cannot live without the other, making this symbiosis an obli-gate mutualism (Weber 1966; Howe 1984). For more than a century, substantial knowledge has accumulated about this intriguing symbiosis. However, most of the research has been focused on the natural history of the ants and the system in general (Wheeler 1907; Weber 1966; Mueller et al. 2005), or on the adaptations of the ants to maintain their crops. Some of these studies have in-vestigated differences in worker polymorphism (e.g. Weber 1966; Mehdiabadi and Schultz 2010), the domestication of actinobacteria at the origin of fungus farming 50 MYA to protect the fungus against parasites (e.g. Currie et al. 1999; Currie et al. 2003; Poulsen and Currie 2009), the devel-opment of antibiotic-producing metapleural glands (see e.g. Do Nascimento et al. 1996; Hughes et al. 2008; Yek et al. 2012), followed by studies on biological control by the ants versus these evolutionary derived chemical controls (see e.g. Fernandez-Marin et al. 2007; Fernandez-Marin et al. 2009).

However, far less is known about the fungal adaptations to a mutualistic life with ants, and the research to date has tended to focus on the role of the fungus from the ants perspective (e.g. Martin 1970; Martin 1974; Quinlan and Cherrett 1979; Bass and Cherrett 1995). In recent years, research-ers have shown an increased interest in how the fungal symbiont adapted to make this mutualism one of the dominant herbivores in the neo-tropics (e.g. Schiøtt et al. 2008; Schiøtt et al. 2010; Semenova et al. 2011; De Fine Licht et al. 2013; Aylward et al. 2013). Therefore, in this thesis I attempt to further explore fungal adaptations by investigating fungal morphology, genetics and biochemistry. The first section will give an overview of the research presented in this thesis within a broad perspective of mutualistic life-style, commitment to mutualistic partners and the need for co-adaptations to sustain and elaborate mutualisms that have become obligate. Next, I will introduce my model system of fungus-growing ants and what is already known about the roles of the fungal symbiont in the mutualism. Finally, I’ll conclude with an outline of how I investigated fungal adaptations to mutualistic life with ants.

Mutualism, commitment and co-adaptation

“Despite their different fundamental organization, ant colonies and mycelia of fungi exhibit strik-

ing similarities in their social organization.” (Rayner and Franks 2003)

In order for a stable mutualistic symbiosis to be maintained it is necessary to have a high degree of partner commitment and functional complementarity (Janzen 1985; Keeler 1985). This can

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mostly be observed in endosymbioses where the levels of interdependency are so high that the symbiont becomes part of the cells and tissues of the host. However, also in ectosymbioses like the fungus-growing ant mutualism, where the fungus lives outside the ants, these high levels of partner commitment and functional complementarity can be seen, because the fungus acts as an ectosymbiont for the individual ants, but as an endosymbiont for the colony (Poulsen and Booms-ma 2005). This can lead to a reduced genetic diversity and sometimes even asexuality of the endo-symbiont compared to its host (Law and Lewis 1983; Law 1985), something that was later shown to be the result of co-speciation, which is present in tight mutualisms (Herre et al. 1999). Another condition to maintain a stable mutualism, is that the host should only benefit relatives of the other partner, which will then stabilize the mutualism, but can also lead to reduced population-wide genetic variability in the partner that becomes domesticated (Frank 1994; Doebeli and Knowlton 1998; Sachs et al. 2004; Foster and Wenseleers 2006).

Hosts and symbionts are potentially in conflict over the direction of symbiont transmission (hori-zontal vs vertical), and the magnitude of symbiont dispersal (Frank 1996a; Frank 1996b; Douglas 1998; Douglas 2008). Hosts may therefore reduce the production of sexual symbiont investments, because this may incur cost to the host and because the host risks being confronted with intro-ductions of new symbiont strains that may reduce host fitness (Frank 1996a; Frank 1996b; Leigh 2010). Symbiont internal conflicts (antagonism or incompatibility) might be beneficial for hosts to maintain control of symbiont acquisition (Frank 1996a; Frank 1996b; Bot et al. 2001b). Due to these factors the symbionts can get so integrated with their hosts that their genomes reduce in such manner that a life without the host is merely impossible (McCutcheon and Moran 2012). At this point the host-symbiont conflicts seem to be resolved, so that high levels of cooperation and low levels of conflict will lead the symbiosis towards organismality (Queller and Strassmann 2009) which in turn is maintained by life-time commitments of both partners (Boomsma 2013).

The fungus-growing ants as model system

“Among the multitudinous activities of insects, none are more marvelous than the fungus-growing

and fungus-eating habits of Attiine ants.” (Wheeler 1907)

The tribe of fungus-growing ants, the Attini, which contains ca. 14 genera and >230 species, evolved approximately 50 MYA from a hunter-gatherer ancestor towards obligate farming of a fungal crop (Schultz and Brady 2008; Mehdiabadi and Schultz 2010) which they maintain by providing growth substrate and by removing of any contaminations (Möller 1893; Wheeler 1907;

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Weber 1966; Weber 1972; Mueller et al. 2005). The ants cultivate their crop in monoculture and by default transmit their crop vertically with incipient queens to new colonies (Möller 1893; Wheeler 1907; Weber 1955; Weber 1972; Mueller et al. 2005; Poulsen and Boomsma 2005; Mueller et al. 2010). However, horizontal transmission seems to be common between shallow phylogenetic branches (Mikheyev et al. 2006; Mikheyev et al. 2007; Poulsen et al. 2009), but more constrained between older lineages (Mueller et al. 1998; Johnson and Vilgalys 1998; Vo et al. 2009)

The ants can be divided in different groups of farming, with the lower agriculture attines at the base of the phylogenetic tree, mainly growing fungi from the Leucocoprinaea tribe, including the specialized yeast agriculture clade, which evolved to grow the same fungal species in yeast form, and the coral fungus agriculture in the genus Apterostigma, which cultivate a fungal crop from the Pterula family (Mueller et al. 1998; Munkacsi et al. 2004; Schultz and Brady 2008; Mehdiabadi and Schultz 2010). This group of lower attine ants provide their fungal crops with crude substrates such as insect frass, nectar, seeds and dead and dry plant material (De Fine Licht and Boomsma 2010). The ants and fungi in these clades do not show signs of tight coevolution, with still free-liv-ing fungal relatives in both the Leucocoprinaea clades (Chapela et al. 1994; Mueller et al. 1998; Mueller 2002; Vo et al. 2009) and the Pterula clades (Chapela et al. 1994; Mueller 2002; Munkacsi et al. 2004; Villesen et al. 2004; Dentinger et al. 2009), and signs of horizontal transfer between ant species (Mueller 2002; Green et al. 2002; Mikheyev et al. 2010; Kellner et al. 2013). But when focused on terminal clades of the ant phylogeny, such as the Cyphomyrmex wheeleri group, a strict coevolution between the ants and fungi can be seen (Mehdiabadi et al. 2012).

Several changes in the symbiosis occurred with the introduction of the higher attine ants of the genera Trachymyrmex and Sericomyrmex 20 MYA(Schultz and Brady 2008; Mehdiabadi and Schultz 2010). The colonies have an increased size of approximately ten fold (Fernandez-Marin et al. 2004; Schultz and Brady 2008; Baer et al. 2009; Fernandez-Marin et al. 2009; Mehdiabadi and Schultz 2010), the growth substrate for these genera now includes fresh leaves, flowers and fruits (De Fine Licht and Boomsma 2010) accompanied by differences in fungus garden enzyme activities (De Fine Licht et al. 2010). Furthermore, lack of free living close relatives shows in-creased level of domestication (Mikheyev et al. 2006; Mikheyev et al. 2010), and specialized hy-phal structures, gongylidia, which act solely as food for the ants found their origin in these fungal strains (Möller 1893; Weber 1955; Quinlan and Cherrett 1978; Quinlan and Cherrett 1979; Bass and Cherrett 1995).

The final step was the evolution of the leaf-cutting ant genera Atta and Acromyrmex 10 MYA

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(Schultz and Brady 2008; Mehdiabadi and Schultz 2010), which are considered major herbivores in the neo-tropics and are major contributors of nitrogen and phosphorus recycling, but are also often seen as pest species (Cherrett and Peregrine 1976; Fowler et al. 1989). The switch to primar-ily live plant material with the addition of flowers and fruits (De Fine Licht and Boomsma 2010) caused another shift in overall fungus garden enzyme activity towards plant cell wall degradation (De Fine Licht et al. 2010) and allowed the colonies to grow to sizes of several thousands of work-ers up to several millions (Fernandez-Marin et al. 2004; Schultz and Brady 2008; Baer et al. 2009; Fernandez-Marin et al. 2009; Mehdiabadi and Schultz 2010). Along with this increase in colony size, came an increase in worker polymorphism (Weber 1966; Mehdiabadi and Schultz 2010) and queen mating frequency (Villesen et al. 2002; Baer et al. 2009; Mehdiabadi and Schultz 2010) to allow the colonies to become this large. This together shows that the ecological footprint increased with every single step in the evolution of the fungus growing ants, which was supported by the arrival of the gongylidia 20 MYA and a recent horizontal sweep of the cultivars in the leaf-cutting ants 2-4 MYA replacing all fungal symbionts in the ant clade with a new one (Mikheyev et al. 2010).

Open questions and objectives of this thesisThe influence of domestication on fungal genetics

“The study of the Attini and other fungus-growing insects has only just begun, and further ad-

vance in this fascinating subject will be more difficult for the mycologist than for the entomolo-

gist.” (Wheeler 1910)

Domestication of crop fungi can bring complications for the fungal symbiont such as degenera-tion of genetic diversity due to asexuality caused by vertical transmission (Möller 1893; Wheeler 1907; Weber 1955; Weber 1972; Mueller et al. 2005; Poulsen and Boomsma 2005; Mueller et al. 2010), or mushroom castration by the ants (Fisher et al. 1994a; Fisher et al. 1994b; Mueller 2002), personal observations M Schiøtt, HH De Fine Licht and PW Kooij). However, there are signs of horizontal transfer in the fungal symbionts of the higher attine ants (Mikheyev et al. 2006; Mikhe-yev et al. 2007; Poulsen et al. 2009), and recent studies showed signs of polyploidy in these fungi (Scott et al. 2009; De Fine Licht et al. 2013), but no further research has been done specifically on the latter.

For comparison, in the human-domesticated Agaricus bisporus an increase in the number of hap-loid nuclei (up to 26)(Saksena et al. 1976) was shown compared to its free-living sister species

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Agaricus bitorquis (Raper 1976; Hintz et al. 1988). A similar increase in number of nuclei can be seen in the termite-domesticated fungus Termitomyces with up to 12 nuclei per cell (De Fine Licht et al. 2005). The fungus-growing ants have different levels of domestication of fungal crops, from the paleo- and basal agriculture with free living relatives of the fungal symbionts (Mueller et al. 1998; Mueller 2002; Munkacsi et al. 2004; Schultz and Brady 2008), to more complex domesti-cated agriculture by the higher attine ants and aggressive herbivore agriculture by the leaf-cutting ants with no free living relatives of the symbionts (Mikheyev et al. 2006; Schultz and Brady 2008; Mikheyev et al. 2010). Because of these different levels of domestication these ants are a perfect subject to investigate the influence of domestication on increasing number of nuclei in the fungal cells and the level of ploidy of the crop fungi. Thus, the questions remain, how is the average number of nuclei correlated with increased domestication? And, does the level of ploidy correlate with increased domestication?

Incompatibility between fungal crops of different ant genera

“A leaf-cutter ant colony is a mutualist to on large plant (its fungus), parasite to many others, and

commensal to yet others” (Janzen 1985)

Each individual colony of leaf-cutting ants maintains a single strain of the fungal symbiont Leu-

coagaricus gongylophorus in monoculture (Acromyrmex, Poulsen and Boomsma 2005; Atta, Mueller et al. 2010). The maintenance of the monoculture is done though behavioral adaptations of the ants, e.g. weeding of foreign strains, (Bot et al. 2001b; Ivens et al. 2009) or by somatic in-compatibility reactions by the fungal strain itself (Poulsen and Boomsma 2005). In Basidomycete fungi, such as the fungal symbiont of the ants, such stepwise incompatibility reactions regulate the allorecognition, by rejecting a foreign strain using programmed cell death and cell pigmentation at the meeting point of the two different fungal strains (Rayner et al. 1984; May 1988; Rayner 1991; Worrall 1997). The exact genetic mechanisms involved in these reactions are still unknown, but multiple loci are involved, and the reactions are correlated with genetic distance (Worrall 1997).

The ants often kill alien strains that are genetically different from their native strain, but this antag-onism disappears after force feeding the ants with a foreign strains (Bot et al. 2001b; Ivens et al. 2009). Furthermore, when the ants are almost entirely depleted from their native fungus, and pro-vided with a new foreign fungal strain, the original native strain will grow back and take over the whole colony again within two weeks (Seal et al. 2012). It has been suggested that this is caused by imprinting of the ants by the native fungal strain (Seal et al. 2012), which could be

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done by using chemical recognition profiles (Richard et al. 2007). However, it was shown that the fungal symbionts of Acromyrmex show somatic incompatibility on agar plates, which gradually increases with genetic distance (Poulsen and Boomsma 2005). The same study showed that the fecal fluid of the ants showed increasing levels of incompatibility on foreign fungi with increasing genetic distance, but that these reactions disappeared when the ants were force fed with the foreign fungus, suggesting the incompatibility reactions are of fungal origin. The somatic incompatibility in this system could therefore be explained by the lifetime commitment of the symbiont to live as a monoculture with the ants, and to avoid conflicts within the symbionts (Boomsma 2013). However, the questions, remaining are: Does the incompatibility in Atta fungal crops show sim-ilar incompatibility mechanisms as Acromyrmex fungal crops? And, how are the incompatibility mechanisms between fungal symbionts across the two ant genera?

Decomposition enzymes in the fungus-growing ant symbiosis

“I believe that they are, in reality, mushroom growers and eaters.” (Belt 1874)

The leaf-cutting ant mutualism is based in the degradation of fresh plant material, transported by the ants to the colony, by the fungal symbiont, which in turn provides the ants with nutrients (We-ber 1966; Bass and Cherrett 1995; De Fine Licht and Boomsma 2010). However, the degradation of fresh plant material brings many complications due to the complex structure of the plant cell walls, such as the plants defense mechanism in the shape of phenolic compounds (Coley et al. 1985; De Fine Licht et al. 2013), strong cellulose fibers that require a complex mix of decompo-sition enzymes (Béguin 1990; Martin et al. 1991), and pectins and hemicelluloses that bind the cellulose fibers together into a strong matrix (Selvendran 1984). Furthermore, a fungivore lifestyle for the ants (Davidson 2004) as well as a herbivore lifestyle in general is likely to be nitrogen limited (Mattson 1980; Behmer 2009) which might explain why the ants normally collect highly nutritious plant material (Mundim et al. 2009) and the presence of nitrogen fixing bacteria in the fungus gardens (Pinto-Tomás et al. 2009).

However, the fungus-growing ants with their fungal symbiont seem to have found ways to over-come these complications and become one of the dominating herbivore systems in the Neotropics. This is reflected in the patterns of forage material acquisition that are found throughout the phy-logeny of the Attine tribe, with more crude materials at the basal lineages and mainly fresh plant material in the higher attine clades (De Fine Licht and Boomsma 2010). This pattern corresponds with the differences that can be found in enzyme activities of the fungus gardens in each of these

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groups (De Fine Licht et al. 2010) as well as more specific changes in protease activities for the fungal symbionts (Semenova et al. 2011). Even though these patterns have been found, the leaf-cutting ants acquire forage material from a wide range of plant species which correlate with the availability during the different seasons (Wirth et al. 2003).

It was shown in colonies of Atta cephalotes that the fungus gardens are able to deal with this due to a fast and high rate of plasticity in enzyme activity towards different substrates (Kooij et al. 2010). But because these were laboratory colonies rather than colonies in the field, the question remains, how are acquired plant substrate and fungus garden enzyme activities correlated in field colonies? Furthermore, in the locality of Gamboa, Panama, six species of leaf-cutting ants co-occur, three from each of the two genera. Within each type of habitat one species of each genus can be found: Atta cephalotes and Acromyrmex volcanus as canopy foragers, Atta colombica and Acromyrmex

octospinosus as forest edge foragers, and Atta sexdens and Acromyrmex echinatior as open sunlit area foragers. This raises the question; do the leaf-cutting ant genera Atta and Acromyrmex that live in the same area forage for the same material? And, do the fungus gardens in both genera express the same enzyme activity?

Special Acromyrmex echinatior fungal symbiont enzymes

“As to leaf-cutting ants, I have always held the same view as which is proposed by Mr. Belt, viz.

that they feed upon the fungus growing on the leaves, they carry into their nests, though I had not

yet examined their stomachs. Now I find that the contents of the stomach are colourless, showing

under the microscope some minute globules, probably the spores of the fungus. I could find no

trace of vegetable tissue which might have been derived from the leaves they gather; and this, I

think, confirms Mr. Belt’s hypothesis.” (Müller 1874)

Aside from the general enzyme patterns of the fungus gardens described above, it has been shown that the fungal symbiont alone too has made incredible adaptations to live their mutu-alistic life with the ants. It was shown that both xylanase and cellulase activities are particu-larly high in the bottom layers of fungus gardens of Acromyrmex echinatior and Atta cepha-

lotes (Schiøtt et al. 2008; Moller et al. 2011; Aylward et al. 2013; Grell et al. 2013). To show that these activities came from the fungal symbiont ant not from other organisms, a xylanase gene was isolated from the fungus and shown to be functional when expressed in yeast (Schiøtt et al. 2008). Furthermore, fungal genes for cellulases were expressed most-ly in the in the bottom layer of the fungus garden (Aylward et al. 2013; Grell et al. 2013).

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Recently, a total of 33 proteins were identified and sequenced in the fecal fluid of the leaf-cutting ant Acromyrmex echinatior (Schiøtt et al. 2010). The ants mix this fecal fluid as manure with fresh leaf material is chewed to pulp and some tufts of mycelium taken from elsewhere in the fungus garden and put this mixture on top of the fungus garden to ensure new fungal growth (Weber 1966). Of these 33 proteins, seven proved to be pectinases, important in the breakdown of the plant cell wall, of which at least five showed to be active in the fecal fluid of the ants. Further-more, genes for six of these pectinases were upregulated in the gongylidia compared to normal mycelium (Schiøtt et al. 2010). Later, similar patterns were shown for one laccase, important for the breakdown of phenolic compounds, the chemical defense mechanism of plants, present in the fecal fluid, which was both active in the fecal fluid and the gene for this enzyme was upregulated in the gongylidia compared to normal mycelium (De Fine Licht et al. 2013).

Because herbivore systems are nitrogen limited, it is to be expected that proteases are present in the fecal fluid as well, as part of the plant degrading enzyme package. Thus, the raised questions are: Are fungal proteases also found in the fecal fluid? If so, are these proteases active in the fecal fluid? And, are the genes encoding these enzymes upregulated in the gongylidia when compared to normal mycelium? Also, because cellulose is the main component in plant material (Manzoni et al. 2010; Bell et al. 2014), are cellulases present in the fecal fluid and what is the role of these cellulases? And, what is the role of cellulases in the different layers of the fungus gardens?

Summary and future perspectivesThe influence of domestication on fungal genetics

“Indeed, crop domestication in the context of coevolving and codomesticated microbial consortia

may explain the 50-million year old agricultural success of insect farmers.” (Mueller et al. 2005)

I show that the level of domestication in the fungus-growing ants influences the average number of nuclei per fungal cell, with the fungal symbionts of the paleo- and basal agriculture always having two nuclei, the standard for Basidiomycete fungi. However, when domestication intensi-fies in the higher attine ants, the fungal symbionts have between 7-17 nuclei per cell on average. Furthermore, using microsatellite markers, we were able to estimate that these higher attine fungal symbionts have an increased level of ploidy of approximately 5n-6n, meaning that about half the nuclei present in these strains have different genomes. This sudden increase coincides with the arrival of gongylidia as well as the obligate commitment of being a domesticated crop without free-living relatives (Mikheyev et al. 2006; Schultz and Brady 2008; Mikheyev et al. 2010).

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The increase in number of nuclei and ploidy level seems to be a clever solution to resolve compli-cations of asexuality due to loss of possible gene flow with free-living relatives and the increased lack of viable fruiting bodies, which are, though rarely, generally only seen at colonies of the paleo- and basal attine ants (Mueller et al. 1998; Pagnocca et al. 2001; Mueller 2002). However, it would be interesting to obtain a better estimate of the ploidy level in the fungal symbiont, as well as the nuclear ratio of the different nuclear types, i.e. different genomes, by sequencing the nuclei individually. The challenge for this is to find the right techniques to make this possible. One way could be to create protoplasts of the fungal strains, which is a technique that removes the cell walls of the fungal cells to leave each nucleus separate in a suspension (Sonnenberg et al. 1988). This suspension can then be plated out on a growth medium and subsequently single pro-toplast colonies can be isolated and sequenced. Another techniques is the newly developed laser capture microdissection of single nuclei, which then can be isolated a sequenced using single cell sequencing methods (Guo et al. 2012).

In the fungus-growing wasps Sirex, the fungus is asexually and vertically transmitted as in the fungus-growing ants, but recent results have shown that the fungus maintained the possibility for sexuality (Van der Nest et al. 2013). The diversity in recognition genes in the fungal symbiont of the wasps is relatively high compared to other genes, which indicates that the fungal symbiont has not lost its ability for sexual reproduction. However, the genetic variation in these genes is significantly lower than similar genes in free-living sister species. It would be interesting to see whether the same is true for the fungal symbionts of the ants, because even though it is extremely rare fruiting bodies in colonies of the higher attines can be observed occasionally, though mainly in lab colonies (Fisher et al. 1994a; Fisher et al. 1994b; Pagnocca et al. 2001; Mueller 2002), P.W. Kooij, M. Schiøtt, H.H. de Fine Licht, personal observations).

Finally, polyploidy generally has an influence in genetic aspects such as gene redundancy (Comai 2005), higher gene expression diversity (Groth and Christ 1992) and other stress related advantag-es (Lidzbarsky et al. 2009). It would therefore obvious to investigate what the phenotypic conse-quences are of the increase in ploidy level found in this system. One might expect to find increased levels of expression as well as diversity of biodegrading enzymes when comparing transcriptomes of the higher attine ant fungal symbionts with those of the paleo- and basal attine ants, due to a shift in growth substrate from more crude substrates, e.g. insect frass and dry plant material, in the latter group to more complex substrates, i.e. fresh plant material (De Fine Licht and Boomsma 2010).

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Incompatibility between fungal crops of different ant genera

“Nevertheless, on our functional grounds, the interests of ants and fungi in a colony seem best

viewed as organismally merged.” (Queller and Strassmann 2009)

Following up on the incompatibility study on Acromyrmex fungal symbionts by Poulsen & Booms-ma (2005), I present here that the Atta fungal symbionts show similar incompatibility reactions. Even though the strength of the incompatibility reactions is increasing with increasing genetic distance for Atta fungal symbionts, they tend to be lower in general than in Acromyrmex fungal symbionts. Furthermore, this signal is distorted once the fungal symbionts are in confrontations with strains across the ant genus. To achieve a better insight in the differences found between the Atta symbionts and Acromyrmex symbionts, phylogenies based on microsatellite markers and on AFLP were compared. This not only revealed that the symbionts, which are supposed to be the same species (Mikheyev et al. 2010), separate in each of the ant genera, but also revealed that the phylogeny for Acromyrmex symbionts is much more strict and less variable than that of the Atta

symbionts.

There must be interesting differences between Atta and Acromyrmex symbionts, which may be due to conditional gene expression as these are essentially the same cultivars. This could be relat-ed to the fact that Atta ant species have claustral colony founding, where the queen never leaves the colony and closes of the entrance until the first workers open it to go out foraging 80-100 days later (Weber 1966), securing life time commitment. In contrast, the Acromyrmex queens go out to forage and are therefore most likely to exchange cultivars in this founding phase of the col-ony (Poulsen et al. 2009). Another possible explanation might lie in the possibility that the Atta

symbionts might use imprinting of the ants (Seal et al. 2012), e.g. with chemical cues (Richard et al. 2007), rather than the incompatibility reactions shown in Acromyrmex symbionts (Bot et al. 2001b; Poulsen and Boomsma 2005). A final explanation might lie in the extra AFLP bands that were found in the Atta symbionts compared to those of Acromyrmex, which indicate there might be a presence of RNA/DNA from other organisms such as mycovirusses (Pearson et al. 2009), bacteria (Suen et al. 2010) or prions (Wickner et al. 2007). However, no DNA was amplified with bacteria specific 16S primers, which would exclude the possibility of any bacteria left to explain the extra AFLP bands.

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In order to get a better understanding of how this incompatibility system works it would be inter-esting the measure the overall gene expressions in the interaction zones of both compatible as well as incompatible reactions, by sequencing the full transcriptome in these interaction zones. This has proven to be insightful in the fungus-growing wasp mutualism, and showed clear upregula-tions of multiple genes involved in recognition, stress response as well as programmed cell death (Van der Nest et al. 2011). Furthermore, similar analyses might give an insight in the difference in incompatibility reactions between the Atta and Acromyrmex symbionts at the gene level.

Decomposition enzymes in the fungus-growing ant symbiosis

“Virtually every facet of the ants’ behavior and life cycle has been shaped by their association

with the fungus they culture.” (Martin 1970)

Foraging behavior in May 2011 by the six sympatric species of leaf-cutting ants in Gamboa, Pan-ama, was different between the two genera, however, it was similar between the species within these genera. The Atta species focus their foraging primarily on tree-leaves, whereas the Acromyr-

mex species focused more on collecting flower parts and herbal leaves. This separation in foraging behavior might help explain why the six Panamanian leaf-cutting ant species can be found in strict groups of two species per habitat in this locality (Atta cephalotes and Acromyrmex volcanus as canopy foragers, Atta colombica and Acromyrmex octospinosus as forest edge foragers, and Atta

sexdens and Acromyrmex echinatior as open sunlit area foragers), because competition for forage material is higher between species rather than between genera. The question that raises now is whether similar separation between genera can be found in other smaller communities.

Furthermore, the fungus garden enzyme activities in the colonies of Atta and Acromyrmex cor-related with the forage material collection by the ants, showing that even though these ant species grow the same species of crop fungus, the expression of enzymes is being adjusted according to the substrate provided. A more striking finding is that the overall enzyme activity in Acromyrmex

fungus gardens is higher than that in Atta fungus gardens. Aside from that, Atta fungus gardens seem to have a more specialized activity than Acromyrmex. These difference could be a possible explanation why it seems that Acromyrmex colonies produce less waste material than Atta colo-nies (Bot et al. 2001a; Hart and Ratnieks 2002), and it would therefore be interesting to investigate whether the amount of generated waste material is the same when the genera are provided with the same forage material.

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Special Acromyrmex echinatior fungal symbiont enzymes

“…it is not their biochemical machinery which the ants are contributing to this mutualistic as-

sociation, but rather their capacity to serve as vehicles of transport.” (Boyd and Martin 1975b)

In the first proteomic analysis of the fecal fluid of the leaf-cutting ant Acromyrmex echinatior six proteases and one cellulase were identified and sequenced. The protease fall into three groups of biodegrading enzymes, metalloendoproteases, serine proteases and aspartic proteases, of which the first two groups showed activity in the fecal fluid of the ants. Furthermore, the genes coding for five of these enzymes (peptidyl-Lys metallopeptidase I and II, subtilisin, grifolisin and sac-charopepsin) showed to be upregulated in the gongylidia compared to normal mycelium. This shows that the fungal symbiont made a special adaptation to use the ants as vehicles to transport enzymes from the middle part of the fungus garden where there is enough mycelium to produce the gongylidia as well as enzymes (De Fine Licht et al. 2013) to the top of the fungus garden to degrade new substrate. The new techniques used confirm results from earlier studies, though with much more detail (Boyd and Martin 1975a; Boyd and Martin 1975b). Furthermore, it shows that the tightly regulated intake of proteins by insects in this nitrogen limited environment (Behmer 2009), has been taken over by the fungal symbiont, something that is reflected in the fact that the fungus growing ants are the only ants with extremely high protease activities in the rectum rather than in their midgut (Martin 1970).

The single cellulase that was identified and sequenced was identified as an endocellulase, Lg-Cel12, from the Glycoside Hydrolase family 12 (GH12), and has previously not been described (Aylward et al. 2013; Grell et al. 2013). Endocellulases are the first in a series of three cellulases that complete the degradation of cellulose (Béguin 1990; Martin et al. 1991), the main component in plant cell walls (Manzoni et al. 2010; Bell et al. 2014). Even though activity was present for all types of cellulases in the fecal fluid of the ants, this particular enzyme showed to be upregulated in the gongylidia compared to normal mycelium. And other than previously studied genes (Aylward et al. 2013; Grell et al. 2013), this gene was mostly expressed in the middle layer of the fungus garden rather than the bottom layer. This shows that the LgCel12 has an important function in the fecal fluid of the ants, even though the highest cellulase activity can be found in the bottom layer of the fungus garden and the debris material (Moller et al. 2011; Aylward et al. 2013; Grell et al. 2003; Chapter 5 of this thesis).

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Together with previously studied pectinases (Schiøtt et al. 2010) and laccases (De Fine Licht et al. 2013), now proteases and a cellulase proved to have an important role in the initial stages of plant cell wall degradation in this mutualistic symbiosis, which all together show interesting parallels to the enzyme package used by phytopathogens to attack plants (St Leger et al. 1997). Furthermore, because this system is most likely nitrogen limited due to the herbivore way of living (Mattson 1980; Behmer 2009), it is of interest that the combination of proteases and cellulases is in balance. The proteases are necessary to utilize as much nitrogen as possible, but the cellulases also play an important role in keeping the C:N ratio in balance. When this balance gets disrupted the efficiency of utilizing both carbon and nitrogen decreases (Manzoni et al. 2010; Hartman and Richardson 2013; Sinsabaugh et al. 2013), which in turn would have a detrimental effect on the development of the colony. A reason to keep the cellulase activities particularly high in the bottom layer of the fungus as suggested by Grell et al. (2013) is to maintain the water balance in this section, so to keep the fungus healthy and prevent it from obtaining any diseases before it is discarded by the ants.

These results however, only test the presence and activity of biodegrading enzymes in the leaf-cut-ting ant Acromyrmex, but earlier studies show increased protease levels in the fecal fluid in ants from the whole phylogeny of fungus-growing ants when compared to other ant species (Martin 1970; Martin and Martin 1971; Martin 1974). An obvious next step would thus be to investigate how these fungal enzymes are regulated in the fecal fluid of the leaf-cutting ant genus Atta, which have even larger colonies (Schultz and Brady 2008; Mehdiabadi and Schultz 2010), in the higher attine genera Trachymyrmex and Sericomyrmex, which feed their fungal crop cruder substrates but where gongylidia are present (De Fine Licht and Boomsma 2010), and in lower attines, which do not have gongylidia, but where the ants too produce fecal fluid (Martin and Martin 1971).

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chaPter 1

aDvanceD farming ants rear PolyPloiD croP fungi

PePijn W kooij, Duur k aanen, morten schiøtt, jacobus j boomsma

(manuscriPt)

34

Advanced farming ants rear polyploid crop fungi

Pepijn W. Kooij1, Duur K. Aanen2, Morten Schiøtt1 and Jacobus J. Boomsma1

1Centre for Social Evolution, Department of Biology, University of Copenhagen, Universi-tetsparken 15, DK-2100 Copenhagen, Denmark

2Laboratory of Genetics, Wageningen University, PO Box 309, 6700 AH, Wageningen, The Neth-erlands

Phone: +45 35321239 Fax: +45 35321250

35

Abstract Polyploidy is common in plants, presumably because advantages of increased functional heterozy-gosity often surpass costs of destabilized mitosis or epigenetic instability, but rare in fungi, where ‘diploid’ cells retain the two parental haploid nuclei rather than merging them into a zygote. Such dikaryotic mycelia are actively maintained in Basidiomycota where clamp connections ensure honest propagation of nuclei during cell division, but the same mechanism may constrain the evo-lution of multinucleate cells. Previous research has indicated that the domesticated basidiomycete crop-fungus of leaf-cutting ants, which lacks clamp connections, is a functional polyploid, but without pursuing this further. Here, we used microscopy to estimate the mean number of nuclei per somatic cell for 42 fungal symbionts reared by 14 species (eight genera) of fungus-growing ants in Panama, and mapped these numbers on a fungal symbiont phylogeny generated by ITS and LSU sequencing. This showed that all higher attine ants that rear specialized fungi without free-living close relatives had 7-17 nuclei per cell, whereas non-specialized fungal crops of the basal paleo- and lower attines were dikaryons. Analysis with ten microsatellite markers revealed that ca. 40% of the additional nuclei represented novel haplotypes, yielding an estimate of average ploidy in higher attine crop fungi of 5-6. The fungal symbionts reared by Atta and Acromyrmex

leaf-cutting ants were distinct from the clade reared by other higher attine ants, but we found no differences in nuclei number between these lineages. We hypothesize that functional polyploidy in these asexual and vertically transmitted crop symbionts evolved because benefits of increased heterozygosity outweighed costs that could be met by the farming ants. Polyploid crops may thus represent a form of symbiont chimerism for the sake of enhanced genetic diversity, similar to mul-tiple queen-mating that evolved in the leaf-cutting ants to generate higher heterozygosity among colony workers. The transition to crop-polyploidy coincided with at least an order of magnitude increase in colony size of farming ants.

KeywordsAttini, mutualism, co-evolution, multi-nucleate, Leucoagaricus, Basidiomycota, fungus-growing ants

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IntroductionPolyploidy, the possession of more than two sets of haploid chromosomes, is widespread in flow-ering plants (Masterson 1994; Otto and Whitton 2000), with 15% of speciation events having likely been facilitated by polyploidization, a figure that is even twice as high (31%) in ferns (Wood et al. 2009). Ploidy levels can be as high as 10x in Fragaria (Hummer et al. 2009) and 12x in Rhodondendron (Jones et al. 2007), but have almost always remained even. Polyploidy may be advantageous because it increases functional heterozygosity, which helps masking the phenotypic effects of deleterious recessives, but this may come at the cost of more likely chromosome loss during mitosis, spindle irregularities and other imbalances during meiosis, and more epigenetic instability (Comai 2005). In contrast, polyploidy is rare in metazoans (Wertheim et al. 2013), suggesting that the benefits of polyploidy outweigh the costs more consistently in plants, which retained modular growth in spite of sharing zygote formation and embryonic development with animals (De Mendoza et al. 2013).

Polyploidy is also rare in fungi, but this may be primarily related to fungal cells not being haploid or diploid in the normal sense, but having one (monokaryon) or two (dikaryon) haploid nuclei per cell, with just a few exceptions such as the unicellular Saccharomyces cerevisiae and the multi-cellular Armillaria mellea (honey fungus) which occasionally have merged diploid nuclei (Rizzo and May 1994; Carvalho et al. 1995). Monokaryon hyphae emerging from germinating spores produced after meiosis are thus analogous to gametophytes in vascular plants, so the sexual pro-cess remains uncompleted until two monokaryons merge to form a dikaryon. When such contact is initiated and proves to be compatible, the nuclei of the parental monokaryons distribute them-selves evenly throughout the hyphal system so all cells end up having both types of nuclei that will henceforth be passed jointly when cells divide. This makes dikaryons functionally analogous to diploid vascular plant sporophytes (Raper 1955; Ellingboe and Raper 1962).

As the original haploid nuclei of sexual spores do not become life-time committed in a zygote, their association needs to be actively reinforced with every new cell division of a dikaryon to en-sure that nuclei retain equal probabilities of taking part in sexual reproduction wherever mycelia will initiate the formation of sexual organs. The Basidiomycota that occur mostly as dikaryons have resolved this challenge by evolving clamp connections, specialized bridges between the new cell, which has received a copy of one nucleus, and the old cell that is about to become separated by a new cell wall (Buss 1987). This septum forms simultaneously in the clamp connection where it separates the two copies of the second nucleus, so one of these is predictably returned to the old cell (Moore et al. 2011). Once a basidiomycete dikaryon has formed, incompatibility mechanisms

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will normally prevent that additional nuclei from another monokaryon or dikaryon can be accept-ed unless they are genetically identical or very closely related (Adaskaveg and Gilbertson 1987; Wilson 1991; Marçais et al. 2000; Poulsen and Boomsma 2005).

Multinucleate hyphal cells have only rarely been observed in Basidiomycota. In some cases such as Agaricus bisporus there may be up to 25 nuclei per cell, but these are all clonal copies of the two original nuclei (Saksena et al. 1976) and thus an example of autopolyploidy. However, rare examples of allopolyploidy are documented for Heterobasidion species that have three (James et al. 2009) or four types of nuclei in a single mycelium (Johannesson and Stenlid 2004). These studies used neutral microsatellites markers to estimate the number of haplotypes in excess of the standard two and achieved high resolution, but there is only indirect evidence that the usual expla-nation of increased heterozygosity might apply. The particularly demanding necrotrophic life his-tory of Heterobasidion phytopathogens, relative to saprotrophic fungi, might offer unusual fitness rewards to the higher gene expression diversity that allopolyploidy would likely allow (Groth and Christ 1992), but stress-related advantages of this kind have only been documented for polyploid yeasts (Lidzbarsky et al. 2009).

In a recent genome sequencing project of the basidiomycete fungal symbiont of the leaf-cutting ant Acromyrmex echinatior, we found strong indications for functional polyploidy (De Fine Licht et al. 2013). This was consistent with earlier findings (Scott et al. 2009) and effectively precluded that a draft genome could be assembled. This fungus, Leucoagaricus gongylophorous, is a special-ized domesticated crop organism that the ants rear as their almost exclusive food source in special underground gardens (Mueller et al. 2005). The evolutionary history of fungus farming goes back ca. 50 million years when the ancestor of all extant attine ants gave up its hunter-gatherer life style to start farming vertically transmitted leucocoprinaceous fungi for food (Schultz and Brady 2008). However, it took 30 million years until a single lineage of these crop fungi was irrevers-ibly domesticated, and evolved specialized hyphal tips (gongylidia) to feed the ants (Quinlan and Cherrett 1978; Quinlan and Cherrett 1979; Bass and Cherrett 1995) while losing genetic exchange with free-living relatives (Mueller et al. 2005; Schultz and Brady 2008; De Fine Licht et al. 2010). This gave rise to four genera of so called ‘higher’ attine ants that all rear cultivars belonging to this clade of gongylidia-bearing crop fungi, of which the most advanced Atta and Acromyrmex

leaf-cutting ants arose only 10 million years ago and rear a single, genetically polymorphic spe-cies of crop fungus (Mikheyev et al. 2010).

Given the rarity of basidiomycete polyploidy, and the fact that the human domesticated mushroom

38

Agaricus bisporus has multinucleate cells while its free-living close relative Agaricus bitorquis has normal dikaryons (Raper 1976; Hintz et al. 1988), we decided to test the hypothesis that do-mestication might have induced functional polyploidy, to compensate for the absence or extreme rarity of sexual reproduction (Pagnocca et al. 2001; Mueller 2002; Mikheyev et al. 2006) and pos-sibly also to meet novel requirements of enzyme production (Schiøtt et al. 2010; De Fine Licht et al. 2013). We therefore set out to estimate the average number of nuclei per cell across the fungal symbionts of 14 species of attine ants from Panama and used internal transcribed spacer (ITS) and large subunit rRNA (LSU) ribosomal RNA sequencing and microsatellite genotyping to map nuclei-numbers on the symbiont tree and to estimate functional ploidy.

Materials and methodsBiological materialFungal symbionts of attine ants were isolated on Potato Dextrose Agar (PDA) from 42 lab-col-onies collected in Panama from 2004 to 2012. Samples included representatives of the different stages of ant fungus farming (see Table S1, for details on species and laboratory nest numbers), such that we had 25 samples of higher attine ants rearing domesticated gongylidia- bearing fun-gi, and 17 samples of lower- and paleoattine ants that rear fungi without obvious adaptations to being crops (Mueller et al. 1998; Schultz and Brady 2008). Plates were maintained at ca. 25°C, and DNA was obtained with 5% Chelex extractions when sufficient biomass had been produced.

Nuclei countingSmall pieces of mycelium from each colony were stained with DAPI on a microscope slide, after which we counted the number of nuclei per cell in five randomly chosen cells under a fluores-cence microscope. For analysis the data were grouped in five categories of fungus-farming that are generally recognized as characterizing the diversification and progression of attine fungus farming: coral fungus agriculture, yeast agriculture, lower agriculture, domesticated agriculture, and leaf-cutting agriculture (Schultz and Brady 2008; Mehdiabadi and Schultz 2010). Differences in number of nuclei per cell were subsequently analyzed with a General Linear Model in SAS with colonies as random factor nested within species, species nested within genera, and genera nested within the five categories of fungus farming.

Internal transcribed spacer (ITS) and large subunit rRNA (LSU) sequencingTo make sure that the plated fungi were the same strains as the ones maintained by the fungus growing ants and to verify whether the two known clades of lower agriculture crops (Clade 1 and Clade 2)(Mueller et al. 1998; Schultz and Brady 2008; Mehdiabadi and Schultz 2010) were

39

both represented, we amplified and sequenced two conserved regions: Internal transcribed spacer (ITS4: 5’-TCC TCC GCT TAT TGA TAT GC-3’; ITS5: 5’-GGA AGT AAA AGT CGT AAC AAG G-3’) and the nuclear large subunit rRNA (LR0R: 5’-ACC CGC TGA ACT TAA GC-3’; LR5: 5’-TCC TGA GGG AAA CTT CG-3’), using PCR with 8µL dNTP’s, 2µL gold buffer, 2µL 10mM MgCl

2, 0.1µL BSA, 1µL forward and reverse primer each, 0.2µL gold Taq polymerase, 4.7µL ddH2O and 1µL DNA. The PCR program had 10 min denaturing at 94°C followed by 35 cycles of 1 min denaturing at 94°C, 30 sec annealing at 54°C and 1 min extension at 72°C, and finally a 10 min extension at 72°C. All PCR products were sequenced at BGI Europe in Copenhagen and sequences were deposited in GenBank (see Table S1 for accession numbers). In cases of bad quality sequences due to multiple divergent copies with small length mutations, PCR products were cloned in pCR4-TOPO before sequencing them again using the TOPO TA cloning method (Invitrogen, Carlsbad, CA, USA). A phylogenetic tree similar to Mueller (1998) was constructed using Maximum Likelihood calculations with 500 bootstrap replicates, using MEGA5.1 software (Tamura et al. 2011).

Microsatellite screeningAs we were finding multinucleate cells throughout the higher attine ant symbionts, we decided to screen these fungi for allelic variation at ten microsatellite loci: A128, A1030, A1132, A1151, B12, B447, C101, C126, C647 and D115 taken from Scott (2009), using PCR with 5µL VWR Red Taq DNA polymerase Master Mix (VWR International, Haasrode, Belgium), 0.25µL forward and reverse primer each, 1.5µL ddH2O and 1µL DNA, and a program of 5 min denaturing at 95°C, fol-lowed by 14 cycles of 30 sec denaturing at 95°C, 30 sec annealing at 68-58°C with a touchdown of -0.5°C per cycle, and 30 sec extension at 72°C followed by 20 cycles of 30 sec denaturing at 95°C, 30 sec annealing at 58°C and 30 sec extension at 72°C, and finally a 15 min extension at 72°C. 1µL of each PCR product was then mixed with 8.75µL formaldehyde and 0.25µL LIZ500 and analyzed on an ABI3130xl (Applied Biosystems, Nærum, Denmark) sequencer. Chromato-grams were analyzed with Genemapper 4.0 (Applied Biosystems, Nærum, Denmark) to obtain specific allele scorings, after which a phylogenetic tree was constructed using pairwise Fst values, followed by Neighbor Joining analysis with 500 bootstrap replicates using Populations 1.2.32 software (Langella 2001).

Estimating ploidyThe microsatellite screenings often showed more than two allelic peaks, indicating that excess nuclei were not all copies of dikaryon nuclei, but at least partly represented allopolyploidy. If we would have had infinite variation at a single microsatellite marker, the number of alleles observed

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41

would have given us exact estimates of ploidy. However, the limited cumulative detection power of our 10 moderately variable marker loci necessitated that we estimated the detection error of not observing genetically different haplotypes (because alleles are equal by chance) before estimating the average rate at which haplotype numbers increase with the number of nuclei per cell. To do this, we first plotted the number of different alleles observed against the mean number of nuclei for each of the ten markers and calculated the slopes of regression lines forced through the origin. As our dikaryon data for lower attine fungi showed that cells were sometimes counted after having initiated division, we use that quantified error rate of each nucleus being counted as 1.03 to cor-rect scores along the x-axis (Fig. S2). Because ‘the origin’ was the dikaryon expectation of two nuclei giving an expected allele number score of 1+h, where h is the expected heterozygosity at that particular locus, we subtracted these values from the respective x and y scores before running the analyses. Samples with null alleles were excluded, since they provide incomplete information about heterozygosity. As expected the positive slopes of these regression lines increased with lo-cus-specific heterozygosity for the eight most variable marker loci, so we next plotted these slopes against the locus-specific expected heterozygosity (based on observed allele frequencies). As the slopes entered could only vary between zero and one, we fitted a logistic regression through these data points, while weighting them according to their standard error, to obtain an overall estimation of the number of haplotypes we should have detected if we would have had a marker locus with infinite allelic variation.

Figure 1Ancestry, diversification and mean number of nuclei per cell in garden symbionts of 14 Panamanian fungus-growing

ants. A) Phylogenetic tree of attine ant fungal symbionts based on ITS and LSU sequences and Maximum Likelihood

analysis with 500 bootstrap replicates and Agaricus bisporus (ITS: JX684008.1, LSU: AY635775.1) as outgroup (black

branch). Different levels of agriculture are presented in different colors: coral fungus agriculture gray, lower agriculture

yellow, yeast agriculture red, domesticated agriculture blue, and leaf-cutting agriculture green. Numbers at nodes are

percentages consensus support and colony numbers with species names are given next to branches. Because Apterostig-

ma dentigerum grows a completely different (secondarily acquired) coral fungus its tree was generated in a separate

analysis with the same human domesticated A. bisporus as outgroup. B) Histogram of the mean (± SE) number of nuclei

per cell of the fungal symbionts, showing a clear separations between the three basal levels in the phylogeny (2.40 ±

0.29 nuclei per cell for coral fungus farming, 2.07 ± 0.05 for lower attine fungus farming, and 2.00 ± 0.00 for yeast

farming) and the two advanced levels of fungus farming that both rear clades of gongylidia-bearing fungi (10.80 ± 0.43

for Trachymyrmex and Sericomyrmex symbionts and 12.46 ± 0.41 for Atta and Acromyrmex symbionts). Representative

pictures of stained nuclei are given towards the right: C) Atta sexdens (14 nuclei per cell), D) Sericomyrmex amabilis

(8 nuclei per cell), E) Cyphomyrmex costatus (2 nuclei per cell), and F) Apterostigma dentigerum (2 nuclei per cell).

Arrows indicate the septae that separate the cells.

42

ResultsThe ITS and LSU sequences of the strains obtained from the different colonies were compared to the NCBI GenBank database using blastn, which showed that all sequences had their highest similarities with previously sequenced fungal symbionts from each of the different attine ant spe-cies. Consistent with these earlier results, the phylogenetic tree showed distinct clades among the fungal symbionts comparable to those published previously (Mueller et al. 1998; Munkacsi et al. 2004; Vo et al. 2009; Mikheyev et al. 2010), with all leaf-cutting agriculture strains clustering together, all other gongylidia-bearing domesticated strains clustering together, and all lower- and paleoattine ant strains appearing as basal branches when using the closely related human domes-ticated Agaricus bisporus as outgroup (Figure 1A). A separate analysis with the same outgroup provided a complementary tree for the three pterulaceous fungal symbionts of Apterostigma denti-

gerum. Although microsatellite markers are known to evolve much faster than ribosomal sequenc-es, these markers also recovered the split between the gongylidia-bearing fungi reared by Atta

and Acromyrmex on one hand, and those reared by Trachymyrmex and Sericomyrmex on the other (Figure S1). This indicates that there is no gene flow between these fungal clades at our study site in Panama.

Practically no variation was found in the number of nuclei per cell across the lower agriculture lineages. With only two exceptions we always observed a constant number of two, with only two slightly higher counts (Cyphomyrmex longiscapus [Cylo005] and Apterostigma dentigerum

[Aden003]) that were likely due to hyphal tips being observed while cells were dividing so that nuclei were already duplicated but a new septum had not yet been formed. Mapping the number of nuclei per cell on the branches of the phylogenetic tree produced a clear pattern, with all low-er- and paleo-attine strains, coral fungus strains and yeast strains having two nuclei per cell, and all higher attine ant strains having high numbers (7-17) of nuclei per cell (Figure 1B). This sharp transition thus coincided with the origin of the fungal gongylidia, the uniquely modified hyphal tips whose only known purpose is to be eaten by the ants, an irreversible adaptation to obligate symbiotic life. Statistical analyses showed that the number of nuclei per cell differed between higher (domesticated) fungus-farming on one hand and lower fungus-farming on the other (colors in Figure 1A: F4,27 = 74.46, p < 0.0001), but not between the ant genera within each of these farm-ing types (F4,27 = 1.64, p = 0.1941) or between species within ant genera (F6,27 = 1.71, p = 0.1560)(see also Figure S2).

Microsatellite allele numbers per locus varied between 2 and 10 (Table S2) and all loci produced higher allele number scores when the number of nuclei per cell was higher, although the slopes for

43

the two least variable loci (B447 with 3 alleles and C126 with a total of 4 but maximally 2 alleles per fungal clone) were very shallow and thus considered to be hardly informative (Figure S3). The regression line predicting the rate at which haplotypes would increase with the number of nuclei per cell for an ideal marker locus with an infinite allele number (h = 1) produced an estimate of 0.37 ± 0.21 (95% CL 0.09 – 0.77)(Figure 2). As this regression slope estimated the rate of ploidy increase beyond the dikaryon level of two, this indicates that ca. two out of every five nuclei in excess of the dikaryon starting point represented genetically distinct haplotypes. This implies that the average ploidy of the higher attine fungal symbionts was estimated to be 6.2 (95% CL 3.0 – 10.8).

DiscussionOur results corroborate that the emergence of fungal gongylidia ca. 20 MYA is an even more pronounced irreversible transition on the attine fungus farming symbiosis than previously appre-ciated. It characterized the true domestication of fungal crops and produced four new genus-level branches in the ant phylogenetic tree, of which one split into the two extant genera of leaf-cutting ants 10 MYA (Schultz and Brady 2008; Mehdiabadi and Schultz 2010). Constant high levels of cultivar auto- and allopolyploidy throughout the higher attine ants suggests that evolutionary innovation in the crown group of the attine ants has been driven by significant improvements of crop quality related to dosage advantages and chimeric genetic diversity. It seems reasonable to hypothesize that autopolyploidy evolved first as that remains compatible with occasional sex-ual reproduction (Udall and Wendel 2006), but that later allopolyploidy effectively precluded most if not all meiotic cell division. The emergence of gongylidia induced about an order of magnitude increase in scale of operations, with colony sizes increasing to maximally ca. 3000 in

●

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Figure 2Regression coefficients (± SE) for the locus-specif-

ic increase of alleles scored with increasing number

of nuclei plotted against the expected locus-specific

heterozygosities. The fitted logistic regression line is

based on the eight loci with the highest allelic diver-

sity: p(x) = e-11.077+10.553x / (1 + e-11.077+10.553x) (QAIC =

13.150). The two excluded loci, shown in light grey,

had standard errors that overlapped with the 95% c.l.

of the fitted regression line, corroborating that their

contribution just lacked information rather than devi-

ating from the overall trend in a significant manner.

44

Trachymyrmex and Sericomyrmex higher attine ants, quite possibly because the truly domesticat-ed cultivars were better able to decompose freshly shed leaves and flowers with higher nutritious value (De Fine Licht and Boomsma 2010). Leaf-cutting agriculture allowed a further increase in scale of operations by 1-2 orders of magnitude (Schultz and Brady 2008; Baer et al. 2009; Meh-diabadi and Schultz 2010), but how much of this was driven by evolutionary changes in the ants or the acquisition of a novel clade of crop symbionts 2.3 MYA (Mikheyev et al. 2010) remains unknown.

As far as mushrooms originating from attine fungus gardens have been found, they seem to be most prevalent in the lower attine ants (Mueller 2002), which still have closely related free-living sister lineages (Mueller et al. 1998; Pagnocca et al. 2001). This implies that the capacity to prog-ress towards sexual reproduction, particularly in abandoned fungus gardens, is unlikely to have been lost (Mueller 2002). It has been suggested that higher attine fungi have retained the capacity for meiosis (Mikheyev et al. 2006), but observations of mushrooms are extremely rare, although the initiation of primordia can occasionally be observed in lab colony where they generally appear to fail (Fisher et al. 1994; Mueller 2002; P.W. Kooij, M. Schiøtt, H.H. de Fine Licht, personal ob-servations). Whether consistent combinations of auto- and allopolyploidy would allow occasional recombination without the production of sexual mushrooms remains an open question, but seems a possibility as unrelated fungi are not always somatically incompatible when grown on the same agar plate (Poulsen and Boomsma 2005; Lind et al. 2007). However, without regular recombina-tion fungal clones may accumulate deleterious mutations, which will tend to affect phenotypic performance in spite of polyploidy and will thus retain selection for occasional horizontal transfer (Mikheyev et al. 2007; Poulsen et al. 2009) in a system where vertical transmission via virgin queens is the norm (Weber 1972).

While the absence or rarity of recombination may be a long-term challenge for maintaining op-timal phenotypic performance even in a well-protected crop symbiont, this problem is further aggravated by monoculture farming, a mutualism-stabilizing principle that convergently evolved in both the fungus-growing ants and termites via very different mechanisms (Poulsen and Booms-ma 2005; Aanen et al. 2009). Long-lived colonies thus expose the same fungal phenotypes to possible parasites, which would tend to increase disease pressure over time (Hamilton 1982). Allopolyploidy may be a helpful mechanism to alleviate this type of vulnerability, as it will allow the expression of many more resistance factors than a functionally diploid dikaryon could achieve. Analogies of this chimerism principle can be found in the expression of many MHC alleles in otherwise diploid vertebrate immune systems (Potts and Wakeland 1993) and in the evolution of

45

multiple queen-mating in hymenopteran eusocial insects, which allows the number of haplotypes expressed at the colony level to increase from three (under haplodiploid monogamy) to 10 when a queen mates with eight males. Recent studies have shown that this increase in colony-level het-erozygosity indeed reduces vulnerability towards infections (Seeley and Tarpy 2007; Hughes et al. 2010).

Apart from ameliorating the absence of recombination and disease pressure, allopolyploidy in higher attine crop fungi may also have offered advantages that ultimately allowed the attine ants to abandon the brown decomposition food chain (Shik and Kaspari 2010) to become functional herbivores. This transition started with the Trachymyrmex and Sericomyrmex higher attines that can use modest amounts of soft leaves and petals to manure their fungus gardens, and culminated in the leaf-cutting ants that almost exclusively collect fresh plant material (Weber 1972; Hervey et al. 1977; De Fine Licht and Boomsma 2010). This chain of events implied progressing from litter substrate to live substrate actively defended by secondary plant compounds (De Fine Licht et al. 2013), a challenge that may have been easier to meet with an allopolyploid chimeric spectrum of fungal haplotypes that express larger arrays of complementary enzymes (Soltis et al. 1993), increase biochemical flexibility in other ways (Roose and Gottlieb 1976), or merely allow better growth (Otto and Whitton 2000). The latter may also be achieved with autopolyploidy, as gene dosis advantages may generally enhance performance of multinucleate cells even though nuclei are identical copies (Albertin and Marullo 2012). Fungal analogies of these plant examples could possibly also facilitate the degradation of more complex or recalcitrant substrates, as shown in the allopolyploid brewing yeast Saccharomyces pastorianus (Pope et al. 2007; Minato et al. 2009; Libkind et al. 2011). It is interesting that the only known other examples of fungal allopolyploidy are Heterobasidion species that attack live conifer trees (Hodges 1969; Johannesson and Stenlid 2004; James et al. 2009) and that the spectrum of leaf-cutting fungal decomposition enzymes evolved to become similar to that of many plant pathogens after the attine ants became herbivores (Schiøtt et al. 2010; De Fine Licht et al. 2013).

How the 7-17 alleles that we found are distributed across the multiple nuclei in the crop fungi of higher attine ants remains to be resolved, but two hypotheses for cases like this have been proposed (Scott et al. 2009): 1. The same primer pair amplifies the same set of multiple loci in each haploid nucleus, which would be likely if L. gongylophorus underwent recent genome duplications, and 2. There is genetic variation among the haploid nuclei in multinucleate mycelia. The consistent polyploidy across all lineages of higher attine cultivars suggests that recent duplications are un-likely to explain more than a small fraction of the variation, but explicit studies would be needed

46

to verify the validity of this, or the alternative explanation. This will not be an easy task as similar observations of multinucleate somatic cells in arbuscular mycorrhizal fungi have led to different interpretations. Also here multiple alleles can be found in single mycelia, which has been claimed to imply that a single mycelium could have genetically diverse nuclei (e.g. Kuhn et al. 2001; Hijri and Sanders 2005). However, evidence is now accumulating that this type of sequence divergence within mycelia may derive from differences within single nuclei rather than from differences be-tween nuclei (Pawlowska and Taylor 2004; Lin et al. in press). It is interesting that the occurrence of sex has also remained ambiguous in the arbuscular mycorrhizal fungi (Sanders 2011).

Concluding remarksThe adoption of fungus farming 50 MYA by the ancestor of all attine ants implied a radical aban-donment of the normal hunter-gatherer life style of all ants, but our documentation of functional allopolyploidy evolving in the ant crops 20 MYA strongly suggests that the domesticated, gon-gylidia-bearing fungal symbionts drove later evolutionary developments towards large scale fun-gus farming in complex societies of leaf-cutting ants (Bass and Cherrett 1995; Schultz and Brady 2008; Mehdiabadi and Schultz 2010). Our results do not allow us to differentiate between the various selection forces (compensation for lack of recombination, reducing disease vulnerability, performance advantages owing to multiple identical nuclei or disease resistance owing to genetic chimerism) that could have maintained high degrees of crop allopolyploidy, but it is interesting to note that the attine ants domesticated a lineage of fungi that lack clamp connections (Vellinga et al. 2003), so the main constraint for evolving polyploidy was absent, and that this potential was realized as soon as the gongylidia bearing obligate crop-lineage evolved. It is also intriguing that essentially all human agricultural crops are polyploid or originated from polyploid ancestors (Udall and Wendel 2006). Human cultural evolution of farming practices also started with largely vertically transmitted crops and gradual improvement of their quality, to be replaced by global horizontal transmission of industrially innovated cultivars only during the last half century.

AcknowledgementsWe thank the Smithsonian Tropical Research Institute (STRI), Panama, for providing logistic help and facilities to work

in Gamboa, the Autoridad Nacional del Ambiente y el Mar (ANAM) for permission to sample ant colonies in Panama,

R.M.M Adams, A. Illum, J. Liberti, B. Baer, H.H. De Fine Licht, S.P.A. Den Boer and W. Hughes for help with col-

lecting the ant colonies or making some of their own collected colonies available, Gösta Nachmann for suggesting the

statistical analysis for ploidy estimation, and the Danish National Research Foundation for funding (DNRF57).

47

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tinctorium. Can J Bot 69:2716–2723.

Wood TE, Takebayashi N, Barker MS, et al. (2009) The frequency of polyploid speciation in vascular plants. P Natl

Acad Sci USA 106:13875–13879. doi: 10.1073/pnas.0811575106

51

Ae36

0Ac

-200

6-27

Ae37

2As

-13-

BBAe

451

Ac-2

009-

46Ac

-201

1-3

Ao61

6Ao

510

As-2

011-

4Ac

e-16

-BB

Ace2

3Ao

512

Ace-

19-B

BTz

et02

8M

S120

523-

05Tc

orTc

or00

2M

S120

529-

05Tz

etM

S120

523-

01Tz

etRM

MA1

0061

1-11

BigR

edRM

MA1

0062

9-16

BigR

edSa

m01

4RM

MA1

0062

9-17

BigR

edJO

A120

525-

03Sa

ma

AI11

0511

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ama

10059 53

7860

9110

0

85

73

100

4646

33

24

100

25

24

34

13

1312

15

Ae37

2Ao

616

Ae36

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510

Ao51

2Ac

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-BB

Ace2

3Ac

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As-1

3-BB

As-2

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4Ae

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ma

Sam

014

AI11

0511

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ama

MS1

2052

9-05

Tzet

MS1

2052

3-01

Tzet

RMM

A100

611-

11Bi

gRed

RMM

A100

629-

16Bi

gRed

9853

31

62 5756

7837

23

33

52 15 37 247

3

12

24

20

35

14

11

32

Dome

stica

ted ag

ricult

ure

Leaf-

cutti

ng ag

ricult

ure

10 m

icro

sate

llite

sIT

S an

d LS

U

Figu

re S

1C

ompa

rison

bet

wee

n th

e ph

ylog

enet

ic tr

ee fo

r fun

gal s

ymbi

onts

bas

ed o

n IT

S an

d LS

U s

eque

nces

usi

ng M

axim

um L

ikel

ihoo

d w

ith 5

00 b

oots

trap

repl

icat

es (l

eft)

and

the

tree

base

d on

mic

rosa

telli

te F

st v

alue

s usi

ng N

eigh

bor-J

oini

ng w

ith 5

00 b

oots

trap

repl

icat

es (r

ight

). B

oth

trees

pro

duce

the

sam

e se

para

tion

betw

een

the

sym

bion

ts o

f Atta

and

Acr

o-

myr

mex

leaf

-cut

ting

ants

and

the

sym

bion

ts o

f the

oth

er h

ighe

r atti

ne a

nts o

f the

gen

era

Trac

hym

yrm

ex a

nd S

eric

omyr

mex

, but

ther

e is

littl

e co

ngru

ence

with

in th

ese

clad

es. T

his

is c

onsi

sten

t with

mic

rosa

telli

te lo

ci h

avin

g m

uch

high

er m

utat

ion

rate

s tha

n rib

osom

al g

enes

(Sia

et a

l. 20

00),

but a

lso

show

s tha

t the

re is

no

gene

flow

bet

wee

n th

e tw

o fu

ngal

linea

ges a

t our

sam

plin

g si

te in

Pan

ama.

The

re is

a te

nden

cy fo

r the

ribo

som

al R

NA

bra

nche

s to

be st

ruct

ured

acc

ordi

ng to

hab

itat t

ype

(for

est,

open

gra

ssla

nd, o

r bot

h), w

hich

is

unlik

ely

to re

flect

that

thes

e no

n-co

ding

loci

are

affe

cted

by

sele

ctio

n, b

ut ra

ther

that

rare

hor

izon

tal t

rans

fers

of f

unga

l sym

bion

ts c

an o

nly

happ

en w

hen

colo

nies

are

with

in e

ach

othe

r’s fo

ragi

ng ra

nge.

52

0

2

4

6

8

10

12

14

Seric

omyr

mex

am

abilis

Atta

cep

halo

tes

Atta

col

ombi

ca

Trac

hym

yrm

ex c

orne

tzi

Cyp

hom

yrm

ex c

osta

tus

Apte

rost

igm

a de

ntig

erum

Acro

myr

mex

ech

inat

ior

Myr

mec

ocry

pta

edne

lla

Cyp

hom

yrm

ex lo

ngis

capu

s

Acro

myr

mex

oct

ospi

nosu

s

Cyp

hom

yrm

ex ri

mos

us

Atta

sex

dens

Myc

ocep

urus

sm

ithii

Trac

hym

yrm

ex z

etek

i

Mea

n nu

mbe

r of n

ucle

i per

cel

l

Trac

hym

yrm

ex c

f. ze

teki

Figure S2The mean number of nuclei per cell was significantly different for the five different types of fungus farming with the

gongylidia bearing symbionts of the higher attine ants (including leaf-cutting ants) having consistently higher numbers

of nuclei per cell than the symbionts reared by the lower- and paleo-attine ants (including the yeast growers)(F4,27 =

74.46, p < 0.0001). No significant differences were found between the genera within these types of farming (F4,27 = 1.64,

p = 0.1941) or between ant species within genera (F6,27 = 1.71, p = 0.1560). Bars represent means ± SE and colors are

as in Figure 1.

53

Figure S3Regression plots for the number of dif-

ferent alleles observed per symbiont

sample against the average number of

nuclei per cell for each of the ten micro-

satellite loci. Samples with null alleles

were excluded from the analysis, since

they provide incomplete information

about heterozygosity. Regressions were

forced through the ‘origin’ (in this case

2,1+h, the expected number of nuclei

in a dikaryon and the expected number

of alleles to be observed for two nuclei,

which is equal to 1 plus the expected het-

erozygosity), by subtracting these values

from the respective x and y values before

entering them in the analysis. Regression

slopes were expected to vary between

zero (all additional nuclei are copies of

the two unrelated nuclei of the original

dikaryon) and one (the dotted line that

would be obtained if every additional

nucleus would represent an indepen-

dent unrelated haplotype and if a mark-

er locus would have an infinite number

of alleles so that all these nuclei would

also be scored as genetically different).

Text boxes give locus name (Scott et al.

2009), proportion of explained variation

(R2), F and p-values, and the slopes of

the regression lines.

● ●

●

●

●

●

●

●●

●●

●

● ●

● ●

●

●

Locus A128

R = 0.4964F = 16.76p = 0.0007575

Coef = 0.13373

2

1, 17

●● ●

●

●

●

● ●

●● ●●

●

●

●

● ●

●

●

Locus A1030

R = 0.7363F = 50.25p = 1.312e-06

Coef = 0.23212

2

1, 18

●

●

● ●

●

●

●

●

●● ●●

●

● ●

●

●

●

●

Locus A1132

R = 0.6018F = 27.21p = 5.83e-05

Coef = 0.1273

2

1, 18

●

●

● ●●

●

●

●

● ●

●

●

●

●

●● ●

●● ●

●

●

●

●

●

Locus A1151

R = 0.6078F = 37.19p = 2.678e-06

Coef = 0.29174

2

1, 24

● ●

●● ●●● ●

●

● ●

●

●

●

●

●

Locus B12

R = 0.2323F = 4.54p = 0.05007

Coef = 0.05812

2

1, 15

●

●● ●●

●

●● ●

●

●

● ●● ●● ●

Locus B447

R = 0.5229F = 17.53p = 0.0006969

Coef = 0.06881

2

1, 16

●

●

● ●●● ●

●

●

●●

●● ●●● ●●

●

●

●

●

●● ●

Locus C101

R = 0.5391F = 28.07p = 1.956e-05

Coef = 0.07107

2

1, 24

● ●● ●●● ●

●

● ●●

●● ●●● ●●● ●

●

●

●

● ●

Locus C126

R = 0.0695F = 1.793p = 0.1931

Coef = -0.01431

2

1, 24

●●

● ●●

●

●

●

● ●

●

●

●

●

● ●

●

●

05

1015

05

1015

05

1015

05

1015

05

1015 Locus C647

R = 0.3775F = 10.31p = 0.005126

Coef = 0.11224

2

1, 17

●

●● ●

●

●

●

●

●

●

●

●

●

●

●●

●

●

● ●

●

●

●

●

●

0 5 10 15 0 5 10 15

Locus D115

R = 0.4448F = 19.23p = 0.0001985

Coef = 0.15535

2

1, 24

05

1015

05

1015

05

1015

05

1015

05

1015

Num

ber o

f alle

les

per c

olon

y

Average number of nuclei per colony

54

Acc

essio

n nu

mbe

rA

nt sp

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ber

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enBa

nkLa

rge

subu

nit r

RN

A (L

SU)

ITS

ITS

Gen

Bank

acc

essio

n nu

mbe

rs

Atta

cep

halo

tes

Atta

col

ombi

ca

Atta

sexd

ens

Acro

myr

mex

ech

inat

ior

Acro

myr

mex

oct

ospi

nosu

s

LSU

Tabl

e S1

Ove

rvie

w o

f the

col

onie

s use

d in

this

stud

y w

ith c

orre

spon

ding

Gen

Ban

k ac

cess

ion

num

bers

and

the

best

mat

ches

with

Gen

Ban

k fo

r ITS

and

LSU

sequ

ence

s. A

ll bu

t one

hit

with

Gen

Ban

k sh

owed

clo

sest

sim

ilarit

y to

oth

er fu

ngus

-gro

win

g an

t sym

bion

ts. T

he IT

S se

quen

ce fo

r Tze

t028

show

ed c

lose

st si

mila

ritie

s to

a fr

ee-li

ving

Lep

iota

sp.,

but t

he se

cond

best

hit

was

a fu

ngus

-gro

win

g an

t (JQ

6175

61.1

, E-v

alue

= 2

E-10

2).

55

Acc

essio

n nu

mbe

rA

nt sp

ecie

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valu

eA

cces

sion

num

ber

Spec

ies

E-va

lue

AI1

5051

1-04

Cyco

KF5

7199

3K

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951

JQ61

7738

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phom

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uelle

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1190

6.1

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etop

hyla

x co

nfor

mis

0.00

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1006

10-0

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981

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7193

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6177

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61

chaPter 2

somatic incomPatibility of fungal croPs in symPatric AttA anD Acromyrmex leaf-cutting ants

PePijn W kooij, michael Poulsen, morten schiøtt, jacobus j boomsma

(manuscriPt)

62

Somatic incompatibility of fungal crops in sympatric Atta and Acromyrmex leaf-cutting ants

Pepijn W. Kooij, Michael Poulsen, Morten Schiøtt, and Jacobus J. Boomsma

Centre for Social Evolution, Department of Biology, University of Copenhagen, Universi-tetsparken 15, 2100 Copenhagen, Denmark

Phone: +45 35321239Fax: +45 35321250

63

Abstract Obligate mutualistic symbioses rely on mechanisms that secure host-symbiont commitments to maximize host benefits and prevent symbiont cheating. Evolutionary theory predicts that incom-patibility mechanisms between clonal symbiont lineages may enforce such commitments, but the most advanced endosymbioses have exclusive uniparental and vertical transmission of symbionts so that resident symbiont lineages are never challenged. Although attine ants also transmit their fungus-garden symbionts vertically via colony founding queens, horizontal transmission is possi-ble. Resident garden symbionts may therefore face competition by alternative symbionts, which does not serve their fitness interests and is also unlikely to offer short-term benefits to the host colony. Previous studies showed that somatic incompatibility mechanisms correlate with neutral marker genetic distance between fungal symbionts in Panamanian Acromyrmex leaf-cutting ants, but the extent to which this result applies more generally has remained unclear. We hypothesized that somatic incompatibility mechanisms are only likely to evolve between fungal genotypes that compete for the same niche, i.e. for being the clonal symbiont of one or a few ant species that share the same clade of fungal symbionts. We test this by comparing the expression of somatic in-compatibility among plated fungi reared by sympatric Acromyrmex and Atta colonies in Panama. We show that genetic distances for neutral microsatellite and AFLP markers accurately predict somatic incompatibility for Acromyrmex symbionts, but that these correlations are weaker (mi-crosatellites) or absent (AFLP) in sympatric Atta colonies. Neither set of genetic markers explains any incompatibility between combinations of symbionts from Atta and Acromyrmex on the same plates. Further analysis showed that the symbiont clades maintained by Atta and Acromyrmex

are likely to represent separate gene pools, so that neutral markers are unlikely to be correlated with incompatibility loci that have experienced different selection regimes. We discuss possible reasons for somatic incompatibility among Atta symbionts being only modest, based on the like-lihood of resident symbionts being challenged by competing symbionts during colony founding and when colonies are mature.

KeywordsLeucoagaricus gongylophorus, commitment, mutualism, fungus-growing ants

64

Introduction Endosymbionts are normally asexual and transmitted by uniparental vertical inheritance (Sachs et al. 2011). Multicellular organisms thus have a single mitochondrial genotype and those that have photosynthesis rely on a single clone of plastids. The evolution of such obligate symbiotic mutualisms has strong elements of partner commitment driven by kin selection, because exclusive association of hosts with a single symbiont genotype ensures that any services to growth and sur-vival of the host will benefit clone mates of symbionts that are vertically transmitted when hosts reproduce (Frank 1994; Doebeli and Knowlton 1998; Sachs et al. 2004; Foster and Wenseleers 2006). The same logic implies that hosts and symbionts are potentially in conflict over the mode of symbiont transmission (Frank 1996; Douglas 2008), as symbionts would always benefit from additional horizontal transmission. However, hosts might suffer fitness losses from this form of commitment disloyalty and therefore suppress symbiont investments in sexual reproduction even if that incurs only a marginal cost (Frank 1996; Leigh 2010). When, in spite of such host efforts, symbiont lineages manage to co-infect hosts and compete for host commitment, hosts will be un-der selection to monitor this conflict and eliminate it when symbiont mixing implies a net loss of cumulative symbiont service to the host (Frank 1996).

The classical mitochondrial and plastid endosymbioses are so completely integrated with the host that their reduced genomes preclude any form of non-symbiotic life, and the same is true for many obligate and facultative endosymbionts with less reduced genomes (McCutcheon and Moran 2012). Many of these interactions likely represent adaptive endpoints of host-symbiont coevolution, where host-symbiont conflicts were resolved in favor of the hosts (McCutcheon and Moran 2012; Wernegreen 2012) or symbiont (Werren et al. 2008), but their advanced stage of symbiosis normally precludes direct tests of evolutionary conflict theory over symbiont mixing because symbionts cannot be reared in vitro. However, the fungus-growing ants offer a feasible model system to do such tests, because they have multiple obligate mutualisms including fungus gardens (Schultz and Brady 2008; Mikheyev et al. 2010) and cuticular Actinobacteria (Cafaro et al. 2011; Andersen et al. 2013) that are ectosymbionts for individual ants, but endosymbionts for the ant colonies. This condition implies that partners can be separated and reared in vitro without each other’s interference for sufficient periods of time to quantify antagonism between symbiont clones, monitor host reactions to alternative symbionts, and relate observed differences to the genetic characteristics of the interactants (Bot et al. 2001; Armitage et al. 2011; Seal et al. 2012).

As far as populations of fungus-growing ants have been studied, attine ant colonies have nev-er been found to rear a multi-clone fungus garden (Acromyrmex, Poulsen and Boomsma 2005;

65

Apterostigma, Dentinger et al. 2009; Atta, Mueller et al. 2010; Cyphomyrmex, Green et al. 2002, Mehdiabadi et al. 2012). In all of these studies, there was substantial genetic variation among fungus-garden clones across sympatric colonies, consistent with the normal patterns of variation of mitochondrial and plastid organelles across individual animals and plants (e.g., Embley and Martin 2006). However, in contrast to these cellular endosymbionts, there may be considerable horizontal transfer of symbionts between attine colonies that have similar ecologies, with species belonging to the same genus often (but not always) sharing clades of symbionts (Green et al. 2002; Poulsen and Boomsma 2005; De Fine Licht and Boomsma 2011; Mehdiabadi et al. 2012), while sympatric attine ant genera normally rear distinct fungal symbiont clades (Mueller and Gerardo 2002; Vo et al. 2009; Dentinger et al. 2009; Mehdiabadi et al. 2012; Kooij et al. Chapter 1, in prep).

Horizontal swaps of fungus-garden symbionts between colonies of the same or closely related at-tine ant species reduce the efficiency of co-evolutionary adaptation at the lowest taxonomic level, but may allow ant lineages to replace any asexual crop symbiont that is compromised by genetic load or other forms of maladaptation to prevailing ecological conditions. However, as long as fun-gus-garden clones are thriving, they will also be under selection to actively defend their ant-care monopoly (Bot et al. 2001). Such defenses are expected to evolve when the threat to be replaced is real, i.e. any hostility of this kind should target non-self symbiont genotypes belonging to the clade of symbionts that can in fact partake in a viable symbiosis with a focal attine ant species.

Sympatry and co-exploitation of the same clade of fungus-garden symbionts characterize the Ac-

romyrmex echinatior and Acromyrmex octospinosus populations in Gamboa, Panama (Bot et al. 2001; Richard et al. 2007a; Poulsen et al. 2009), where resident fungus gardens have been shown to maintain their clonal integrity by a combination of behavioral adaptations in the ants to remove and kill alternative fungus clones (Bot et al. 2001; Ivens et al. 2009) and the expression of somatic incompatibility reactions between clones from different Acromyrmex colonies reared together on the same agar plates (Poulsen and Boomsma 2005). These expressions of incompatibility correlat-ed with AFLP genetic distances between pairs of fungal symbionts, a pattern that also applied to the fecal fluid of Acromyrmex large workers fed with fungus from their own and other colonies (Poulsen and Boomsma 2005). However, founding queens of these Acromyrmex species readily accepted fungal clones from other colonies, suggesting that horizontal transfers may be successful at this stage even though new combinations may imply reduced symbiont performance (Poulsen et al. 2009). This study also suggested that incompatibility mechanisms might be different for sym-patric Atta leaf-cutting ants, which tend to rear different fungal symbionts (Mikheyev et al. 2006;

66

Mikheyev et al. 2007), and whose queens never forage during colony founding and therefore have negligible likelihood of encountering alternative symbionts at that stage.

In the present study, we focus explicitly on comparing sympatric mature colonies of Atta and Acromyrmex leaf-cutting ants to address the following questions: 1. Do plated fungal symbionts of Panamanian A. echinatior and Atta colombica express similar somatic incompatibility reac-tions when they are confronted with symbionts from other colonies? 2. To what extent does the intensity of these reactions differ within and between the two genera? 3. To what extent is the intensity of these reactions correlated with genetic distance between the fungal symbionts? 4. Are sympatric colonies of Acromyrmex and Atta rearing the same or overlapping set(s) of fungal symbionts or are they associated with segregated lineages of Leucoagaricus gongylophorus? The same set would be expected if horizontal symbiont transmission between genera is more frequent than natural divergence of lineages via mutation and genetic drift. Segregated lineages would be expected when horizontal transmission between genera is rare or absent because co-adaptations in L. gongylophorus strains reared by Acromyrmex and Atta would preclude that horizontal symbiont swaps between genera are viable. At the same time, we also compared two different sets of genetic markers (AFLP and microsatellites) and different time spans between plate inoculation and scor-ing of incompatibilities to evaluate the robustness of our conclusions.

Natural history summary of attine ants and their fungiAttine ants evolved ca. 50 MYA from a hunter-gatherer ancestor to become obligate farmers of fungal crops, with strict domestication of the cultivar fungus evolving ca. 20 MYA and aggressive herbivory in leaf-cutting ants ca. 10 MYA ago (Mueller et al. 1998; Schultz and Brady 2008; Me-hdiabadi and Schultz 2010). Similar to all other fungus-growing ants, leaf-cutting ants rely on the successful maintenance of their fungus garden crop as main food source (Weber 1966; Quinlan and Cherrett 1978; Quinlan and Cherrett 1979). The large workers of Panamanian Acromyrmex

species are known to actively remove fungal symbiont fragments from other congeneric colonies when they are genetically different from their resident garden fungus (Bot et al. 2001; Ivens et al. 2009), but whether foragers or nurses of Atta do the same is unknown.

The fungal symbiont reared by Atta and Acromyrmex leaf-cutting ants is considered to be a single species (Leucoagaricus gongylophorus; Agaricales, Basidiomycota), specifically associated with these two higher attine genera that appear to have obtained this symbiont via a continent-wide hor-izontal selective sweep ca. 2-3 MYA (Mikheyev et al. 2010). As all other fungal symbionts of the higher attine ants, L. gongylophorus is vertically and asexually transmitted by newly inseminated

67

colony-founding queens (Möller 1893; Wheeler 1907; Weber 1955; Weber 1972). Horizontal ge-netic exchange has been documented (Mikheyev et al. 2006; Mikheyev et al. 2007) but has only been studied systematically for Panamanian Acromyrmex (Poulsen et al. 2009) and not for Atta

(see Green et al. 2002 and Mehdiabadi et al. 2012 for comparable studies in the lower attine genus Cyphomyrmex).

Somatic (in)compatibilities between plated fungi of Panamanian Acromyrmex are expressed in a gradual manner that correlates with AFLP genetic distances between pairs of clones (Poulsen and Boomsma 2005). In basidiomycete fungi, such somatic incompatibilities regulate allorecognition so that strains are increasingly likely to be incompatible when they are more genetically different (May 1988; Worrall 1997). These reactions tend to be stepwise (Rayner et al. 1984; Rayner 1991; Worrall 1997) and usually involve dark pigmentation in the interaction zone (Rayner et al. 1984), changes in septal maintenance, or blockage of septa precluding the movement of cytoplasma, and can lead to programmed cell death (Rayner 1991). The underlying genetic mechanisms remain largely unknown, but multiple loci appear to be involved (Worrall 1997) and their expression may be linked to sexual incompatibility genes (Van der Nest et al. 2009; Van der Nest et al. 2011). Sex may be present but very rare in the fungal symbionts of the higher attine ants (Fisher et al. 1994a; Fisher et al. 1994b; Pagnocca et al. 2001; Mueller 2002; Mikheyev et al. 2006) and recent work indicates that these symbionts are invariably functionally polyploid (Kooij et al. Chapter 1, in prep), which likely explains why somatic incompatibility patterns between fungal symbionts of Panamanian Acromyrmex colonies appear to be gradual (Poulsen and Boomsma 2005).

Materials and methodsBiological materialFungal cultivars were isolated from nine A. echinatior colonies (Ae150A, Ae160, Ae168, Ae263, Ae266, Ae322, Ae356, Ae394, Ae488) and nine A. colombica colonies (Ac-2006-27, Ac-2009-42, Ac-2009-46, Ac-2011-2, Ac-2011-3, Ac-2012-1, Ac-2012-2, Ac-2012-8, Ac-2012-31) living sympatically in Gamboa, Panama, and grown on 39 g/L Potato Dextrose Agar (Sigma-Aldrich) with the addition of 5 g/L yeast extract, 15 mg/L Tetracycline and 12 mg/L Streptomycin. DNA of each fungal strain was extracted using the Qiagen DNeasy Plant Tissue extraction kit and stored at -20°C until further analysis. The Gamboa sampling site was the same as where most previous colonies of the Copenhagen fungus-growing ant research program have been collected, including the Acromyrmex colonies studied by Bot et al. (2001), Poulsen & Boomsma (2005), Mikheyev et al. (2007), Richard et al. (2007a), Ivens et al. (2009) and Poulsen et al. (2009).

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Genetic analysesTo calculate the genetic distance between each of the fungal strains we used two different methods: Amplified Fragment Length Polymorphism (AFLP) and ten microsatellite markers (A128, A1030, A1132, A1151, B12, B447, C101, C126, C647 and D115 developed by Scott et al. (2009)). AFLP was performed as described by Vos et al. (1995) with two selective primer combinations (Eco-ACC + Mse-CAT and Eco-ACC + Mse-CAC). Microsatellite markers were analyzed using PCR with 5 µL VWR Red Taq DNA polymerase Master Mix (VWR International, Haasrode, Belgium), 0.25 µL forward and reverse primer each, 1.5 µL ddH2O and 1 µL DNA, and a program of 5 min denaturing at 95°C, followed by 14 cycles of 30 sec denaturing at 95°C, 30 sec annealing at 68-58°C with a touchdown of -0.5°C per cycle, and 30 sec extension at 72°C followed by 20 cycles of 30 sec denaturing at 95°C, 30 sec annealing at 58°C and 30 sec extension at 72°C, and finally a 15 min extension at 72°C.

Both AFLP and microsatellite amplification products were analyzed on an ABI 3130xl (Applied Biosystems) sequencer. Specific allele scorings were obtained by analyzing chromatograms in Genemapper 4.0 (Applied Biosystems). The program Populations 1.2.32 (Langella 2001) was used to calculate Fst values for the microsatellite data and Nei’s standard genetic distance (Ds) for the AFLP data, followed by Neighbor Joining phylogenetic analyses with 500 bootstrap replicates for each of the two types of markers.

Somatic incompatibilityTo test whether plated fungi showed (in)compatibility, cultures of all 18 fungi were paired in all possible (171) combinations with four replicate pairings for each combination. For each pair, small tufts of mycelium (ca. 2 mm3) were placed at a distance of 1.5 cm from each other on a 5 cm Petri dish with 39 g/L Potato Dextrose Agar (Sigma-Aldrich) with the addition of 5 g/L yeast extract and 35 g/L Agar. The growth medium was selected in a pilot study testing somatic incom-patibility reactions for control (self) encounters on this and three alternative media, which showed that the used PDYA medium most consistently avoided any unexpected discolorations in controls (Figure S1).

Incompatibility reactions were assessed after six, eight and ten weeks and scored using the semi-quantitative scale described by Poulsen and Boomsma (2005): 0 = demarcation zone absent, 1 = demarcation zone weak but present, 2 = demarcation zone broad and distinct, and 3 = strong demarcation zone with consistent brown or black coloration of mycelium. These four scores are consistent with the variation in somatic (in)compatibility reactions that are typically found in

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free-living basidiomycetes, although usually in a more stepwise manner as explained above (Wor-rall 1997). All scorings were done blindly by randomly assigning numbers to each plate, after which two of the authors did the initial assessment and the third author blindly checked combina-tions for which the first two authors did not agree on the score. Degrees of (in)compatibility were subsequently compared with genetic distances between pairs of fungal clones with R (R Core Team 2013) using Mantel and Partial Mantel Tests for Dissimilarity Matrices (“mantel”) with 99999 permutations in the Community Ecology Package: Ordination, Diversity and Dissimilar-ities “vegan” (Oksanen et al. 2013). Figures were created using Plot With Repeated Symbols by Size (“sizeplot”) in the Plotrix package (Lemon 2006).

ResultsFor Acromyrmex symbionts, consistent scorings of somatic incompatibility were obtained eight weeks after agar plates were inoculated, as coloration contrasts had not fully developed after six weeks, so Mantel correlation coefficients between incompatibility and genetic distance after six weeks remained low (Figure S2). Beyond eight weeks, Mantel correlation coefficients continued to improve, but contaminations and medium desiccation problems affected scoring accuracy 10 weeks after inoculation (Figure S2) so that we lost almost 5% of the replicates. As eight weeks was approximately the same as the two-months period between inoculation and scoring in Poulsen & Boomsma (2005), we decided to present our main results for the eight weeks observation pe-riod, to remain as comparable as possible with that previous study on somatic incompatibilities between A. echinatior and A. octospinosus fungal symbionts from the same site. However, for Atta symbionts we only obtained a comparable result when using microsatellite markers, as AFLP markers produced Mantel correlation coefficients close to zero for all observation periods (Figure S2).

Using the eight weeks data, somatic incompatibilities increased with increasing AFLP genetic distances between fungi (Mantel r = 0.463, p = 0.003, Figure 1B) in Acromyrmex symbionts, but not in Atta symbionts (Mantel r = -0.164, p = 0.792, Figure 1D). When using genetic distances based on microsatellite markers, both Acromyrmex (Mantel r = 0.469, p = 0.003, Figure 1A) and Atta (Mantel r = 0.312, p = 0.032, Figure 1C) symbionts had incompatibilities that increased with genetic distance, but less of the incompatibility variance was explained in Atta than in Acromyr-

mex. The A. echinatior results were consistent with the results obtained by Poulsen & Boomsma (2005), but the variance explained by the Mantel coefficient was larger in that study (r = 0.855; p < 0.0001). However, when we used our ten-weeks scorings for Acromyrmex symbiont pairings we obtained a correlation closer to the one obtained after two months by Poulsen & Boomsma (2005)

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(Microsatellites: Mantel r = 0.596, p < 0.001; AFLP: Mantel r = 0.654, p < 0.001).

Overall, the Atta fungi had smaller genetic distances when calculated from the microsatellite marker data (0.09 ± 0.01 SE), but larger genetic distances when using AFLP markers (0.26 ± 0.03 SE) compared to Acromyrmex (0.13 ± 0.01 SE and 0.17 ± 0.02 SE, respectively), which is reflect-ed in the average number of AFLP bands observed (Atta: 37.3 ± 1.8 SE; Acromyrmex 30.9 ± 1.5 SE; t15.49 = -2.724, p = 0.015). This implied that AFLP and microsatellite genetic distances (Fst)

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Figure 1. Mutual somatic incompatibility reactions for Acromyrmex and Atta-associated fungal symbionts plotted against genetic

distances calculated from genetic variation at ten microsatellite markers and AFLPs, with the size of each circle rep-

resenting the number of times a particular combination was found. Correlations were significant for Acromyrmex with

both microsatellites (Mantel r = 0.469, p = 0.003) and AFLP markers (Mantel r = 0.463, p = 0.003), and for Atta with

microsatellites (Mantel r = 0.312, p = 0.032) but not AFLP markers (Mantel r = -0.164, p = 0.792). All Mantel tests were

performed with 99999 permutations.

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were only comparable for Acromyrmex symbionts (r = 0.932; Figure S3), whereas correlations decreased in comparisons with Atta symbionts and became very low in comparisons involving only colonies of A. colombica (Figure S3). Independent of the type of genetic marker used, mean genetic distances were higher in comparisons across the ant genera (Microsatellites: 0.20 ± 0.01 SE; AFLP: 0.36 ± 0.02 SE) than in comparisons within genera (means ± SEs given above; mi-crosatellite markers: F2,168 = 45.127, p < 0.0001; AFLP: F2,168 = 22, p < 0.0001). Also the average incompatibility scores were different for comparisons within ant species and comparisons across ant species (genera) (χ2 = 12.236; Df = 2; p < 0.01). This difference was mostly due to less strong-ly expressed somatic incompatibilities between Atta symbionts, as the stronger mean reactions among Acromyrmex symbionts alone and between Acromyrmex and Atta symbionts were not sig-nificantly different from each other (W = 1635, p = 0.338). After pooling data across this entire range of genetic distances, the increase in somatic incompatibility with genetic distance was no longer significant (Microsatellites: Mantel r = 0.153, p = 0.213, Figure 2A) or absent (AFLP: Mantel r = -0.045, p = 0.595, Figure 2B).


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Figure 2. Somatic incompatibilities in confrontations between fungal symbionts across the Acromyrmex and Atta ant-genus-level

barrier plotted against genetic distances calculated using microsatellite and AFLP markers, respectively. Each circle

represents the presence of a combination of incompatibility score and genetic distance with the size of the circles rep-

resenting the number of times a particular combination was found. Correlations were not significant for microsatellites

(Mantel r = 0.1518, p = 0.2064) or AFLP (Mantel r = -0.1034, p = 0.7063). All mantel tests were performed with 99999

permutations.

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Specific evaluation of the microsatellite genetic differences between the fungal symbionts of the sympatric Atta and Acromyrmex colonies showed that they were completely separated (Figure 3A), consistent with earlier findings of Mikheyev et al. (2007) for the same sampling site. Rooting the symbiont tree with a sympatric fungal symbiont of Trachymyrmex zeteki confirmed that the symbionts belonged to separate clades although bootstrap values were low (Figure 3B). Mirror im-aging of trees obtained by microsatellite and AFLP markers showed almost complete congruence for the Acromyrmex fungi, but more noisy correspondence for the Atta fungal symbionts (Figure S4), confirming that these markers are less reliable when used to characterize Atta symbionts.

DiscussionThe results of our study show that sympatric Panamanian colonies of A. echinatior and A. colom-

bica rear genetically different lineages of the leaf-cutting ant garden symbiont L. gongylopho-

rus and that microsatellite markers appear to predict genetic (in)compatibility better than AFLP markers. However, even when using microsatellite markers, the correlation between somatic in-compatibility and neutral marker genetic distance in Atta is noisier than in Acromyrmex. These

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Figure 3. (A) Circular phylogenetic tree based on Fst values estimated from the ten microsatellite loci showing a clear separation

between fungal strains associated with Acromyrmex and Atta. (B) Rooted tree of the same Acromyrmex and Atta fungal

symbionts with the symbiont of a sympatric colony of Trachymyrmex zeteki as outgroup. Both trees were calculated

with Neighbor Joining and the numbers at nodes are percentage consensus support for 500 bootstrap replicates.

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incompatibility reactions correlate only with genetic distances among fungal strains that have a realistic probability of being horizontally transferred, and not between more distant clades that are apparently unsuitable as symbionts for the sister genus of leaf-cutting ants. We discuss these findings in more detail below.

Somatic incompatibility only between fungal symbionts that can be exchanged? The results of our study confirm the earlier findings by Poulsen and Boomsma (2005) showing that somatic (in)compatibility of Panamanian Acromyrmex symbionts is predictable from pairwise genetic distances for AFLP markers (Figure 1A, B) and show that the same result can be obtained with more specific microsatellite markers. They also indicate that incompatibility reactions be-tween separate clades of fungal symbionts, maintained by different genera of leaf-cutting ants, can no longer be predicted by neutral genetic markers. This may be due to incompatibility being ultimately caused by allelic variation at unknown loci (Worrall 1997) that only correlate with neutral markers when there is recent common ancestry. This is only likely for local lineages of fungal symbionts that are exploited by a single or a few populations of attine ants that share the same pool of symbionts because each ant colony can in principle establish a viable symbiosis with each fungal genotype.

It is only in this situation that we expect somatic incompatibility to be actively maintained, be-cause resident fungus-garden symbionts are selected to defend their monopoly and farming ants loose fitness when maintaining multiple lineages of the same symbiont that are equally viable but compete rather than serve their hosts (Frank 1996; Bot et al. 2001). When symbionts belong to different clades that no longer mix or exchange genes, each symbiont lineage is only a viable sym-biont for one lineage of ant farmers or the other, but not for both. Our finding that A. colombica

and A. echinatior maintain 100% separated clades of fungal symbionts (Figure 3) suggests that L. gongylophorus has in fact been split into an Atta and Acromyrmex clade after its monophyletic origin 2-3 MYA (Mikheyev et al. 2010), and that the two species of leaf-cutting ants that we in-vestigated rear representatives of these symbiont lineages that are adapted to being, respectively, an Atta and Acromyrmex symbiont.

These findings are consistent with the results of a recent study that experimentally swapped fun-gus-garden symbionts between sympatric Trachymyrmex septentrionalis and Atta texana from Texas, USA (Seal et al. 2012). Although these ants are at the northern edge of the attine ant distribution (Mueller et al. 2011a), and likely to have lower genetic variation among their symbi-onts, they share even less common ancestry than the Panamanian fungal symbionts of our present

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study (Mueller et al. 2011b). The swapped fungal symbionts thus could only serve as viable mu-tualists for either Trachymyrmex or Atta, but not both, which was confirmed in the experiments showing that: 1. Alternative fungus gardens were not always rejected by the ants, but they were never adopted as a viable alternative symbiont, because the ants were able to grow their original symbiont back from minuscule remnants that the authors had been unable to remove. 2. None of the T. septentrionalis colonies ever produced virgin queens when they maintained an A. texana

fungal symbiont, consistent with the new combination being non-viable for transmitting ant of fungal genes to future generations. A follow up study transplanting A. texana fungus to colonies of both T. septentrionalis and T. turrifex confirmed that these Trachymyrmex cannot enter into viable symbiosis with L. gongylophorus symbionts and that, even though some virgin queens were produced on swapped gardens, they had poor fat reserves making it unlikely that they could successfully found colonies (Seal and Mueller 2013).

To make further progress, it would be highly desirable to identify the genes that are directly re-sponsible for somatic incompatibility, as this would allow direct studies on the signatures of selec-tion and specialization across the clades of higher attine ant symbionts. In another, non-eusocial model system of fungus-growing insects, the Sirex woodwasp, a range of genes are involved in somatic incompatibility reactions, including fusion and recognition genes, and genes that mediate cellular damage, stress response, and programmed cell death (Van der Nest et al. 2011). Whether these genes have homologs in attine ant fungal symbionts remains to be seen, as the Sirex symbi-ont Amylostereum areolatum belongs to a distantly related clade of basidiomycete fungi (Binder and Hibbett 2002), and their respective domestication histories may have implied that recognition systems were lost and gained over evolutionary time.

Why do Atta fungal symbionts express weaker incompatibility reactions? Incompatibility reactions among Atta symbionts were significantly weaker and less predictable from neutral marker Fst values than similar reactions between Acromyrmex-associated fungi. One possible explanation for this difference could be that there is a fundamental difference in colony founding in the sense that Acromyrmex queens forage during colony founding similar to all more basal attine ants, whereas Atta queens have secondarily evolved claustral colony founding. This implies that newly-mated Atta queens close off their nest cavity to raise the first worker cohort purely on their body reserves, so that new colonies will only be opened by these workers 80 to 100 days after they were founded (Weber 1966; Fernandez-Marin and Wcislo 2005). As a con-sequence, Acromyrmex queens may not only forage for leaf fragments to manure their incipient fungus garden, but also for gardens of other incipient colonies whose queens are out foraging

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(Poulsen et al. 2009), an option that is unavailable for founding Atta queens. Although swapping of incipient fungus gardens with a fungus garden fragment from a mature Acromyrmex colony seems relatively unconstrained during early colony founding, it is likely that stronger mutual com-mitment between a founding queen and her resident fungus garden builds up in a matter of weeks, including relatively strong incompatibility reactions in case a queen or one of her first workers bring in an unrelated fungus garden fragment from a neighboring nest (Poulsen et al. 2009). This can never happen in the 80-100 days during which Atta colonies remain closed, removing selec-tion for expressing incompatibility mechanisms during colony founding.

Why neutral markers should be less reliable predictors of somatic incompatibility in Atta colonies after workers start foraging remains unclear. Imprinting of workers on the odor of a resident fun-gus garden is a possibility (Seal et al. 2012), but it seems unclear why such mechanisms should differ between Atta and Acromyrmex symbionts and why that should reduce selection for more direct defenses by fungal symbionts against being replaced. Fungus gardens of Panamanian Acro-

myrmex colonies differ in chemical profiles (Richard et al. 2007a; Richard et al. 2007b), but these differences do not correlate with genetic distances and comparable data for sympatric Atta colo-nies are lacking. The study by Richard et al. (2007a) also showed that feeding Acromyrmex work-ers with the fungus of another colony increased their chance of being accepted by members of that other colony. A compelling hypothesis may also be that mature Atta colonies in Panama have hundreds of fungus gardens, whereas sympatric mature Acromyrmex colonies have one or a few at best. This may make a difference in the likelihood of a resident fungus garden symbiont being replaced by an accidentally imported small fragment of fungus garden from a neighboring colony, so that less accurate recognition systems suffice in mature colonies of Atta but not in Acromyrmex.

Finally, there could also be technical explanations for the incompatibility differences between the fungal symbionts of Atta and Acromyrmex. First, specific growth conditions may be needed to switch on incompatibility genes and the agar medium that we used may have approached these conditions for Acromyrmex better than for Atta symbionts. Second, we found similar variation for the Panama-symbiont-specific microsatellite markers (Scott et al. 2009) and the general AFLP markers for Acromyrmex symbionts, but enhanced variation in AFLP peaks for Atta symbionts, relative to Acromyrmex symbionts (Figure S3). This suggests that DNA from other organisms may have been amplified with the AFLPs and that such other organisms were only present in Atta sym-biont cultures. In principle, this could be viral (Pearson et al. 2009), bacterial (Suen et al. 2010) or prion (Wickner et al. 2007) DNA. However, universal bacterial 16S primers did not amplify the DNA samples (P.W. Kooij, unpublished data), making bacterial contaminations highly unlikely.

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a STENO grant from The Danish Council for Independent Research | Natural Sciences to MP, and the Danish National

Research Foundation (DNRF57) and the ERC (323085) for funding.

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PDA PDA+ PDYA PDYA+

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Acromyrmex vs Acromyrmex (C), and the cross-confrontations between Atta and Acromyrmex symbionts

(B). All media were Potato Dextrose Agar (PDA) with either 5 g/L Yeast Extract (PDYA) or 35 g/L Agar

(PDA+) or both (PDYA+). The average incompatibilities of the intraspecific controls were significantly

lower than the scores obtained in the interspecific test combinations Ac-Ae (F2, 21 = 5.88, p < 0.01). The

PDYA medium was selected for the remaining experiments because it had consistent zero scores for the in-

traspecific controls, even though there were no significant overall differences between the four media (F3, 21 =

1.49, p = 0.25). Also the medium x type-of-confrontation interaction term was not significant (F6, 21 = 1.19, p

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points (six, eight and 10 weeks) at which incompatibilities were scored. Correlations increased over time for the mi-

crosatellite markers, indicating stronger relationships between genetic distance and level of incompatibility, but this

was only true for Acromyrmex when AFLP markers were used, and cross-confrontations never produced significant

correlations. We decided to base our main analyses on the week 8 data, because in week 10 a total of 32 plates (4.7%)

had to be discarded due to infections (6) or because they had dried out (26), so that fungal interactions could no longer

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Figure S3. Comparisons of genetic distances calculated with AFLP and microsatellite markers. Pearson’s correlation coefficients

were 0.580 for all comparisons (A), 0.932 for comparisons between Acromyrmex symbionts (B), 0.400 for comparisons

between Atta symbionts (C), and 0.418 for the cross-comparisons between Acromyrmex and Atta symbionts (D).

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Figure S4. Comparison of unrooted phylogenetic trees based on Fst values estimated from the ten microsatellite loci (left; the same

data as in Figure 3) and Nei’s standard genetic distances (Ds) calculated from AFLP markers (right), indicating that

phylogenies are almost identical for Acromyrmex-associated fungal strains, but less consistent between the two markers

for Atta-associated strains.

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chaPter 3

Differences in forage-acquisition anD fungal enzyme activity contribute to niche segregation in Panamanian leaf-cutting ants

PePijn W. kooij, joanito liberti, konstantinos giamPouDakis, morten schiøtt,

jacobus j. boomsma

(submitteD)

86

Differences in forage-acquisition and fungal enzyme activity contribute to niche segregation in Panamanian leaf-cutting ants

Pepijn W. Kooij*, Joanito Liberti, Konstantinos Giampoudakis, Morten Schiøtt, Jacobus J. Boomsma

Centre for Social Evolution, Department of Biology, University of Copenhagen, Universi-tetsparken 15, 2100 Copenhagen, Denmark

Phone: +45 35321239 Fax: +45 35321250

*Corresponding author: [email protected]

87

Abstract The genera Atta and Acromyrmex are often grouped as leaf-cutting ants for pest management assessments and ecological surveys, although their mature colony sizes and foraging niches may differ substantially. Few studies have addressed such interspecific differences at the same site, which prompted us to conduct a comparative study across six sympatric leaf-cutting ant species in Central Panama. We show that foraging rates during the transition between dry and wet season dif-fer about 60 fold between genera, but are relatively constant across species within genera. These differences appear to match overall differences in colony size, especially when Atta workers that return to their nests without leaves are assumed to carry liquid food. We confirm that Panamanian Atta specialize primarily on tree-leaves whereas Acromyrmex focus on collecting flowers and herbal leaves and that species within genera are similar in these overall foraging strategies. Spe-cies within genera tended to be spaced out over the three habitat categories that we distinguished in Central Panama (forest, forest edge, open grassland), but each of these habitats normally had only a single predominant Atta and Acromyrmex species. We measured activities of twelve fungus garden decomposition enzymes, belonging to the amylases, cellulases, hemicellulases, pectinases and proteinases, and show that average enzyme activity per unit of fungal mass in Atta gardens is lower than in Acromyrmex gardens. Expression profiles of fungal enzymes in Atta also appeared to be more specialized than in Acromyrmex, possibly reflecting variation in forage material. Our results suggest that species- and genus-level identities of leaf-cutting ants and habitat-specific foraging profiles may give predictable differences in the expression of fungal genes coding for decomposition enzymes.

KeywordsAZCL-assay, behavior, mutualism, plant degradation, Acromyrmex, Atta, Leucoagaricus gongy-

lophorus

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IntroductionMaximizing the acquisition of high quality food under varying ecological conditions is expected to be under continuous natural selection. This notion has inspired many studies addressing opti-mal foraging strategies [1] and the extent to which related species realize niche segregation [2] and character displacement [3] to avoid interspecific competition. These processes often lead to (sub)habitat segregation [4-6] or food specialization, but few comparative studies have focused on generalist insect herbivores as it often remains unclear whether specialization within gener-alist strategies does in fact occur and what the decisive axes are along which niches and habitats may segregate [7, 8]. This question is particularly relevant for social insects, as they are central place foragers and often have a large impact on their surrounding communities. For wood eating termites that live in their food, pest management agencies will automatically accumulate compara-tive data on habitat and niche segregation among species and genera [9, 10], but such comparative studies have remained rare in the leaf-cutting ants.

Atta [FABRICIUS, 1804] and Acromyrmex [MAYR, 1865] leaf-cutting ants originated between 8 and 12 million years ago as the most specialized crown-group of the fungus growing ants (Attini [EMERY, 1913]) [11]. Their extant distribution ranges from warm-temperate South America up to the southern regions of the United States [12-14]. Throughout this range these ants are import-ant (often dominant) herbivores and significant accelerators of nitrogen and phosphorus cycling [15]. They decompose the harvested live plant material through the mutualistic services provided by their fungus-garden symbiont Leucoagaricus gongylophorus [SINGER, 1986], which feeds the ants in exchange for the plant substrate provided [16]. Weber [14] estimated that ca. two kg of fresh plant material is needed to build one fungus garden in an Atta cephalotes [LINNAEUS, 1758] colony and that almost 6000 kg of fresh vegetation had been processed by the collective fungus gardens of a 6.5 year old colony of Atta sexdens [LINNAEUS, 1758].

Many species of leaf-cutting ants are considered pests in agricultural and urban areas [17]. For economic damage assessments, the genera Atta and Acromyrmex are often considered indiscrim-inately, in spite of large differences in colony size [14, 18], degree of worker polymorphism [18-20], fungus garden enzyme activity [21], and foraging behavior [14, 19, 22, 23]. For example, Cherrett [24] showed that forage material of an Atta cephalotes colony in Guyana consisted mostly of leaves with flowers as a distinct minority class, similar to a later studied colony of Atta colombi-

ca [GUÉRIN-MÉNEVILLE, 1844] in Panama [25], whereas forage of Costa Rican Acromyrmex

octospinosus [REICH, 1793], Acromyrmex coronatus [FABRICIUS, 1804] and Acromyrmex vol-

canus [WHEELER, 1937] is known to consist of leaves, flowers and some fruit fragments [26].

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However, to our knowledge no studies have been done to quantify differences of this kind simulta-neously at the same site for an entire local guild of leaf-cutting ants. This implies that habitat-spec-ificity, foraging efficiency, and leaf-processing in fungus gardens have not been compared with formal statistical analyses.

So far, 40 species of leaf-cutting ants have been described [11, 18], and they have all been hy-pothesized to rear the same polymorphic species of L. gongylophorus as fungal symbiont [27], despite the enormous distribution range mentioned above [12, 28] and the highly variable habitats and forage availabilities [18, 22, 24, 28-32]. A recent study [33] has indicated that the extant L.

gongylophorus species is only 2-3 million years old, inferring that it must have swept through all leaf-cutting ant species while replacing the original fungus garden symbiont(s) that they had retained after coming into existence 8-12 million years ago. Other recent studies have shown that the L. gongylophorus fungus garden symbiont is highly plastic in its enzymatic responses to the various leaf-substrates that the ants deposit on their fungus gardens [21, 34], suggesting that for-age type may systematically affect the expression of decomposition enzymes.

The objective of our study was to design a sampling scheme that allows the key characteristics of forage acquisition and processing to be compared across an entire guild of leaf-cutting ants. To achieve that goal, we quantified the diversity of forage material and the absolute and relative foraging rates for six sympatric leaf-cutting ants in the month of May, around the start of the rainy season, in Gamboa, Panama: Atta cephalotes, Atta sexdens, Atta colombica, Acromyrmex

echinatior [Schultz, Bekkevold & Boomsma, 1998], Acromyrmex octospinosus, and Acromyrmex

volcanus. Based on field surveys at this mosaic landscape of secondary growth forest and subur-ban areas for a period of two decades the following generalizations of habitat differentiation [35] appear to apply in Gamboa: Atta cephalotes and Acromyrmex volcanus are forest canopy foragers, whereas Acromyrmex octospinosus forages on the forest-floor. Atta colombica occurs both in the forest (usually at lower elevations) and in moist open grassland habitats, while Atta sexdens and Acromyrmex echinatior prefer open and sunlit nesting habitats for foraging. The latter two species extend their distributions towards the Pacific coast where annual rainfall is less than in Gamboa and natural habitat resembles savannas rather than a mosaic of forest patches [12, 28], matching their preference for open habitat in Gamboa. We supplemented our comparative data on foraging rates and substrate diversity with field measurements on the activity of extracellular enzymes in the fungus gardens maintained by the six leaf-cutting ant species to assess whether foraging pref-erences might be related to specific garden processing activities.

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Material and methodsAnt foraging behaviorIn May 2011 we located 9 – 11 foraging trails each for five of the six ant species (Table 1), always < 30m from the nest for Atta and < 5 m from the nest for Acromyrmex. The sixth species, Acro-

myrmex volcanus, was so rare that only one trail was found. We observed Acromyrmex trails for 15 to 30 min and Atta trails for 2 min (or 4 times 0.5 min when trails were very busy) to obtain comparable data when counting ants that passed an imaginary line perpendicular to the trail. We replicated observations by sampling either trails of different colonies or multiple trails of the same colony going in different directions so they could be considered as independent samples of foraging habitat (Table S1). Diversity of forage material was classified in six categories: (parts of) flowers, (pieces of) fruit, herbaceous leaves, tree-leaves, other material (always rare), and ants carrying nothing on their way back to the colony. When in doubt, we verified the origin of forage particles by back-tracking the trail to the source. Observations were repeated across parts of the day (morning 9 AM-12 PM, afternoon 12 PM-5 PM, evening in the dark 10 PM – 11 PM) and compared statistically to see whether this made any difference.

To assess rates of foraging, we expressed our records in numbers of workers on trails per hour, counting both workers carrying material back to the nest and those without. Data were log-trans-formed to approximately equalize variances and analyzed with R [36], using Linear Mixed-Ef-fects Models (“lme”) [37] and generating p-values with General Linear Hypotheses (“glht”) in the package “multcomp” [38]. Proportional distributions of forage types (tree-leaves, herbaceous leaves, flowers, fruit, other) were analyzed with the same tests, with separate trails being consid-ered as a random factor in both analyses. To test for heterogeneity across trails within species we created a sub dataset consisting of four Atta trails from two colonies (one Atta colombica and one Atta cephalotes) and compared the foraging category scores between trails and species (with trails nested within species, and 0.5 min replicate observations for each trail). This was only possible for these Atta species as we did not have replicate samples within single trails for Acromyrmex (Table S1).

Results for the proportional distribution of forage types were visualized using “heatmap.2” in the R package “gplots” [39]. Dendrograms were calculated with the “pvclust” package [40] using 1000000 bootstrap iterations. Final clustering plots were based on the overall similarities in mean proportions (p) between species and supplemented by estimates of the inverse Simpson Diversity index (D = 1/[Σpi

2]) to allow an explicit analysis of the degree of evenness (high D) between the different forage or expressed enzyme categories across species and genera of leaf-cutting ants.

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The denominator of the index decreases when more categories (p) enter the equation, but when the number of categories is constant (as in our analyses) more even distributions will give lower sums in the denominator and thus higher values of D. For the purpose of our study, D therefore func-tions as an index of generalist foraging or equal enzyme expression, so that high values indicate that all categories are important and low values indicate specialization either on a subset of forage categories or on a subset of expressed enzymes that were most active.

AZCL enzyme activity assaysFor each of the six ant species the garden enzyme profiles were analyzed for five different colo-nies, with the exception of Acromyrmex volcanus for which only one colony was available, but where we could add data for another colony obtained in the previous year by H. H. De Fine Licht (pers. com.). For each colony, fungus gardens were dug up, and about equal size fragments (ca 80 mg) from top, middle and bottom layers of fungus gardens were collected and immediately ho-mogenized together to obtain representative average enzyme activity measures per colony. These measurements were subsequently performed using previously published methods, which are easi-ly applicable in the field and give repeatable results [21, 34, 41]. In short, fungus garden material (240 mg) was crushed with a pestle in a 1.5 ml eppendorf-tube containing 1000 µl 0.05 M TRIS-HCl buffer (pH 7.0), vortexed immediately and then centrifuged for 15 minutes (15000g) after which the supernatant was removed and applied immediately to each of 12 different assay-plates containing 0.1g/L of the Azurine-Crosslinked (AZCL) substrates: amylose, arabinoxylan, barley ß-glucan, casein, collagen, debranched arabinan, galactan, galactomannan, HE-cellulose, rham-nogalacturonan, xylan and xyloglucan that were chosen because they yielded positive enzyme activities in an earlier study [21].

The assay-plates of 6 cm diameter were prepared separately for each substrate using an agarose medium (1% agarose, 23 mM phosphoric acid, 23 mM acetic acid, 23 mM boric acid), and pH adjusted according to the manufacturer’s description (Megazyme, Bray, Ireland). After the medi-um had solidified, round wells (area of ca. 0.1 cm²) were made in each plate with a cut-off pipette tip and 12 µl of the supernatant was applied to each well in triplicate. After 22 hours of incuba-tion at 25°C the plates were photographed and the area of the blue halo surrounding each well (a quantitative measure for the absolute amount of enzyme activity [21, 34]) was measured using the software program ImageJ ver. 1.43u for Macintosh. Enzyme activity measurements were grouped into categories based on which plant cell wall component is the main target of the enzymes ([42] and Megazyme, Bray, Ireland): amylases (measured with amylose), cellulases (measured with barley ß-glucan and HE-cellulose), hemicellulases (measured with arabinoxylan, galactomannan,

92

xylan and xyloglucan), pectinases (measured with debranched arabinan, galactan and rhamnoga-lacturonan), and proteases (measured with casein and collagen). We present these data grouped for five categories of enzymes, implying that each of these categories had data from 2-3 enzymes, except for amylases that were represented by only a single enzyme amylose.

The enzyme activity scores were analyzed using a General Linear Model in SAS, with “colony” nested within “species” and “species” nested within “genus”. Colony was then treated as having a random enzyme class activity, composed of specific enzyme activities nested within enzyme activity class. This procedure implied that, we had to omit amylase, because we only had a single substrate (amylose = starch) for testing activity and because starch is not a primary challenge in the degradation of plant material [34]. We also left out rare Acromyrmex volcanus and thus report original mean values for enzyme activity in Figure 2 for all species and enzyme classes, whereas statistics given in the text refer to the reduced data set of four enzyme classes and the gardens of five ant species. In our final analyses (Figure 3) we combined the enzyme and foraging datasets and visualized patterns of association with the plot.PCA function after Principal Component Anal-ysis (“PCA”) using the “FactoMineR” package [43].

ResultsThe three Atta species had an average foraging rate of 5014 ± 555 SE ants/h, with 3374 ± 476 SE ants (67%) returning to the nest carrying forage material and 1640 ± 203 returning unloaded, whereas the three Acromyrmex species had an average foraging rate of 80 ± 14 SE ants/h (t52.444 = -19.347, p < 0.0001) and no workers returning without forage (Table 1). Separate analyses, using the probability of a foraging trail having loaded ants, showed that the genus-level differences in loaded and unloaded returning foragers per hour were highly significant (χ2

1 = 7.567, p < 0.01), but that the differences in loaded returning workers between species within genera were not sig-nificant (χ2

4 = 1.257, p = 0.87).

Across genera, Atta foragers harvested significantly more tree-leaves than Acromyrmex workers (z = 6.420, p < 0.0001), while Acromyrmex foragers collected significantly more (pieces of) flowers than Atta workers (z = -4.894, p < 0.0001). However, there were also differences in the most abun-dant forage category within genera. All Acromyrmex species preferred some combination of flow-ers and herbaceous leaves, but Acromyrmex volcanus was more flower-biased and Acromyrmex

echinatior more herbaceous-leaves-biased (Figure 1A). Similarly, while Atta cephalotes primarily harvested tree-leaves (sexdens vs cephalotes, z = -4.987, p < 0.001; cephalotes vs colombica, z = 6.782, p < 0.0001), Atta colombica brought in more herbaceous leaves (cephalotes vs colombica,

93

z = -7.109, p < 0.0001), and Atta sexdens had approximately equal shares of all forage categories (Figure 1A), which confirmed earlier findings by De Vasconcelos [44].

We validated the statistical independence of our trail samples, using a subset of two Atta colonies (one Atta colombica and one Atta cephalotes), for which we had four replicated samples of the same trails (0.5 min each) and two separate trails per colony (Table S1). This recovered our ear-lier result that the two Atta species have different fractions of forage categories (F4,60 = 89.48, p < 0.0001), but also showed that different trails of the same colony yielded similar results in spite of covering non-overlapping fractions of the colony’s foraging habitat (F8,60 = 0.75, p = 0.65). Fur-ther ANOVA showed that frequencies of forage types between the different times of the day were significantly different for Atta species (F8,218 = 4.451, p = 0.0001), and a post-hoc test indicated this was due to a higher share of herbaceous leaves in the afternoon compared to the evening (z = -3.929, p = 0.009). Acromyrmex species did not show any activity in the dark (evening) and frequencies of forage types between morning and afternoon observations were not different (F4,76

= 1.457, p = 0.224).

Ranking the six species according to the diversity of forage material (Figure 1B) gave no signifi-cant difference between species within genera in evenness of forage category use (F5,45 = 1.776, p = 0.137), but the pooled Atta species had a lower evenness in forage category use (D = 1.42±0.08 SE) than the pooled Acromyrmex species (D = 1.86 ± 0.13 SE; F1,49 = 5.435, p = 0.024). After excluding Acromyrmex volcanus where sample size was very small and two forage categories completely missing, the evenness trends in Figure 1B corresponded fairly well with the relative proportional forage acquisition data in Figure 1A with, from left to right, a clearly increasing

Genus Species Trails Observation Total ants Loaded ants per hour Unloaded ants per hour

Acromyrmex

echinatior 10 (10) 285 min 26580 ± 14

58 ± 100 ± 0

0 ± 0octospinosus 10 (10) 285 min 458 102 ± 26 0 ± 0

volcanus 1 (1) 30 min 43 86 ± NA 0 ± NA

Atta

cephalotes 11 (6) 26 min 15193374 ±

476

2740 ± 4561640 ± 203

2408 ± 373colombica 10 (7) 24.5 min 1724 5383 ± 1003 1115 ± 188

sexdens 9 (7) 22 min 801 1173 ± 447 1012 ± 301

Table 1. Differences in foraging rate between loaded and unloaded foragers.Differences in foraging rate between loaded and unloaded foragers of Atta and Acromyrmex species in Gamboa, Pan-

ama, with summary statistics on the number of trails observed per species (number of colonies in brackets), the total

number of minutes of observation per species, the total number of ants counted while returning to their nests, and the

foraging rates for loaded and unloaded returning workers: means (± SE) per genus and per species (see Table S1 for

details).

94

trend in tree-leaf use, a decreasing trend in the use of flowers, and hump-shaped trends in the use of herbaceous leaves and fruit. These inferences were supported by moderately high overall Ap-proximately Unbiased (AU) p-values and Bootstrap Probability (BP) values (Figure 1). As night foraging tended to decrease the acquisition of herbaceous leaves by Atta species, we probably underestimated the difference in dependence on tree leaves between Acromyrmex and Atta by obtaining most of our Atta observations at daytime.

In absolute quantities, fungus gardens of Acromyrmex showed a higher overall enzyme activity than gardens of Atta (F1,20 = 8.54, p < 0.01)(Figure 2A), but species within genera did not show significant additional differences (F3,20 = 1.32, p = 0.29). To further investigate the differences in

0 0.3 0.90.6Value

0.512 0.491 ± 0.104

0.140 0.062 ± 0.031

0.352 ± 0.105

0.115 ± 0.083

0.027 ± 0.008

0.036 ± 0.016

0.149 ± 0.093

0.188 ± 0.121

0.054 ± 0.032

0.002 ± 0.001

octovolc echi sex cepcol

0.349 0.007 ± 0.007

0.000 ± 0.000

0.252 ± 0.099

0.323 ± 0.116

0.790 ± 0.054 tree

other

0.000 0.298 ± 0.081

0.417 ± 0.100

0.565 ± 0.098

0.251 ± 0.093

0.000 ± 0.000 herbaceous

0.000 0.142 ± 0.082

0.115 ± 0.073

0.121 ± 0.057

0.089 ± 0.058

0.153 ± 0.051 fruit

flowers

A Forest

Forest edge

Open area

B

Acr

omyr

mex

vol

canu

s

Atta

cep

halo

tes

Atta

col

ombi

ca

Atta

sex

dens

Acr

omyr

mex

ech

inat

ior

Acr

omyr

mex

oct

ospi

nosu

s

1.37 ± 0.141.77 ± 0.19 1.63 ± 0.221.88 ± 0.182.48 1.36 ± 0.08D =

D = 1.86 ± 0.13 D = 1.42 ± 0.08

67 711

99 832

98 713

89 754

au bpedge #

100

92

au

21

30

bp

2

3

edge #

100

76

83

41

1

4

Figure 1. Differences in forage diversity.Differences in forage diversity between leaf-cutting ant species (nested within genera), using solid lines for Atta and

dotted lines for Acromyrmex, and with typical foraging habitat indicated with dark green (forest), yellow (forest edge),

and orange (open sunlit areas): (A) Heatmap showing differences between species and genera in the use of forage cat-

egories, with numbers representing the mean proportions ± SE of the forage types. Darker colors indicate higher mean

acquisition proportions, with the top-dendrogram illustrating similarities between species/genera across means of the

five forage categories (vertical axis). Ant species names are given as abbreviations (volc, octo, echi, col, sex, cep).

(B) Dendrogram based on the Inverse Simpson Diversity Index of the five forage categories, indicating the degree of

evenness across foraging categories (numbers below the branches are mean D-values ± SE per species and means per

genus), showing that Acromyrmex has a broader (more even) spectrum (D = 1.86 ± 0.08 SE) of forage material than

Atta (D = 1.42 ± 0.13 SE; F1,49 = 5.435, p < 0.05). Numbers above the branch nodes represent Approximately Unbi-

ased p-values (AU, red) and Bootstrap Probability values (BP, green).

95

enzyme activity between the two genera, we decomposed the significant interaction term of ant genus and AZCL category (F10,780 = 4.95, p < 0.0001). This revealed significant differences be-tween Acromyrmex and Atta for all substrates except rhamnogalacturonan. The spectrum of rel-ative enzyme activities, as expressed by the inverse Simpson indices (Figure 2B), showed that Acromyrmex species tend to have more evenly distributed enzyme activities (D = 4.55 ± 0.05 SE) than Atta species who tend to specialize more on the expression of specific classes of enzymes (D = 4.18 ± 0.07 SE; F1,52 = 15.006, p < 0.0001). No significant differences were observed for the evenness of the enzyme activity spectra for species within genera (Atta: F2,12 = 1.618, p = 0.239; Acromyrmex: F2,9 = 0.111, p = 0.896).

BA

D = 4.55 ± 0.05 D = 4.18 ± 0.07

Atta

col

ombi

ca

Atta

sex

dens

Acr

omyr

mex

ech

inat

ior

Acro

myr

mex

vol

canu

s

Atta

cep

halo

tes

D = 4.15 ± 0.10 3.98 ± 0.144.54 ± 0.094.64 ± 0.25 4.42 ± 0.074.54 ± 0.05

Acr

omyr

mex

oct

ospi

nosu

s

proteinases

pectinases

hemicellulases

cellulases

amylases1.716 ±0.031

1.372 ±0.163

1.322 ±0.140

1.670 ±0.134

1.261 ±0.159

1.938 ± 0.113

0.980 ± 0.108

1.652 ± 0.086

1.205 ± 0.087

1.426 ± 0.103

1.581 ± 0.109

1.038 ± 0.094

1.074 ± 0.097

0.942 ± 0.073

1.014 ± 0.074

1.823 ± 0.109

0.915 ± 0.135

1.081 ± 0.071

0.736 ± 0.076

0.968 ± 0.097

1.382 ± 0.081

0.549 ± 0.116

0.826 ± 0.054

0.611 ± 0.064

0.650 ± 0.097

1.454 ± 0.150

0.551 ± 0.075

0.992 ± 0.047

0.741 ± 0.065

0.907 ± 0.086

octovolcechi sexcep col

0.5 1 1.5 2Value

100 100

6666

au

44 44

5959

bp

1 2

34

edge #

99 99

28

66

au

51 22

21

41

bp

1 2

3

4

edge #

Figure 2. Differences in fungus garden enzyme activity.Differences in fungus garden enzyme activity between species grouped as in Figure 1 with solid lines for Atta and dot-

ted lines for Acromyrmex, and with dark green, yellow and orange indicating the same habitat categories: (A) Heatmap

showing differences between species and genera in fungus garden activity of enzyme classes, expressed as the mean

area in cm2 ± SE of colored halos on AZCL plates across all assays for enzymes belonging to the amylases (1), cellulases

(2), hemicellulases (4), pectinases (3) and proteinases (2). Darker colors in the heatmap indicate higher means, and the

top-dendrogram illustrates similarities between species across all means for the five groups of enzymes, estimated by

“pvclust” with 1000000 bootstraps. (B) Dendrogram based on the inverse Simpson Diversity Index of proportional

enzyme activity showing that Acromyrmex fungus gardens have more even secretions across enzyme categories (D =

4.55 ± 0.05 SE) than Atta (D = 4.18±0.07 SE, F1,52 = 15.006, p < 0.0001). Numbers above the branch nodes represent

Approximately Unbiased p-values (AU, red) and Bootstrap Probability values (BP, green).

96

Comparative analyses (PCA), with either fungus garden enzyme expression as a predictor variable and forage diversity as a response variable (Figure 3A and B) or vice versa (Figure 3C and D), confirmed a separation between the genera Atta and Acromyrmex (Figure 3B and D). Taking the fungus garden enzyme activities as predictor variables produced a first axis explaining 69.04% of the variation and a second axis explaining 12.02% of the variation. The first axis corresponded to overall enzyme activity and illustrates that general fungus garden enzyme activity is lower towards the left (predominantly Atta) and higher towards the right (predominantly Acromyrmex) (Figure 3A and B), confirming the results given in Figure 2. The vertical axis reflects higher amounts of pectinases (positive scores) versus higher amounts of proteases (negative scores). However, this does not correspond in any obvious way with genus-level differences, but may correlate with colony-level differences in the proportions of flowers and fruit in the forage (Figure 3A and B).

A similar pattern was obtained when the predictor and response variables were reversed, (Figure 3C and D). The first axis (explaining 33.11% of the variation) illustrates a preference for tree-leaves (mostly Atta) towards the left and a preference for herbaceous leaves (mostly Acromyrmex) towards the right (Figure 3C and D). The second axis (explaining 25.54% of the variation) indi-cates higher intake of fruit (negative scores) in weak association with cellulases, and of flowers (positive scores) mostly in association with pectinases. Here the leaf-cutting genera are also sepa-rated to some extent (centroid squares) with Acromyrmex having some preference for flowers and Atta for fruit, confirming the results depicted in Figures 1 and 2.

The PCA comparisons also revealed correlations between forage preference and fungus garden enzyme expression, as both PCA analyses showed a negative correlation between extent of acqui-sition of tree-leaves and overall intensity of enzyme activity. Kendall’s rank test for correlation showed that this negative trend was significant for most enzymes: amylases (z = -3.185, p < 0.01); pectinases (z = -2.608, p < 0.01); proteases (z = -3.465, p < 0.001), but not cellulases (z = -1.429, p = 0.153) and hemicellulases (z = -1.953, p = 0.051). The same analyses also found a negative correlation between fruit foraging and expression of amylases (z = -2.311, p < 0.05) and positive correlations between foraging on herbaceous leaves and expression of amylases (z = 3.643, p < 0.001), pectinases (z = 2.130, p < 0.05) and proteases (z = 3.404, p < 0.001) and between flower foraging and expression of the same enzymes: amylases (z = 2.712, p < 0.01), pectinases (z = 3.066, p < 0.01) and proteases (z = 3.769, p < 0.001). Foraging on other materials was only (pos-itively) correlated with the expression of pectinases (z = 2.366, p < 0.05).

97

A B

C D

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Variables factor map

Dim 1 (69.04%)

Dim

2 (1

2.02

%) Amylases

Cellulases

Hemicellulases

Pectinases

Proteinases

Flowers

Fruit

HerbacousOther

Tree

-2 0 2 4

-4-2

02

4

Individuals factor map

Dim 1 (69.04%)

Dim

2 (1

2.02

%)

AcromyrmexAtta

AcromyrmexAtta

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

Variables factor map

Dim 1 (33.11%)

Dim

2 (2

5.54

%)

Flowers

Fruit

Herbacous

Other

Tree

Amylases

CellulasesHemicellulases

Pectinases

Proteinases

-4 -2 0 2

-3-2

-10

12

3Individuals factor map

Dim 1 (33.11%)

Dim

2 (2

5.54

%)

Acromyrmex

Atta

AcromyrmexAtta

Enzy

me

activ

ityFo

rage

Figure 3. Correlations between forage diversity and enzyme activity.Principal Component Analyses (PCA) using either the five enzyme groups of Figure 2 (A and B) or the five forage

material categories of Figure 1 (C and D) as predictor variables (black arrows). Red arrows represent response vectors

for forage material (A) or enzymes (C). The B and D panels complement the respective A and C panels by plotting

PCA’s scores across the fungus garden measurements (B, 45 for Atta and 32 for Acromyrmex) the sampled ant trails

(D, 30 for Atta and 21 for Acromyrmex; Table 1), largely separating the ant genera along the x-axes, confirming that

Atta primarily focuses on tree-leaf material (compare panels C and D) and Acromyrmex on herbaceous leaves, flowers

and (less pronounced) fruit. Comparison of the A and B panels illustrates that enzyme activity was generally higher in

Acromyrmex (towards the right).

98

DiscussionGenus-level niche segregation between Atta and AcromyrmexAlthough it is widely appreciated that Atta and Acromyrmex differ by more than two orders of magnitude in their scale of operations (see e.g. [14, 18, 26, 45]), systematic comparative studies similar to the present analyses have to our knowledge not been done. Although our snapshot re-sults for the month of May cannot be generalized, we believe to have achieved our objective of demonstrating that larger scale studies like this can be done in principle. Our analyses illustrate that the statistical tools to analyze such data are available and can easily be expanded for use in more encompassing field surveys, as extra seasons and/or sampling sites can be added as new factors within which genera, species, and colonies can be nested. In the sections below, we offer tentative interpretations and compare them with available literature.

Our results confirm that the two genera of leaf-cutting ants operate at different scales, show that their foraging niches are different and that the enzymatic processing activities of fungus gardens appear to reflect these differences. The differences in foraging preferences quantified our intuitive expectations based on two decades of fieldwork in Gamboa, but the enzymatic activity differences were more substantial than we expected, because the two genera rear fungus-garden symbionts that belong to a single species L. gongylophorus [33]. This suggests that studies of phenotypic plasticity in enzyme gene expression will be worthwhile to enhance our understanding of the ver-satility of the leaf-cutting ant symbiosis. We will return to this in more detail below.

The finding that average genus-specific foraging rates show a 42 fold difference in loaded-worker return rates and a 63 fold difference in total worker return rates per hour, seems to match the ca. two order of magnitude difference in colony size between Atta and Acromyrmex. The fact that the differences do not quite reach 100 fold [14, 18, 26, 46] may be due to our primary focus on the largest Acromyrmex colonies (smaller colonies have too little foraging activity making the type of sampling that we did less feasible), whereas our selection of Atta colonies mostly contained me-dium size colonies. It is also conceivable that the Atta workers that returned to their nests without carrying plant material may have had their crops filled with plant sap as suggested by Littledyke & Cherrett [47], Quinlan & Cherret [48] and Hölldobler & Wilson [19], but the present setup did not allow any measurements on this. This suggests that considering only loaded workers may underestimate foraging effort, and that larger scale comparative studies should include sampling of liquid food in the crops of returning foragers. In spite of these limitations, we will also return to tentative inferences on species- and genus-level niche segregation that our snapshot data for Gamboa may allow.

99

Garden enzyme activity and forage material – is there a connection?It has long been known that the fungus is a major producer of decomposition enzymes for the plant material that leaf-cutting ant foragers provide, and recent work has shown that these decom-position services are supplemented by several other microorganisms that live in attine gardens [49, 50]. Other recent studies have emphasized that the expression of enzymes can be remarkably plastic and substrate dependent [21, 34]. This is consistent with earlier notions that there are active feedback loops between forager supply and symbiont demand, such that foragers may discard some forage material under specific conditions where its excess processing would not be optimal [51].

Our present results suggest that Atta and Acromyrmex represent ecologically distinct ant genera, both with regard to forage acquisition/diversity and garden enzyme activity/diversity. The latter confirms recent results showing differences in proteomes between Acromyrmex echinatior and Atta cephalotes fungus gardens [52]. It is remarkable that our results indicate that Atta gardens generally produce lower amounts of enzymes, even though these ants forage mostly on tree-leaves (Figure 3), which one would expect to be more demanding to decompose. It also appears that Atta gardens tend to overproduce two classes of enzymes, cellulases and pectinases, in addition to amylases (Figure 2), whereas Acromyrmex tended gardens produce higher amounts of all enzyme categories. This suggests that Atta gardens may somehow extract necessary nutrients more effi-ciently, but further work will be needed to understand the details of these processes. An additional factor to consider in this context is that Atta colonies produce conspicuous waste heaps or under-ground compost chambers, whereas this is rare for Acromyrmex (J.J. Boomsma & P.W. Kooij pers. obs.; CSE logbooks). This is consistent with Panamanian Atta discarding a much larger fraction of not fully degraded older fungus garden biomass than Acromyrmex [53, 54], perhaps because average enzyme activity per unit of fungus garden mass is lower and fresh tree leaves are more abundantly available than flower parts.

An earlier study [21] has hypothesized that Atta species focus on the rapid degradation of starch and proteins, but discard fungus garden material before most of the cellulose and hemicellulose is degraded. This is consistent with other recent studies showing that high amounts of cellulose and hemicellulose are still present in the bottom layer of the fungus gardens [50, 55, 56] and that only cellulases from L. gongylophorus remain highly active in this bottom layer [52, 57]. The larger scale and more wasteful substrate processing practiced by Atta may thus leave more substantial niches for additional bacterial and/or yeast [49] decomposition, similar to the domestication of specialized gut bacteria in large ungulates [58] that rely on residues of leaf-material that were

100

hard to digest even for ruminants. Focused, comparative transcriptomics to investigate conditional gene expression in fungus gardens of the two leaf-cutting ant genera could shed further light on possible differences of this kind and metagenome sequencing could identify the microbial com-munities involved, similar an earlier yeast study on the fungus gardens of Acromyrmex and Atta [49].

Niche partitioning in Panamanian Atta and Acromyrmex

The data provided in our study are a snapshot of year-round foraging, which is known to vary across the seasons [25]. This implies that we cannot be sure that sampling in other seasons or at other sites would have yielded similar results. However, we also note that the five forage catego-ries that we distinguished are very general and likely to be available throughout the year and that medium-size colonies of Atta and large colonies of Acromyrmex are unlikely to move over sub-stantial distances (but see [59]), so their central place for foraging would tend to cover the same (sub) habitat over time. In spite of these caveats, our study shows that genus- and species-level differences across leaf-cutting ants can be quantified with the statistical tools we developed during this study.

The results of our study suggest that direct competition for forage material between the two genera of leaf-cutting ants is likely to remain limited, because Atta and Acromyrmex species target rather different types of forage, in spite of some overlap consistent with earlier reports that mostly re-port allopatrically collected data[18, 22, 24, 28-30, 32]. The correlations between garden enzyme activity and genus-level difference in forage use that we uncovered for the Gamboa community of leaf-cutting ants, may be reinforced or supplemented differences in salivary gland secretions between the two ant genera [60], a variable we were unable to measure. However, comparisons at species level suggested that both Atta and Acromyrmex species tend to have optimal habitats that are largely mutually exclusive, with Acromyrmex volcanus and Atta cephalotes foraging in the canopy, Acromyrmex octospinosus and (somewhat less specifically) Atta colombica foraging on the forest floor, and Acromyrmex echinatior and Atta sexdens foraging in the open landscape. Although it is possible that these differences are less pronounced in other seasons or sites, these results seem consistent with ecological theory predicting that interspecific competition is more pronounced when species are more similar, so that habitat partitioning may evolve [4-6].

The only case in which habitat segregation was somewhat less pronounced was between Atta

sexdens and Atta colombica, which often overlap in park-like and man-made habitats. Although there is a clear gradient across the isthmus of Panama, Atta sexdens is the dominant Atta species

101

along the Pacific coast and becomes less abundant towards Gamboa in central Panama, whereas the pattern is opposite for Atta colombica [28]. It is interesting that these are the only two spe-cies for which we once observed neighboring trails and active avoidance behavior, i.e. trails of a colony stopping ca. one meter from the trail of another colony (P.W. Kooij, pers. obs.), behavior expected for all Atta spp. For the two common Panamanian Acromyrmex species, of which our research group has dug up ca. 500 colonies over the last two decades, habitat segregation (forest for A. octospinosus and open grassland areas for A. echinatior) is so pronounced that they will rarely encounter each other, similar to what is seen in Costa Rica [61]. In this way mature colonies of these species are unlikely to compete for the same type of plant forage. As far as we are aware distributions of incipient (founding) colonies are similar in Gamboa, but this is harder to quantify.

AcknowledgmentsWe thank the Smithsonian Tropical Research Institute (STRI), Panama, for providing logistic help and facilities to work


Anders Illum, Mattias Lange Nielsen, Sämi Schär and Nicholas Westwood for help with collecting the foraging data,

Henrik De Fine Licht for supplying data on enzyme activity in an Acromyrmex volcanus colony and for helping to

interpret the results, Gøsta Nachmann for suggesting the statistical analysis of the enzyme experiment, and the Danish

National Research Foundation for funding (DNRF57). The collection of the observational data reported in this paper

was facilitated by the Gamboa Graduate Course in Tropical Behavioral Ecology and Evolution, hosted by the Copenha-

gen Centre for Social Evolution and the Smithsonian Tropical Research Institute.

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105

Species

Colony

TrailTime of day

Observatio

n tim

e (m

in)Minutes observatio

n pe

r spe

cies

Minutes observatio

n pe

r colon

yMinutes observatio

n pe

r trail

Num

ber o

f ants c

ounted

Ae1

5110

:30 AM

3030

3041

Ae2

1711

:30 AM

3030

3015

Ae3

1811

:30 AM

3030

3010

Ae4

3210

:20 AM

1515

1525

Ae5

282:55

PM

3030

3021

Ae6

293:35

PM

3030

3021

Ae7

1610

:30 AM

3030

3021

Ae8

304:10

PM

3030

3024

Ae9

401:00

PM

3030

3025

Ae10

319:40

AM

3030

3062

TOTA

L28

528

528

528

526

5

Ao1

509:55

AM

3030

3014

Ao2

142:00

PM

3030

3065

Ao3

153:00

PM

3030

3023

Ao4

423:00

PM

3030

3014

Ao5

4311

:00 AM

3030

3023

Ao6

442:15

PM

3030

3042

Ao7

459:45

AM

1515

1550

Ao8

4610

:15 AM

3030

3013

5Ao

948

10:45 AM

3030

3073

Ao10

499:10

AM

3030

3019

TOTA

L28

528

528

528

545

8

Acromyrmex volcanu

sAv

147

11:00 AM

3030

3030

43TO

TAL

3030

3030

43

Acromyrmex echinatior

285

Acromym

rex octospinosus

285

Table S1. Sample sizes for each colony.

106

Spec

ies

Colo

nyTr

ail

Tim

e of

day

Obs

erva

tion

time

(min

)M

inut

es o

bser

vatio

n pe

r spe

cies

Min

utes

obs

erva

tion

per c

olon

yM

inut

es o

bser

vatio

n pe

r tra

ilN

umbe

r of a

nts c

ount

ed11

5:00

PM

22

9812

5:00

PM

22

7213

5:00

PM

22

713:

00 P

M2

903:

00 P

M2

688

3:00

PM

22

4811

:15

AM2

6011

:15

AM2

5310

:50

AM0.

550

10:5

0 AM

0.5

2410

:50

AM0.

548

10:5

0 AM

0.5

4310

:30

PM0.

582

10:3

0 PM

0.5

9410

:30

PM0.

591

10:3

0 PM

0.5

9121

10:3

0 AM

22

4910

:00

PM0.

542

10:0

0 PM

0.5

4310

:00

PM0.

545

10:0

0 PM

0.5

99Ac

e625

10:0

0 PM

22

215

8TO

TAL

2626

2626

1519

Atta cepha

lotes

Ace1

26

6

Ace2

76

4

Ace3

234

4 2

Ace5

427

2

Ace4

22

4

2

26

107

Spec

ies

Colo

nyTr

ail

Tim

e of

day

Obs

erva

tion

time

(min

)M

inut

es o

bser

vatio

n pe

r spe

cies

Min

utes

obs

erva

tion

per c

olon

yM

inut

es o

bser

vatio

n pe

r tra

ilN

umbe

r of a

nts c

ount

ed10

:30

AM2

130

10:3

0 AM

298

510

:30

AM2

226

36

10:3

0 AM

22

724:

00 P

M0.

529

4:00

PM

0.5

974:

00 P

M0.

510

24:

00 P

M0.

594

4:00

PM

0.5

994:

00 P

M0.

587

4:00

PM

0.5

954:

00 P

M0.

511

39:

15 A

M2

229:

15 A

M2

3910

:00

AM2

6610

:00

AM2

58Ac

ol5

331:

30 P

M2

22

118

Acol

636

3:50

PM

22

261

Acol

737

4:00

PM

0.5

0.5

0.5

81TO

TAL

24.5

24.5

24.5

24.5

1724

19:

00 A

M2

210

52

9:30

AM

22

172

9:30

AM

269

9:30

AM

210

311

:45

AM2

2011

:45

AM2

5As

334

3:15

PM

22

210

7As

435

3:40

PM

22

27

As5

384:

15 P

M2

22

88As

639

12:3

0 PM

22

253

As7

413:

00 P

M2

22

72TO

TAL

2222

2222

801

4

Acol

2

9

4

2

Atta colom

bica

Acol

14

24.5

8

102

Acol

319

44

Atta se

xdens

As1

22

83

As2

244

4

Acol

420

44 4

109

chaPter 4

LeucoAgAricus gongyLophorus uses leaf-cutting ants to vector Proteolytic enzymes toWarDs neW Plant substrate

PePijn W kooij, aDelina rogoWska-Wrzesinska, Daniel hoffmann, Peter roePstorff,

jacobus j boomsma, morten schiøtt

(in Press at the isme journal)

110

Leucoagaricus gongylophorus uses leaf-cutting ants to vector proteolytic enzymes towards new plant substrate

Running title: Protease activity in ant fungus gardens

Pepijn W Kooij*a, Adelina Rogowska-Wrzesinskab, Daniel Hoffmanna, Peter Roepstorffb, Jacobus J Boomsmaa, Morten Schiøtt*a

aCentre for Social Evolution, Department of Biology, University of Copenhagen, Universi-tetsparken 15, DK-2100 Copenhagen, Denmark

bProtein Research Group, Department of Biochemistry and Molecular Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark.

*Corresponding authors: [email protected] and [email protected] at the above address

Accepted for publication at The ISME Journal, December 2013

111

AbstractThe mutualism between leaf-cutting ants and their fungal symbionts revolves around processing and inoculation of fresh leaf pulp in underground fungus gardens, mediated by ant fecal fluid deposited on the newly added plant substrate. As herbivorous feeding often implies that growth is nitrogen-limited, we cloned and sequenced six fungal proteases found in the fecal fluid of the leaf-cutting ant Acromyrmex echinatior and identified them as two metalloendoproteases, two ser-ine proteases and two aspartic proteases. The metalloendoproteases and serine proteases showed significant activity in fecal fluid at pH values of 5-7, but the aspartic proteases were inactive across a pH range of 3-10. Protease activity disappeared when the ants were kept on a sugar water diet without fungus. Relative to normal mycelium, both metalloendoproteases, both serine proteases and one aspartic protease were upregulated in the gongylidia, specialized hyphal tips whose only known function is to provide food to the ants. These combined results indicate that the enzymes are derived from the ingested fungal tissues. We infer that the five proteases are likely to accelerate protein extraction from plant cells in the leaf pulp that the ants add to the fungus garden, but regulatory functions such as activation of proenzymes are also possible, particularly for the aspartic proteases that were present but without showing activity. The proteases had high sequence similarities to proteolytic enzymes of phytopathogenic fungi, consistent with previous indications of convergent evolution of decomposition enzymes in attine ant fungal symbionts and phytopathogenic fungi.

Keywords:Acromyrmex echinatior/nutrition/mutualism/phytopathogens/proteases

112

IntroductionObligate symbioses are characterized by high degrees of partner commitment and functional complementarity (Janzen 1985). Such interactions normally show only minor remnants of the potential reproductive conflicts that needed to be overcome when these partnerships evolved, allowing them to ultimately become single adaptive units with organismal properties (Queller & Strassmann 2009; Boomsma 2013). Such high levels of interdependency are most characteristic for endosymbioses where the symbiont lives in cells or tissues of the host, but may also occur in ectosymbioses like the eusocial fungus-farming mutualisms of ants and termites where the fungus gardens are ectosymbionts for the individual ants but endosymbionts for the colony (Poulsen & Boomsma 2005; Aanen et al. 2009). We are intuitively inclined to consider the insect colony as the active farming host and the fungus gardens as passive crops, but the contention that fungal clones have contracted ant families to propagate them across generations appears to be equally valid, especially in the attine ants where the fungal symbiont is vertically transmitted (Poulsen & Boomsma 2005).

In terms of biomass conversion efficiency, the functional complementarity between attine ants and their fungus garden symbionts achieved spectacular progression, moving from initial stages of ant-driven litter-based decomposition farming that evolved 50 Million Years Ago (MYA) to advanced farming of irreversibly domesticated crop fungi 20 MYA (Mueller et al. 1998; Schultz & Brady 2008; Mehdiabadi & Schultz 2010), culminating in aggressive herbivorous leaf-cutting farming 10 MYA (De Fine Licht et al. 2010; Schiøtt et al. 2010). Particularly the latter transition allowed the ants to evolve much larger and complex societies (Mueller et al. 1998; Villesen et al. 2002; Schultz & Brady 2008; Fernandez-Marin et al. 2009; Mehdiabadi & Schultz 2010) after they had managed to overcome a series of challenges that are associated with a herbivorous life-style (Schiøtt et al. 2008; 2010; De Fine Licht et al. 2010; 2013). Although depending on live veg-etation rather than dead leaf litter undoubtedly implied better access to the proteins that normally limit insect growth, an almost exclusive fungal diet is generally poorer in protein than the carnivo-rous diets of many hunter-gatherer species in the Myrmicinae subfamily of ants (Davidson 2004). Hosts and symbionts can thus be assumed to have been under consistent selection to maximize protein yield as this will determine colony growth rate and the production of winged virgin queens that transmit both their own genes and clonal copies of the fungal symbiont to future generations.

Specialized foraging on live plant material as growth substrate for the fungal symbiont Leucoag-

aricus gongylophorus implies that leaf-cutting ants are major herbivores in the Neo(sub)tropics with substantial roles in recycling nitrogen and phosphorus (Fowler et al. 1989). A recent study

113

has confirmed that the leaf-cutting fungus-farming mutualism is likely to be nitrogen-limited as Atta fungus gardens harbour substantial levels of nitrogen fixing bacteria (Pinto-Tomás et al. 2009). Studies on the other genus of leaf-cutting ants, Acromyrmex, have shown that the fungal symbiont uses the ants to vector pectinases and a laccase from the most productive middle layers of fungus gardens to the top where they are most useful for the symbiosis but where fungal bio-mass to produce these enzymes is in short supply (Schiøtt et al. 2010; De Fine Licht et al. 2013). The fungal gongylidia, special hyphal tips produced only to feed the ants and their brood (Bass & Cherrett 1995), are instrumental in this process as they upregulate the expression of genes produc-ing these enzymes (Schiøtt et al. 2010; De Fine Licht et al. 2013). These studies indicate that the fungal symbiont uses the ants for vector enzymes for the neutralization of phenolic compounds (laccase) and for loosening up the cell wall matrix (pectinases) to new garden sections. However, whether these initial decomposition steps are followed by a similar boost in protease activity to decompose intracellular proteins has remained unknown.

The ants process freshly cut leaf fragments into a mass of leaf pulp while mixing it with droplets of fecal fluid and subsequently deposit the new substrate on fungal ridges in the top of the garden where they inoculate it with small tufts of mycelium from the older part of the garden (Weber 1966). In the 1970’s, leaf-cutting ant fecal fluid was shown to contain active proteases (Martin 1970; Martin & Martin 1970a; Martin & Martin 1970b; Martin & Martin 1971; Martin 1974) with similar chemical properties as enzymes originating from the fungal symbiont (Boyd & Martin 1975a; 1975b), and recent work has shown that fungus garden endo-protease activity in evolu-tionarily derived leaf-cutting ants is much higher than in sister ant lineages that do not use fresh leaves to make their gardens grow (De Fine Licht et al. 2010), activity that could subsequently be assigned to metalloproteases and serine proteases (Semenova et al. 2011).

Recently, we obtained the proteome of Acromyrmex echinatior fecal fluid and were able to iden-tify 33 proteins. Among these were seven pectinases (Schiøtt et al. 2010), one laccase (De Fine Licht et al. 2013), and seven proteases. Only a single of these proteases is known to be produced by the ants, whereas the other six are contributed by the fungal symbiont. We hypothesized that a number of these proteases might serve to either process the intracellular proteins of freshly opened plant cells or to assist in the breakdown of secondary plant defences (Christeller et al. 1992; Mer-cado-Flores et al. 2003; Ievleva et al. 2006). The latter is a distinct possibility as plants are known to use protease inhibitors to counteract the harmful effect of proteases produced by phytopathogic fungi and to generally decrease the digestibility of their tissues to discourage herbivores (Habib & Fazili 2007). The objectives of the present study were to: 1. measure the specific protease activity

114

in fecal fluid of gardening A. echinatior workers, 2. confirm that they are derived from the fungal symbiont and have different expression levels and pH optima, 3. assess the extent to which the expression of genes coding for these enzymes was enhanced in the fungal gongylidia, as would be expected when vectoring these enzymes to the top of fungus gardens is a specific adaptation of the symbiosis, and 4. discuss the implications of our results for understanding the co-adaptations between partners that has allowed this symbiosis to evolve its substantial ecological footprint in the Neo(sub)tropics.

Materials and methodsBiological materialWe used seven colonies of A. echinatior (Ae263, Ae280, Ae322, Ae332, Ae335, Ae349 and Ae370), collected in and around Gamboa, Panama, between 2004 and 2007 and kept in rearing facilities in Copenhagen under controlled conditions of ca. 25°C and ca. 70% humidity, where they were fed twice a week with fresh bramble leaves, apple pieces and dry rice. Fecal fluid was obtained by gently squeezing the abdomen of large workers with a forceps on a microscope slide. Each fecal droplet was then mixed with 0.5 µl demineralised water, collected with a micropipette, and stored in an Eppendorf tube on ice. Sixty droplets from five colonies each (Ae263, Ae280, Ae322, Ae332 and Ae349) were collected this way, pooled per colony and diluted with deminer-alised water to a final volume of 250 µl.

For the gene expression measurements, gongylidia clusters (staphylae) and normal mycelium were collected separately under a stereomicroscope at 40x magnification from each of five colonies (Ae263, Ae280, Ae322, Ae335 and Ae370) in 2 mL Eppendorf tubes floating in liquid nitrogen. After collecting app. 100 µL for each type of tissue, samples were stored at -80°C for subsequent RNA extraction.

Protein identification and gene cloningSDS-PAGE and mass spectrometry were performed as described previously (Schiøtt et al. 2010; De Fine Licht et al. 2013). RNA extraction from the fungal symbiont, followed by cDNA con-struction, was done as in Schiøtt et al. (2010). Aspartic protease 2 (AspII), Metallopeptidase 1

(MetI), Serine protease 1 (SerI) and Serine protease 2 (SerII) were initially identified by PCR amplification from cDNA using degenerate primers constructed from the mass spectrometry data using the PCR scheme: one cycle of 95°C for 5 min, 35 cycles of 94°C for 20 sec, 50°C for 30 sec and 72°C for 2 min, and ending with one cycle of 72°C for 7 min. Aspartic protease 1 (AspI) and Metallopeptidase 2 (MetII) were identified at a later stage by a Blast search of the ca. 100x

115

coverage A. echinatior genome (Nygaard et al. 2011) and a low coverage genome sequence of the A. echinatior fungal symbiont (De Fine Licht et al. 2013) using the mass spectrometry data as queries. Sequencing of full-length gene transcripts was done using a RACE (Rapid Amplification of cDNA Ends) strategy. 3’ and 5’-RACE libraries were made from ca. 1 µg of purified RNA with the SMART RACE cDNA kit (CLONTECH, Mountain View, California, USA), and gene sequences were PCR amplified from these libraries using specific primers designed from the PCR amplified gene fragments (AspII, MetI, SerI and SerII) or BLAST search identified sequence reads (AspI and MetII) along with the primers enclosed in the SMART RACE cDNA kit. In some cases the 5’RACE had to be done in several steps to get a full transcript sequence. The following PCR scheme was used in the RACE experiments: one cycle of 95°C for 5 min, 10 cycles of 94°C for 20 sec, 72°C for 30 sec (with a decrease in temperature of 0.5°C in every cycle) and 72°C for 2 min, followed by 35 cycles of 94°C for 20 sec, 67°C for 30 sec and 72°C for 2 min, and ending with one cycle of 72°C for 7 min. All PCR products were cloned in pCR4-TOPO using the TOPO TA cloning method (Invitrogen, Carlsbad, California, USA) and sequenced at Eurofins MWG Op-eron (Ebersberg, Germany). Gene sequences are deposited in GenBank with accession numbers KF571927-KF571932. Primer sequences for cloning of the genes are provided in Table S1.

Enzyme assaysProtease activity was measured for four different protease inhibitors and two controls (without inhibitors, one of which incubated and the other not incubated) at eight different pH levels. The inhibitors used in the experiment were 5mM 1,10-phenanthroline to inhibit metalloendoproteases, 2µM pepstatin to inhibit aspartic proteases, 10mM phenylmethylsulfonyl fluoride to inhibit serine proteases and 10µM trans-epoxysuccinyl-L-leucylamido(4-guanidino)butane to inhibit cysteine proteases. In the assays, 4 µl of the diluted fecal fluid was mixed with 1 µl of inhibitor or distilled demineralised water and incubated on ice for one hour. Next, 35 µl of 0.1M Britton Robinson buffer (Britton & Robinson 1931) with 2% azoalbumin was added and the mix was incubated at 30°C for one hour. For the blank control (not incubated) the azoalbumin mix was added at the last moment without incubation to prevent any protease activity to occur. To stop the activity of the proteases, 120 µl 10% trichloroacetic acid was added, after which the mix was vortexed and incubated for 15 minutes before centrifugation for 5 minutes at 8000g. The supernatant (86 µl) was added to 100 µl freshly prepared 1M NaOH on a 96 well cell culture plate.

Absorbance was measured at 440 nm on a Versamax ELISA microplate reader (Molecular Devic-es, Sunnyvale, USA) and scored after subtracting the corresponding blank-control measurements to correct for background absorbance. To find the enzyme activity in each of the four protease

116

classes, measurements obtained with specific inhibitors for each protease class were subtracted from the total protease activity in identical assays without addition of inhibitors. Protease activity was defined as the amount of enzyme required to cause a unit increase at 440 nm across a 1 cm path length (Sarath et al. 2001). The number of units of protease activity data were plotted and analyzed with R (R Core Team 2013), using Linear Mixed-Effects Models (“lme”) (Pinheiro et al. 2013), with Colony as random factor, and generating p-values with General Linear Hypoth-eses (“glht”) in the Simultaneous Inference in General Parametric Models package “multcomp” (Hothorn et al. 2008).

For measuring protease activity in control ants, 100-200 workers of the colonies Ae322, Ae332 and Ae349 were transferred to subcolonies with no access to fungus garden material, but with an ad libitum diet of 10% sucrose and bramble leaves. After ca. two weeks protease activity was measured in pooled samples of fecal droplets from two ants for each colony at pH 6 in three rep-licates per colony.

Quantitive real-time PCRPrimers for the six different genes (Table S2) were designed by matching the obtained cDNA sequences to a database of a partially sequenced genome of the A. echinatior fungal symbiont (De Fine Licht et al. 2013) using BLASTn to identify intron and exon sequences. Primers were designed to span an intron to avoid amplification of genomic DNA. The qPCRs were run on an Mx3000P QPCR System (Agilent, Santa Clara, CA, USA) in 20 µL reactions (0.5 µL cDNA, 10 µL 2x SYBR Premix Ex Taq [TaKaRa Bio Inc., Otsu, Japan] and 0.4 µL of each primer [10µM]) with the following PCR conditions: one cycle of 95°C for 2 min, followed by 40 cycles of 95°C for 30 sec, 55°C (AspI, AspII, MetII, SerI, SerII, and EF1-α) or 57°C (MetI, GAPDH, Ubc) for 30 sec, and 72°C for 30 sec, and ending with a melting curve cycle of 95°C for 30 sec, 55°C or 57°C for 30 sec and 95°C for 30 sec.

For each of the ten samples (one gongylidia and one mycelium sample from five colonies), three technical qPCR replicates were done for nine genes: three housekeeping genes (GAPDH, Ubc

and EF1-α) and six target genes (MetI, MetII, AspI, AspII, SerI and SerII). All Ct values from the RT-qPCR analysis were analyzed in R with packages “ReadqPCR” (Perkins & Kohl 2011) and “NormqPCR” (Kohl & Perkins 2011) using averages across the three technical replicates. Stabili-ty of housekeeping genes was assessed across the entire data set, which showed that GAPDH and Ubc were more stable than EF1-α, so the latter gene was discarded in the further analyses. GAP-

DH and Ubc were then used to calculate the normalized expression (2∆Ct) of the target genes in

117

Table 1 Peptide sequences of proteases in fecal fluid of A. echinatior and corresponding molecular weights (Da) of full-length

proteins. The letter J symbolises the amino acids leucin and isoleucin, which have the same molecular mass and are

therefore indiscernible by mass spectrometry. The single discrepancy between mass spectrometry data and sequence

data (letters D and H in bold) may have been caused by deamination of asparagine into aspartic acid.

Protein Mass spectrometry data Sequence data MW (Da)

Aspartic protease 1 DENDGGEATFGGJNPSSYR

NAYWEV

YFTVYDJGR

DENDGGEATFGGINPSSYR

NAYWEV

YFTVYDLGR

44788

Aspartic protease 2 FTGAJNFTPR

SSGFEGTDGJJGJGPVDJTR

GTJSPAVNSJVPTVTDNJFSSGR

TSTSPSSJFWGJNQSVR

VJDNNTGJJR

FTGALNFTPR

SSGFEGTDGILGIGPVDLTR

GTLSPAVNSLVPTVTDNLFSSGR

TSTSPSSLFWGLNQSVR

VLDNNTGLLR

43158

Metallopeptidase 1 TYASNAATYJSSHSSSSTR

YTTWFGTYTSAR

TYASNAATYLSSHSSSSTR

YTTWFGTYTSAR

37207

Metallopeptidase 2 TYASNAJJYJR TYASNALIYLR 37511

Serine protease 1 AWAGJHFAVAAGNDNR

GSDGSGSTSDVJAGVQWAAR

AADAGLHFAVAAGNDNR

GSDGSGSTSDVIAGVQWAAR

50114

Serine protease 2 ADJQTFFR

AGYJDEFANR

AJYNTVNYVPSQTSR

DGJGVAGYJDEFANR

GVAGYJDEFANR

GSVGGTSASSPTVAGVFAJJNDFR

FRPDAAGSSFTTVR

YFSTPSYQSAAVSR

ADLQTFFR

AGYLDEFANR

ALYNTVNYVPSQTSR

NGLGVAGYLDEFANR

GVAGYLDEFANR

GSVGGTSASSPTVAGVFALLNDFR

FRPDAAGSSFTTVR

YFSTPSYQSAAVSR

63388

Carboxypeptidase A (EGI65848)

QNNPGVFFESGJHAR QNNPGVFFESGIHAR 46943

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gongylidia and mycelium. Values of 2-∆∆Ct (Livak & Schmittgen 2001) for each gene subsequently produced estimates of fold changes in relative gene expression between gongylidia and mycelium. This sequence of procedures allowed us to identify genes with significantly different expression levels and to obtain estimates of their normalized and relative expression.

ResultsThe mass spectrometry data of fecal fluid proteins (Table 1) described previously (Schiøtt et al. 2010) allowed us to clone and sequence six protease genes encoded by the fungal symbiont ge-nome, and to identify an ant encoded protease (EGI65848) belonging to a family of M14 carboxy-peptidases, which is significantly expanded in the genome of A. echinatior (Nygaard et al. 2011). Comparison with the MEROPS database release 9.8 (http://merops.sanger.ac.uk/ (Rawlings et al. 2012)) (Table S3) showed that all six proteases (AspI, AspII, MetI, MetII, SerI and SerII) were similar to peptidases known from other fungi; AspI with saccharopepsin (A01.018) from Laccar-

ia bicolor (78.57%, E-value = 1.10e-142); AspII with polyporopepsin (A01.019) from L. bicolor (71.60%, E-value = 1.80e-125); MetI with peptidyl-Lys metallopeptidase (M35.004) from Pleuro-

tus ostreatus (56.55%, E-value = 1.50e-53); MetII with peptidyl-Lys metallopeptidase (M35.004) from P. ostreatus (69.23%, E-value = 2.60e-42); SerI with an unassigned peptidase from the S8A subfamily from L. bicolor (61.28%, E-value = 6.90e-111); SerII with grifolisin (S53.010) from L.

bicolor (66.67%, E-value = 4.40e-136).

To test the overall protease activity in the ant fecal fluid and to determine whether the fungus garden is responsible for this activity, protease activity was measured in fecal droplets of worker ants taken directly from their colonies and from purged ants that had been kept on sugar water and bramble leaves for ca. two weeks with no access to fungal garden material. This showed that protease activity is present in fecal fluid when the ants have their normal diet, but disappears al-most completely when the ants are deprived of fungus garden material (t17 = 14.8972, p < 0.0001), indicating that the protease activity in the ant fecal fluid is caused by ingested proteases from the fungal symbiont and not by proteases produced by the ants themselves (Figure 1).

Next, we tested the activity of the four main types of proteases using different inhibitors spe-cifically targeted at each category of proteases: metalloendoproteases, serine proteases, aspartic proteases and cysteine proteases. Only the metalloendoproteases and serine proteases had sig-nificant activities, with respective peaks at pH 6 (z = -10.824, p < 0.0001) and pH 7 (z = 4.654, p < 0.0001) and serine protease activity also being significantly enhanced at pH 6 (z = 2.004, p < 0.05), consistent with fecal fluid also having pH 6 (Figure 2). In contrast to serine proteases,

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metalloendoproteases were active at a very broad range of pH values and retained activities up to pH 10. We found no activity for aspartic proteases (present in fecal fluid), and cysteine proteases (absent in fecal fluid).

Relative changes in gene expression in gongylidia versus mycelium (fold-changes: 2-∆∆Ct) showed that the expression of five out of the six fungal protease genes was upregulated in the gongylidia compared to the mycelium (Figure 3a): peptidyl-Lys metallopeptidase I and II (MetI and MetII) (z = -3.979, p = 0.00004; z = -20.010, p = 0.00001), saccharopepsin (AspI) (z = -3.946, p = 0.00004), subtilisin (SerI) (z = -8.927, p = 0.00001) and grifolisin (SerII) (z = -9.041, p = 0.00001), whereas the expression of polyporopepsin (AspII) remained unchanged (z = 2.580, p = 0.99492). This is consistent with all but one of these fungal proteases having their main function after being ingest-ed by the ants, i.e. after they pass the ant guts unharmed to be deposited with the fecal fluid.

Plotting relative gene expression (fold change in gongylidia vs. mycelium) as a function of nor-malized gene expression in the gongylidia (i.e. expression relative to housekeeping genes) (Figure 3b) showed proportionality for four of the proteases (saccharopepsin, subtilisin, grifolisin and polyporopepsin). However, the two metalloendoproteases deviated from the 1:1 proportionality represented by the diagonal in Figure 3b. One of them (MetII) combined a fairly low normalized expression (Table S4) with a 17x upregulation in gongylidia, and the other (MetI) combined a high normalized expression with a relatively low 5x fold upregulation in gongylidia. This implies that

Figure 1Enzyme activity in milliunits of proteases

(mU) in fecal fluid of workers of the leaf-cut-

ting ant A. echinatior kept on natural fun-

gus-garden-diet and on a diet of only sugar

water and possibly plant sap as control (t17 =

14.8972, p < 0.0001). Whiskers are SEs of the

mean.

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the expression of MetII is much more gongylidia-specific than the expression of MetI, which is apparently also needed in high quantities in the undifferentiated mycelium. It also appeared that the single non-differentially expressed protease (AspII) had a low normalized expression level (Figure 3b), indicating that it may not play a very important role under normal conditions, but might have an unknown adaptive function under specific field conditions that did not apply to our lab colonies.

DiscussionIn the present study we used mass spectrometry to identify seven proteases in the fecal fluid of A. echinatior leaf-cutting ants. For all but one of these we could exclude that they are produced by the ants, because we have a high quality reference genome for the ants (Nygaard et al. 2011), to which only one protease matched, and a poorly assembled genome for the fungal symbiont (De Fine Licht et al. 2013) to which the remaining six sequences matched. Our protease assays showed both metalloendoprotease activity and serine protease activity in the ant fecal fluid, but when we deprived the ants of their fungal symbiont by giving them a diet of sugar water and bramble leaves, essentially all protease activity disappeared. We further showed that five out of the six protease genes were overexpressed in the fungal gongylidia that the ants eat. In combination, the results strongly suggest that these proteases mostly belong to a distinct but restricted group of proteins that have been selected to be ingested by the ants for the sole purpose of being passed on to the fecal fluid where they provide functional mutualistic benefits for the farming symbiosis.

Figure 2Activity of metalloendoproteases, serine proteases, aspartic proteases, and cysteine proteases in milliunits (mU). Both

metalloendoproteases (z = 10.824, p < 0.0001) and serine proteases (z = 2.004, p < 0.05) showed significantly enhanced

activity at pH 6 relative to the zero base line. Serine proteases had their peak activity at pH 7 (z = 4.654, p < 0.0001)

and activities of metalloendoproteases at pH levels >7 remained equally significant as those at pH 6. Whiskers are SEs

of the mean.

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The most parsimonious hypothesis to explain this remarkable phenomenon is that the stationary fungus uses the ants to vector crucial enzymes produced in the central parts of the garden, where the ratio between fungal biomass and remaining substrate allows high gongylidia production, to the upper parts of the garden where an abundance of new substrate is available but as yet without much hyphal growth. These findings and interpretations are consistent with earlier results ob-tained for seven pectinases (Schiøtt et al. 2010) and a laccase enzyme (De Fine Licht et al. 2013). It remains possible that the ingested fungal enzymes also interact with unknown other compounds in the ant gut, produced either by the ants or their microbiome, but such additional complexities appear unnecessary to explain our current or previous results obtained for other enzymes in the gongylidia and fecal fluid (Rønhede et al. 2004; Schiøtt et al. 2010; De Fine Licht et al. 2013).

Figure 3Average relative gene expression (fold change) across gongylidia collected from fungus gardens of five colonies of A. ech-

inatior: dark grey = metalloendoproteases, grey = aspartic proteases, light grey = serine proteases. A) Relative expression

fold-changes (mean ± SE) in gongylidia compared to mycelium, with the dashed horizontal line representing no change

in expression. Target genes were significantly upregulated in the gongylidia (P = 0.00004, 0.00001, 0.00004, 0.99492,

0.00001 and 0.00001 for MetI, MetII, AspI, AspII, SerI and SerII, respectively, marked with asterisk (*)), except for poly-

poropepsin (AspII). B) Relative expression fold-change plotted against gene expression normalized for the housekeeping

genes GAPDH and Ubc (means ± SE). The three upregulated serine (SerI and SerII) and aspartic (AspI) proteases had

fold-changes proportional to normalized gene expression (being very close to the dashed line diagonal), but peptidyl-Lys

metallopeptidase I (MetI) combined a high normalized gene expression with a relatively low gene fold-change, and pep-

tidyl-Lys metallopeptidase II (MetII) combined a low normalized gene expression with a relatively high gene fold-change.

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It remains to be seen whether the proteases that we identified may also have functions that are restricted to the gongylidia. If that would be the case, the non-adaptive null hypothesis would be that finding them in the fecal fluid may merely imply that digestion was incomplete, but this seems unlikely given that SDS-PAGE banding patterns of fecal fluid were always consistent, showing a very specific subset of proteins that escape digestion (Schiøtt et al. 2010). Together with our previous work (Schiøtt et al. 2010; De Fine Licht et al. 2013) we have now shown that 12 of the 14 identified fungal pectinase, laccase and protease enzymes in the A. echinatior fecal fluid have upregulated gene expression in the gongylidia, which we believe underlines the key significance of these special hyphal tips as fungal organs mediating a series of symbiotic adaptations.

As it appears, the main function of the fecal fluid proteases is enhanced digestion of the most valu-able nitrogen-containing molecules of the fresh plant material harvested by the leaf-cutting ants. Peptidyl-Lys metallopeptidases (MetI and MetII) and serine carboxyl-peptidases (SerII) have been suggested to be involved in the extraction of nutrients from extracellular proteins (Heck 2012; Oda 2012). As both MetI, MetII and SerII were active in the fecal fluid (Figure 2) and upregulated in the gongylidia (Figure 3), it seems reasonable to hypothesize that these enzymes increase nitro-gen extraction from the chewed-up leaf pulp after other enzymes such as the pectinases (Schiøtt et al. 2010) have partially broken down the cell walls. The other active serine protease, subtilisin, is structurally similar to several unassigned fungal proteases in the subfamily S8A (Table S3). Most of the matches in protein databases for this enzyme were proteases from general pathogenic or phytopathogenic fungi. This finding is intriguing, because most of the subtilisins in subfamily S8A are part of a large gene-family expansion in Angiosperms with functions roles in seed or fruit de-velopment or manipulating cell walls in response to changes in biotic and abiotic factors (Schaller 2012; Schaller et al. 2012).

While subtilisins likely have defensive functions in plants, other studies have shown that subtil-isins have important aggressive functions in saprotrophs, phytopathogens and other infectious pathogens (Sreedhar et al. 1999; Dunaevskii et al. 2006; Ievleva et al. 2006; Bryant et al. 2009; Li et al. 2010). One explanation for this could be that using proteins very similar to those present in the hosts may allow pathogens to avoid detection by immune defences, a phenomenon referred to as molecular mimicry (Elde & Malik 2009; Armijos Jaramillo et al. 2013). Although the fungal symbionts of leaf-cutting ants hardly face any active immune defense in the severed plant material that the ants bring in, they will be challenged by substantial amounts of secondary (defensive) compounds, similar to any functional insect herbivores whose digestive systems need to cope with plant defences that decrease digestibility (Chen 2008). Gongylidial subtilisin might therefore

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have some role in the penetration and digestion of newly harvested plant material, consistent with serine protease activity being also high in the gardens of other higher attine ants. Panamanian rep-resentatives of the genera Trachymyrmex and Sericomyrmex that readily use soft fresh leaves as substrate in the laboratory, tend to have very high levels of serine protease activity in their fungus gardens, whereas these levels tend to be low in attine ant species that do not or only reluctantly use fresh leaves as fungal substrate (Semenova et al. 2011, P. Kooij and M. Schiøtt, unpublished observations).

We found two aspartic proteases in the fecal fluid, but none of them showed activity (Figure 2), even though one of them, saccharopepsin, was upregulated in the gongylidia when compared to mycelium (Figure 3). This suggests that this protein has a function, either for the gongylidia or after ingestion by the ants. Saccharopepsin is known to be important in the activation of yeast vacuolar hydrolases (Parr et al. 2007). This could explain the abundance of this enzyme in the gongylidia as they always have a large vacuole, but it does not explain why this protease is passed on to the fecal fluid. As non-upregulated polyporopepsin is also found in fecal fluid, it might be that these proteases cleave very specific target sequences so they would not show activity in our general activity assays. Possible target sequences could be other fungal enzymes that are produced as inactive precursors (zymogens or proenzymes), which are subsequently activated when regu-latory proteases cleave off part of the precursor amino acid chain. Such an activation mechanism would be particularly appropriate for abundant plant degrading enzymes that are also harmful to the fungus garden but not to the ants. These enzymes could then be produced in large quantities in the gongylidia, but only be activated in the ant gut to be mixed with the plant substrate via fecal fluid before the fungal hyphae are inoculated.

The possibility of leaf-cutting ant fungi having independently evolved adaptations reminiscent of phytopathogens is intriguing, so it is of interest to briefly evaluate recent studies addressing evolutionary arms-races between phytopathogens and their host plants (Misas-Villamil & van der Hoorn 2008). Also here, metalloproteases appear to have a role in the breakdown of cell wall stabilizing proteins (Rauscher et al. 1995) and in targeting specific plant resistance proteins (Xia 2004). Among serine proteases, trypsins appear to be most prevalent in phytopathogens, whereas subtilisins are more important in saprotrophs (Dunaevskii et al. 2006; 2008). However, when they occur, subtilisins may have an important role in the breakdown of cell wall structural proteins (Murphy & Walton 1996), as in infections of Kentucky bluegrass, Poa pratensis (Sreedhar et al. 1999) and in Colletotricum fungi that apparently acquired a subtilisin gene by horizontal transfer from their host plant, encoding a protease which is active during infection (Armijos Jaramillo et al. 2013).

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The possible similarities in attine fungal proteases with those of phytopathogens (St Leger et al. 1997) has interesting parallels to our earlier studies on pectinases (Schiøtt et al. 2010), and a laccase (De Fine Licht et al. 2013) in the fecal fluid of A. echinatior ants. Also here, indications of convergent evolution were found, although the evidence remained largely indirect as in the present evaluation of our proteases results. Although many of the molecular mechanisms remain unknown (but see De Fine Licht et al. 2013), it seems beyond doubt that selection pressure on herbivorous leaf-cutting ant fungi must have been largely in the same direction as in unrelated lineages of necrotrophic fungi. The relatively simple fecal fluid proteome (a total of ca. 33 iden-tified proteins so far in A. echinatior) appears to be a valuable and relatively transparent window through which the molecular adaptations that have shaped fungus-farming in the monophyletic clade of the attine ants can be unravelled.

Acknowledgements

The authors thank the Smithsonian Tropical Research Institute (STRI), Panama, for providing logistic support and facil-

ities to work in Gamboa, the Autoridad Nacional del Ambiente y el Mar (ANAM) for permission to sample ant colonies

in Panama in to export them to Denmark, and the Danish National Research Foundation for funding (DNRF57).

Abbreviations

EF1α - Elongation factor 1-α

GAPDH - Glyceraldehyde 3-phosphate dehydrogenase

SDS-PAGE - Sodium dodecyl sulphate – polyacrylamide gel electrophoresis

Ubc – Ubiquitin

Conflict of interest

The authors declare that they have no competing interests related to this manuscript

Supplementary Information accompanies the paper on The ISME Journal website (http://www.natur.com/ismej)

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Table S1. List of primers used for gene cloning.

Gene Primer name Primer function Primer sequences

AspI Asp-F1 3’ RACE primer 5’-CGTTCCCTTCGTTGTTGGCCTCC-3’

Asp-R1 5’ RACE primer 5’- GCAAGCTTGTGAAGTCTGAGGGTGTG-3’AspII Pro-F4 Forward degenerate primer 5’-ACNGGXAGNTCXAAYACNTGG-3’

Pro-R2 Reverse degenerate primer 5’-TTRTCNGTIACNGTIGGDAT-3’

Pro-F5 3’ RACE primer 5’-CCGTCGATCTCACGCGAGGAACGCTG-3’

Pro-R5 5’ RACE primer 5’-GCGACGCTGTTGCCGGTCTGCTGAC-3’MetI Sp21-F2 Forward 3’RACE degenerate

primer* 5’-GGTGGTACYTAYGTNCAYGA-3’

Met-R1 5’ RACE primer 5’-AGCGAGTGCCTTGGCGCCAGATTG-3’

Met-R2 5’ RACE primer 5’-CGTAGTCTTGTGTTCCGCCATTTCTGAGG-3’

Met-R3 5’ RACE primer 5’-AGGGCTTGAGAGGCTCGATTGAGTAAG-3’

Met-R4 5’ RACE primer 5’-CCGGAGTTGGTGAAGTTGTACATTGTTGAC-3’

Met-R5 5’ RACE primer 5’-CAAGAGCAACAGCACATCGC-3’MetII MetII-full-F3 Forward primer 5’-AATGTCAATTCTATCGAATTCAAG-3’

MetII-R1 Reverse qPCR primer 5’-CGTGGACGATTGTTCCACCTC-3’

MetII-F1 Forward qPCR primer 5’- GGCTATTACTGCAGCGAAGACC -3’

MetII-full-R Reverse primer 5’-CAATTCAAGAATTATATGGCAGG-3’SerI Sub-F1 Forward degenerate primer 5’-GTTGARTTYGARGGXMGNGC-3’

Sub-R1 Reverse degenerate primer 5’-TTRTCRTTACCXGCNGCNAC-3’

Sub1-F1 3’ RACE primer 5’-GATCGCCGACAACAAGTATGAAGATGGTG-3’

Sub1-R1 5’ RACE primer 5’-GCACAGTGAGTACCATGACCAACACC-3’SerII Sp24-F3 Forward degenerate primer 5’-GCTGACCTYCARACNTTYTT-3’

Sp16-R2 Reverse degenerate primer 5’-GAGAAACCACCNCCNGARAA-3’

Ser-F1 3’ RACE primers 5’-TAGGCGGTGGCGACTGCCGTACTAATGAC-3’

Ser-R2 5’ RACE primer 5’-GAAACTAGAACCTGCAGCATCGGGGCGG-3’

Ser-R3 5’ RACE primer 5’-CGGAATCTGGGTCAAAAGGAGGGGAACCG-3’

Ser-R4 5’ RACE primer 5’-GTCCATACCGTGCATGACCAGGATCGC-3’

* This primer was used together with the UPM primers from the SMART RACE cDNA kit

Note: X symbolizes inosines

129

Table S2

The Real Time qPCR primers used in this study.

Enzyme Primer 5’- sequence -3’ Annealing temp °C Size

Saccharopepsin (AspI)

Pep-F2 CCG CCA TTG ACA CTG GTA CC 53.8289bp

Pep-R2 GGC GGA GGA AAA CAT CAC CAA 54.4

Polyporopepsin (AspII)

Pol-F2 CTC GTT GAC ACC GGG AGT TC 55.9243bp

Pol-R3 GAC GGG GCC GAT GCC TAA 54.9

Peptidyl-Lys metallopeptidase

(MetI)

Met-F2 CGG GAC CGA CTC TAA GGG 54.9169bp

Met-R6 TTC ATT GTG AAT ACA TCA TAG ACT TC 51.7

Peptidyl-Lys metallopeptidase

(MetII)

MetII-F1 GGC TAT TAC TGC AGC GAA GAC C 56.7307bp

MetII-R1 CGT GGA CGA TTG TTC CAC CTC 56.3

Subtilisin (SerI)Sub-F2 AGA AAT CTG CTC CAT GGG GC 53.8

300bpSub-R1 CTA CCA TCA GAA CCA AGC ACC 54.4

Grifolisin (SerII)Ser-F3P GCT ACC TTC AAT GGT CTA TAC AAT AAA AC 56.0

310bpSer-R4P TAC CCA GGC CAG TGA CAG GAT 55.4

Elongation factor 1-a (EF1-a)

EF1a-F1 TTG GAG GAA TCT CCC AAC ATG 57.9273bp

EF1a-R1 AAC GGA CTT GAC TTC AGT AGT C 58.4Glyceraldehyde

3-phosphate dehydrogenase

(GAPDH)

GAPDH-F4 TCA ACG GCA AGC TCA CTG GT 53.8253bp

GAPDH-R4 ACA AAA TTC CCG TTC AAA GGA ATC 52.3

Ubiquitin (Ubc)Ubc-F4 AAC GAT AAT GGG ACC CGG TG 53.8

239bpUbc-R4 GAT TGG GAT CAG TCA GCA TTG 52.4

130

Table S3

Comparison of the sequenced proteases with the MEROPS database showing similarities with known fungal

peptidases. For each protease the top ten blast results are shown.

Protease MEROPS code

MEROPS classification

Protease name Species % identity E-value

AspI MER122335 A01.018 saccharopepsin Laccaria bicolor 78.57% 1.10e-142

MER309581 A01.018 saccharopepsin Serpula lacrymans 79.56% 4.40e-141

MER107385 A01.018 saccharopepsin Coprinus cinereus 78.02% 1.70e-139

MER309629 A01.018 saccharopepsin Schizophyllum commune

77.36% 4.10e-138

MER382986 A01.018 saccharopepsin Stereum hirsutum 76.88% 5.20e-138

MER382978 A01.018 saccharopepsin Coniophora puteana 76.42% 8.40e-138

MER382983 A01.018 saccharopepsin Punctularia strigosozonata

77.67% 1.80e-137

MER382989 A01.018 saccharopepsin Trametes versicolor 77.12% 180e-135

MER382982 A01.018 saccharopepsin Dichomitus squalens 76.42% 4.80e-135

MER382984 A01.018 saccharopepsin Fomitiporia mediterranea

74.53% 1.30e-132

AspII MER118464 A01.019 polyporopepsin Laccaria bicolor 71.60% 1.80e-125

MER383691 A01.019 polyporopepsin Pholiota nameko 74.53% 2.70e-122

MER383440 A01.019 polyporopepsin Trametes versicolor 71.60% 2.80e-120

MER383572 A01.019 polyporopepsin Dichomitus squalens 71.74% 3.20e-190

MER000940 A01.019 polyporopepsin Polyporus tulipiferea 69.04% 9.70e-118

MER383537 A01.019 polyporopepsin Stereum hirsutum 70.15% 2.00e-117

MER383829 A01.019 polyporopepsin Punctularia strigosozonata

70.37% 4.20e-117

MER062431 A01.019 polyporopepsin Phanerochaete chrysosporium

67.70% 4.50e-113

MER172923 A01.019 polyporopepsin Laccaria bicolor 65.43% 5.30e-110

MER383761 A01.019 polyporopepsin Stereum hirsutum 64.71% 1.40e-109

MetI MER003546 M35.004 Peptidyl-Lys metallopeptidase

Pleurotus ostreatus 56.55% 1.50e-53

MER005595 M35.004 Peptidyl-Lys metallopeptidase

Armillaria mellea 57.49% 1.20e-51


Grifola frondosa 51.50% 7.90e-50

MER187451 M35.003 EcpA peptidase Corallococcus coralloides

50.00% 4.30e-42

MER019534 M35.003 EcpA peptidase Xanthomonas axonopodis

48.82% 3.60e-38

MER255520 M35.003 EcpA peptidase Collimonas fungivorans

45.51% 5.80e-38

MER089593 M35.003 EcpA peptidase Shewanella loihica 47.56% 2.50e-37

MER071568 M35.003 EcpA peptidase Shewanella denitrificans

48.45% 4.10e-37

MER075809 M35.003 EcpA peptidase Shewanella amazonensis

48.50% 5.20e-37

MER062468 M35.003 EcpA peptidase Xanthomonas campestris

47.62% 2.90e-36

131

MetII MER003546 M35.004 Peptidyl-Lys metallopeptidase


















MER255520 M35.003 EcpA peptidase Collimonas fungivorans

59.18% 2.60e-32

SerI MER169844 S08.UPA Subfamily S8A unassigned peptidases

Laccaria bicolor 61.28% 6.90e-111

MER034994 S08.UPA Subfamily S8A unassigned peptidases

Coprinus cinereus 60.17% 3.10e-108


Aspergillus oryzae 52.47% 3.30e-100


Aspergillus oryzae 36.11% 3.30e-100


Cryptococcus gattii 56.90% 5.80e-98


Neosartorya fischeri 61.65% 9.00e-94


Neosartorya fischeri 36.11% 9.00e-94


Aspergillus fumigatus 61.65% 1.10e-93


Aspergillus fumigatus 36.11% 1.10e-93


Penicillium chrysogenum

60.43% 4.90e-93

SerII MER137045 S53.010 grifolisin Laccaria bicolor 66.67% 4.40e-136

MER137535 S53.010 grifolisin Laccaria bicolor 60.05% 1.90e-119

MER389943 S53.010 grifolisin Stereum hirsutum 56.77% 6.00e-93

MER125099 S53.010 grifolisin Coprinus cinereus 46.44% 1.20e-89

MER178229 S53.010 grifolisin Sclerotinia sclerotiorum

44.78% 1.30e-85

MER178229 S53.010 grifolisin Sclerotinia sclerotiorum

40.80% 1.30e-85

MER405414 S53.UPW Family S53 unassigned peptidases

Agaricus bisporus 66.24% 1.50e-82

MER078639 S53.010 grifolisin Grifola frondosa 45.70% 1.90e-80

MER032166 S53.010 grifolisin Emericella nidulans 44.53% 1.10e-79

MER030252 S53.010 grifolisin Aspergillus oryzea 43.91% 1.50e-78

132

Table S4

Results of statistical analyses in R on normalized (relative to the housekeeping genes GAPDH and Ubc) and relative

gene expression measurements (gongylidia relative to mycelium)

Prot

ease

Rel

ativ

e fo

ld

chan

ge (2

-C

t )2-

Ct m

in -

max

SD

z va

lue

p va

lue

Nor

mal

ized

ex

pres

sion

gong

ylid

ia (2

-C

t )SD

gon

gylid

iaN

orm

aliz

ed

expr

essio

n m

ycel

ium

(2-

Ct )

SD m

ycel

ium

Asp

I3.

8745

1.62

88 -

9.21

7-3

.946

20.

0000

44.

2134

1.25

021.

0875

2.17

65

Asp

II0.

7395

0.22

58 -

2.42

22.

5797

0.99

492

0.13

071.

7113

0.17

672.

6928

Met

I5.

0304

2.26

48 -

11.1

73-3

.979

30.

0000

423

.129

61.

1513

4.59

80.

9403

Met

II16

.842

11.

7703

- 16

0.23

2-2

0.01

020.

0000

12.

4346

3.25

0.14

461.

2406

SerI

2.23

560.

3095

- 16

.146

-8.9

269

0.00

001

2.25

482.

8525

1.00

860.

9072

SerI

I6.

9805

1.47

91 -

32.9

44-9

.041

20.

0000

16.

3952

2.23

860.

9162

0.61

21

133

chaPter 5

cellulose DegraDation Patterns in Acromyrmex echinAtior colonies

PePijn W kooij, jeroen W m Pullens, jacobus j boomsma , morten schiøtt

(manuscriPt)

134

Cellulose degradation patterns in Acromyrmex echinatior colonies

Pepijn W. Kooij1, Jeroen W.M. Pullens1,2, Jacobus J. Boomsma1, Morten Schiøtt1

1Centre for Social Evolution, Department of Biology, Copenhagen University, Universitetsparken 15, DK-2100 Copenhagen, Denmark

2Laboratory of Genetics, Wageningen University and Research Centre, P.O. Box 309, 6700 AH Wageningen, The Netherlands

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AbstractCellulose is the dominant component in plant cell walls, but only few herbivorous animals can digest this compound without help of microbial symbionts. Leaf-cutting ants are special in feeding on a domesticated fungus that is provisioned with freshly harvested live plant material that the ants chew up and mix with fecal fluid. In recent studies we have shown that fungal-derived fecal fluid pectinases, laccases and proteases are particularly produced in the gongylidia, inflated hyphal tips that are the main food source for the ants. Here we complement these results by investigating the presence, activity and gene regulation of a putative fungal cellulase enzyme (LgCel12) in the ant fecal fluid. We show that this cellulase is only active in the fecal fluid when the ants have access to their normal fungal diet, but remains less active than the fecal fluid pectinases and proteases. The Lgcel12 gene is ca. 3-fold upregulated in the gongylidia that the ants ingest compared to normal mycelium, consistent with overall gene expression being highest in the middle layer of fungus gardens where most gongylidia are produced. However, overall cellulase activity was higher in the older, bottom layers of gardens and in discarded debris material than in the upper and middle layers of gardens. These results indicate that LgCel12 endocellulase plays a role in the initial breakdown of plant substrate but without being a major participant in the decomposition process, as overall cellulolytic activity is most prevalent in the bottom layer of the gardens, consisting of material that is about to be discarded by the ants. We offer a stoichiometric interpretation of these results emphasizing that the symbiosis between leaf-cutting ants and their garden symbiont is almost certainly protein limited, so that the acquisition of nitrogen and phosphorus is prioritized by the joint efforts of the mutualistic partners and excess carbon-based resources discarded in considerable quantities.

Keywords:Leaf-cutting ants, Leucoagaricus gongylophorus, gene expression, stoichiometry

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IntroductionCellulose is the dominant constituent of plant cell walls and is considered the most abundant or-ganic compound on earth, but proteins with the capacity to degrade cellulose are encoded in the genomes of very few animals (Watanabe and Tokuda 2001). Instead, most animals that specialize on cellulose-rich food live in symbiosis with cellulose degrading microorganisms allowing them to tap into this enormous nutritional resource. Symbioses of this kind are particularly prevalent among the insects where feeding on dead wood and other plant material has evolved multiple times independently (Breznak 1982; Douglas 2013). These gut symbiont microorganisms are usu-ally bacteria, but protists fulfil similar roles in the lower wood-dwelling termites (Ohkuma 2003; Noda et al. 2007). However, in some other cases, these services are provided by fungi, with gut yeasts in several families of wood-ingesting beetles (Zhang et al. 2003; Grünwald et al. 2010), ambrosia fungi in wood-boring Xyleborine beetles (Biedermann et al. 2009), and Termitomyces

fungi in fungus-growing termites (Aanen et al. 2002).

Also the lower attine fungus-growing ants use dead plant material to manure their fungus-gardens, which likely implies that these fungi must have considerable cellulose degrading capacities (De Fine Licht and Boomsma 2010). However, in contrast to any other insect-fungus symbiosis, the attine ants evolved an evolutionarily derived lineage of active herbivores, which gained access to vast amounts of fresh leaf substrate with an abundance of intracellular sugars and proteins that would typically no longer be available in dead plant cells. Although the fungi of these leaf-cutting ants have retained the capacity to degrade cellulose (De Fine Licht et al. 2010; Moller et al. 2011; Aylward et al. 2013; Grell et al. 2013), the relative importance of cellulolytic enzymes for the herbivorous leaf-cutting ant symbiosis has remained controversial (Abril and Bucher 2002; Erthal et al. 2009; Nagamoto et al. 2011).

The two genera of leaf-cutting ants, Atta and Acromyrmex, live in obligate mutualistic symbiosis with a single, genetically variable fungal species: Leucoagaricus gongylophorus (Weber 1966; Mueller et al. 2005; Kooij et al., Chapter 1). These ants are dominant herbivores in the New World tropics (Della Lucia et al. 2014; Kooij et al., Chapter 3, submitted) and significant accelerators of nitrogen and phosphorus cycling (Fowler et al. 1989). Similar to their lower attine ant relatives rearing saprotrophic fungus gardens, the leaf-cutting ants cultivate their symbiotic fungus in spe-cial underground chambers, but their fungal crop belongs to a unique domesticated lineage that has evolved special symbiotic organs, staphylae, that grow bundles of inflated hyphal tips (gon-gylidia) that provide essentially all nutrition for the ants and their brood (Quinlan and Cherrett 1978; Quinlan and Cherrett 1979; Bass and Cherrett 1995). After the larger foraging worker ants

137

have cut the leaf fragments and transported them back to the nest, smaller workers chew these pieces into a pulp, reducing the crystallinity of the plant cellulose (Martin et al. 1991; Nagamoto et al. 2011), and place this material on top of the fungus garden before manuring it with fecal fluid and inoculating it with fresh tufts of mycelium.

The fecal fluid is used for the rapid maceration of cut leaves (Martin et al. 1975), and contains a wide range of fungal plant degrading enzymes such as pectinases (Schiøtt et al. 2010), laccases (De Fine Licht et al. 2013) and proteases (Kooij et al. in press). These enzymes have retained activity in the fecal fluid after gut passage, and the genes encoding them were almost always upregulated in the gongylidia when compared to the normal mycelium. However, gongylidia up-regulation and fecal fluid expression for cellulases has not been investigated, as cellulases were not particulatly common among the ca. 33 proteins found in the Acromyrmex echinatior fecal fluid (Schiøtt et al. 2010). Only a single protein (LgCel12) belonging to the glycoside hydrolase family 12 (GH12) was identified in the fecal fluid of Panamanian Acromyrmex echinatior (Schiøtt et al. 2010), but remained unconfirmed in a recent study on lignocellolytic enzymes in Leucoagaricus

gongylophorus from other sympatric colonies of the same ant species (Aylward et al. 2013).

Substrates targeted by the members of the GH12 enzyme family are cellulose, xyloglucan and β-1,3-1,4-glucan. As already indicated above, the importance of cellulose degradation for the ant-fungus mutualism has been much debated (e.g. Martin and Weber 1969; Abril and Bucher 2002; Richard et al. 2005; Erthal et al. 2009; Moller et al. 2011; Nagamoto et al. 2011; Aylward et al. 2013; Grell et al. 2013), which makes finding a putative non-digested fungal cellulase in the ant fecal fluid (but missed in the proteome of the fungus garden; Aylward et al. 2013) of interest, as it may suggest that some steps in cellulose degradation in the top of fungus gardens (where fecal fluid is deposited) may be important for the symbiosis even in low quantities. The present study therefore set out to: 1. Measure cellulase activity in both fecal fluid and fungus gardens of Acromyrmex echinatior, 2. Confirm that most of the cellulase activity in the fecal fluid is fungal derived, 3. Measure gene expression levels of Lgcel12 in the different layers of the fungus garden and in the fungal gongylidia relative to normal mycelium, and 4. Discuss the implications of our findings to the ongoing discussion on cellulose degradation by leaf-cutting ant fungus gardens.

Material and methodsBiological materialA total seven ant colonies of Acromyrmex echinatior (Ae226, Ae263, Ae266, Ae280, Ae322, Ae335 and Ae370), collected between 2004 and 2007 in Gamboa, Panama, were used for this study.

138

All colonies were kept under controlled conditions of ca. 25˚C and a relative humidity of ca. 70%, and were fed twice a week with fresh bramble leaves (Rubus sp.), parboiled rice and pieces of ap-ple. For enzyme activity measurements ant fecal fluid and fungus garden material were collected from five of these ant colonies (Ae226, Ae263, Ae266, Ae280 and Ae322).

For the collection of fecal fluid, the gaster of a large worker was gently pressed to a glass mi-croscope slide using a forceps (Rodrigues et al. 2008). Each fecal droplet was mixed with 0.5µl demineralized ddH2O (the fecal fluid “enzyme extract”) and collected with a micropipette in an Eppendorf tube kept on ice until further processing. Fecal fluid was collected from two different groups of large workers in each colony, one having access to the fungus garden and thus likely having ingested fungal gongylidia, and the other being deprived of their fungus garden and pro-vided only with 10% sucrose water for 20 days. We collected 120 mg fungus garden material from each of the three visible layers of fungus garden (Kooij et al. 2011), the fresh and dark top layer, the gongylidia-rich middle layer (De Fine Licht et al. 2013) and the older bottom layer. We supplemented this material with samples from the debris material, i.e. old fungus garden material discarded by the ants, similar to a previous study (Kooij et al. 2011) and added 500 µl of ddH2O to each sample and crushed material with a pestle before vortexing and centrifuging for 15 min (15000 g). The fungus garden supernatant and the fecal fluid extract were then kept on ice until further processing the same day.

For gene expression measurements in gongylidia and mycelium, we collected from five colonies (Ae263, Ae280, Ae322, Ae335 and Ae370) ca. 100 µl of gongylidia clusters (staphylae) and my-celium in separate 2 ml Eppendorf tubes floating in liquid nitrogen using a stereomicroscope at 40x magnification. For gene expression measurements in the different fungus garden layers and the debris pile, these samples were supplemented by 120 mg fungal material from colonies Ae226, Ae263, Ae266, Ae280 and Ae322, after carefully removing any visible ants or ant parts, larvae and eggs. All these gene expression samples were stored at -80˚C for RNA extraction at a later point.

Cellulase activityFecal fluid and fungus garden cellulase activities were measured for β-glucosidase with a pNPG-assay and for endo-1,4-β-glucanase with both a CMC/DNS-assay (Zhang et al. 2009) and the AZO-CM-Cellulose-assay (Megazyme, Wicklow, Ireland). Exoglucanase assays require pu-rified enzymes to avoid interference with other cellulases and could therefore not be performed. β-Glucosidase activity was determined by incubating 3 µl enzyme extract from fecal fluid or fun-gus garden in 42 µl mM 4-nitrophenyl-β-D-glucopyranoside (Sigma N7006) and 0.1 M sodium

139

acetate buffer, pH 4.8, for 30 minutes at 30°C. We added 60 µl 0.4 M glycine buffer, pH 10.8 and read absorbances at 430nm using a Versamax ELISA microplate reader (Molecular Devices, Sunnyvale, USA). Absorbance measurements were converted to enzyme units using a standard curve made with 4-nitrophenol (Sigma 241326), defining one unit as the amount of enzyme re-quired to release 1 µg nitrophenol per minute per fecal droplet or mg fungus garden.

Using the CMC/DNS-assay, endo-1,4-β-glucanase activity was determined by incubating 2.5 µl enzyme extract in 2 % (w/v) Carboxymethylcellulose (Sigma 21902) and 50 mM citrate buffer, pH 4.8 for 30 minutes at 30°C. A quantity of 15 µl DNS solution (0.4 M NaOH, 0.04M 3,5-dini-trosalicylic acid, 1 M potassium sodium tartrate) was added, and the mixture was heated at 99.9°C for 5 minutes in a PCR machine. Samples were put on ice to terminate the reaction and 100 µl distilled water was added, after which the absorbance at 540 nm was read in a Versamax ELISA microplate reader (Molecular Devices, Sunnyvale, California, USA). The absorbance measure-ments were converted to enzyme units using a standard curve made with glucose (Sigma G8270). One unit was defined as the amount of enzyme required to release 1 µg of glucose per minute per fecal droplet or mg fungus garden.

Endo-1,4-β-glucanase activity was also determined using the AZO-CM-Cellulose-assay (Mega-zyme, Wicklow, Ireland) using 50 µl of enzyme extract added to 50 µl pre-equilibrated substrate solution, pH 5. The solution was vortexed and incubated for 30 minutes at 30˚C and the reaction terminated by adding 250 µl of precipitant solution. Samples were then vortexed and centrifuged for 10 minutes at 1000 g, after which the supernatant was measured for absorbance at 590 nm in the Versamax ELISA micro plate reader and converted to enzyme units using the standard curve included in the protocol, once more defining a unit as the amount of enzyme required to release 1 µg of glucose per minute per fecal droplet or mg fungus garden. All data for cellulase activity were analysed with R (R Core Team 2013) using a Wilcoxon test for the fecal fluid data and a Kru-skal-Wallis test for the garden-layer data followed by the multiple comparison test “kruskalmc” from the package “pgirmess” (Giraudoux 2013).

AZCL enzyme activity assaysFecal fluid enzyme profiles were performed similar to previously published methods (Rønhede et al. 2004; De Fine Licht et al. 2010). In short, 50 fecal droplets per colony were diluted 20x with ddH2O and 1 ml of this suspension was applied immediately to 16 different assay-plates contain-ing 0.1 g/l of the Azurine-Crosslinked (AZCL) substrates: amylose, arabinoxylan, barley ß-glu-can, casein, chitosan, collagen, curdlan, debranched arabinan, dextran, galactan, galactomannan,

140

HE-cellulose, pachyman, pullulan, xylan and xyloglucan.

The assay-plates of 6 cm diameter were prepared separately for each substrate using an agarose medium (1 % agarose, 23 mM phosphoric acid, 23 mM acetic acid, 23 mM boric acid) and were pH adjusted according to the manufacturer’s description (Megazyme, Bray, Ireland). After the medium had solidified, round wells of ca. 0.1 cm² were made in each plate with a cut-off pipette tip and 10 µl of the fecal fluid mixture was applied to each well in triplicate. After 22 hours of incubation at 25°C the plates were photographed and the area of the blue halo surrounding each well (a quantitative measure for the absolute amount of enzyme activity) was measured using the software program ImageJ ver. 1.43u for Macintosh (De Fine Licht et al. 2010; Kooij et al. 2011). Enzyme activity measurements were grouped into categories based on the main plant cell wall target of each enzyme (Kacuráková et al. 2000 and Megazyme, Bray, Ireland): amylases (measured with amylose), cellulases (measured with barley ß-glucan and HE-cellulose), hemi-cellulases (measured with arabinoxylan, curdlan, galactomannan, pachyman, xylan and xyloglu-can), pectinases (measured with debranched arabinan and galactan), proteases (measured with casein and collagen), and a rest group (including the substrates chitosan, dextran and pullulan, to measure chitosanases, dextranases and pullulanases respectively). The average halo values were calculated for each triplicate of identical plates after which the data were log transformed and an-alyzed with R, using Linear Mixed-Effects Models (“lme”) in the package “nlme” (Pinheiro et al. 2013) and posthoc analysis with General Linear Hypotheses (“glht”) in the package “multcomp” (Hothorn et al. 2008). We present these data grouped for the six enzyme categories with colony as random effect.

Protein identification and gene cloningSDS-PAGE and mass spectrometry was performed as described previously (Schiøtt et al. 2010; De Fine Licht et al. 2013; Kooij et al. in press). A mix of fifty fecal droplets was subjected to SDS-PAGE, after which individual protein bands were manually excised from the gel. Proteins were extracted from the gel plugs and digested with trypsin. De novo sequencing of the resulting pep-tide fragments was performed on the basis of b and y fragment ions present in MS/MS spectra of derivatized and underivatized samples. Amino acid sequences were obtained by manual analysis of the spectra using the software program AminoCalc (Protana A/S, Odense, Denmark) as support. The obtained amino acid sequences were used as queries in a Blast search of the assembled Ac-

romyrmex echinatior genome (Nygaard et al. 2011) and a low coverage genome sequence of the Acromyrmex echinatior fungal symbiont (De Fine Licht et al. 2013).

141

RNA extraction and cDNA production was performed as described in Schiøtt et al. (2010). A full-length gene transcript sequence of Lgcel12 was obtained using a RACE (Rapid Amplification of cDNA Ends) strategy. 3’ and 5’-RACE libraries were made from ca. 1 µg of extracted RNA with the SMART RACE cDNA kit (CLONTECH, Mountain View, California, USA), and 3’end and 5’end gene sequences were PCR amplified from these libraries using the gene specific primers SYT-F1 (CAG TCG ACA CTT CCC AGC ACT GTG) or SYT-R1 (GGT ATA CTG ACC AGC AGT GAC GGT G) designed from the BLAST-search-identified sequence reads along with the UPM primer enclosed in the SMART RACE cDNA kit. The following PCR scheme was used in the RACE experiments: one cycle of 95°C for 5 min, 10 cycles of 95°C for 20 sec, 72°C for 30 sec (with a decrease in temperature of 0.5°C in every cycle) and 72°C for 3 min, followed by 35 cycles of 94°C for 20 sec, 67°C for 30 sec and 72°C for 3 min, and ending with one cycle of 72°C for 7 min. PCR products were cloned in pCR4-TOPO using the TOPO-TA cloning procedure (Invitro-gen, Carlsbad, California, USA) and sequenced at Eurofins MWG Operon (Ebersberg, Germany). The full length gene sequence will be deposited in GenBank after publication.

Quantitative real-time PCRPrimers for the Lgcel12 gene (Table S1) were designed by matching the obtained cDNA sequenc-es to a partially sequenced genome of the Acromyrmex echinatior symbiont (De Fine Licht et al. 2013) using BLASTn. Matching sequencing reads were assembled and aligned to the cDNA se-quence in question to identify intron and exon sequences. Primers were then designed to span an intron to avoid amplification of genomic DNA. The qPCR was run on an Mx3000P QPCR System (Agilent, Santa Clara, CA, USA) in a 20 µl qPCR reaction (0.5 µl cDNA, 10 µl 2x SYBR Premix Ex Taq [TaKaRa Bio Inc., Otsu, Japan] and 0.4 µl of each primer [10µM]) with the following PCR conditions: one cycle of 95°C denaturing for 2 min, followed by 40 cycles of 95°C denaturing for 30 sec, 57°C annealing for 30 sec, and 72°C extension for 30 sec and ending with a melting curve cycle of 95°C for 30 sec, 57°C for 30 sec and 95°C for 30 sec.

We performed qPCR for three genes in total: the target gene Lgcel12 and two housekeeping genes ubiquitin (Ubc, GenBank HQ174771) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH, GenBank HQ174770) with three technical replicates for each sample. All Ct values from the RT-qPCR analyses were analysed using R with packages “ReadqPCR” (Perkins and Kohl 2011) and “NormqPCR” (Kohl and Perkins 2011). Average expression was calculated from the three technical replicates of each sample for two datasets, one with Ct values for the different fungus garden layers including the debris pile and the other with Ct values for the fungus garden myce-lium and gongylidia.

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Based on calculations for housekeeping gene stability with “selectHKs” from the “NormqPCR” package, only GAPDH was used as reference housekeeping gene to determine the normalized expression values for Lgcel12 in the fungus garden layer samples, which produced 2∆Ct values (Schmittgen and Zakrajsek 2000) for each layer. These results were further analysed with R using ANOVA to generate individual p-values with General Linear Hypotheses (“glht”) in the Simulta-neous Inference in General Parametric Models package “multcomp” (Hothorn et al. 2008). For the gene expression comparison between gongylidia and mycelium, both housekeeping genes (Ubc

and GAPDH) were used to calculate the relative expression levels of the Lgcel12 gene, resulting in 2-∆∆Ct values (Livak and Schmittgen 2001).

ResultsBoth the Megazyme-assay (Figure 1A) and the CMC-assay (Figure 1B) for endo-1,4-β-glucanase showed a substantial reduction in activity when ants had been feeding on sucrose water for 20 days (W = 23, p = 0.0159; W = 22, p = 0.0278), but low activity remained and was significantly higher than zero for the CMC-assay (V = 15, p = 0.0313) but not for the Megazyme-assay (V = 10, p = 0.0502). Also β-Glucosidase activity in the fecal fluid (Figure 1C) was significantly different between the two diets (W = 25, p = 0.0060), but with activity in the purged ants not being signifi-cantly different from zero (V = 6, p = 0.0907).

The cellulase activities were different among the layers of the fungus gardens with a trend of increased activity from top to bottom/debris material (Figure 2). The Megazyme-assay for en-do-1,4-β-glucanase (Figure 2A) showed significantly lower activity in the top and middle layer compared to the bottom layer and debris material (overall: χ2

3 = 85.979, p < 0.0001, see Figure 2 for post-hoc details). The CMC-assay for endo-1,4-β-glucanase (Figure 2B) showed similar re-sults but only the activity in debris material was significantly higher than the middle layer (overall: χ2

3 = 11.480, p = 0.0094, see Figure 2 for post-hoc details). β-Glucosidase activity (Figure 2C) showed larger differences comparable to those for endo-1,4-β-glucanase in the Megazyme essay, with significantly higher activity in the bottom layer and debris material compared to the top layer (overall: χ2

3 = 15.789, p = 0.0013, see Figure 2 for post-hoc details).

To better understand the significance of the cellulase activities in the fecal fluid and be able to compare activities to those found in earlier studies of fecal fluid pectinases (Schiøtt et al. 2010) and proteases (Kooij et al. in press) we measured the overall activities of all enzymes important for plant material degradation with AZCL enzyme assays. This showed significant differences among the enzyme groups (Figure 3, F5,70 = 68.108, p < 0.0001), with pectinases having the

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highest activity, followed by amylases and proteases, and cellulases and hemicellulases having lowest activities. Other enzymes targeting the substrates chitosan, dextran and pullulan showed no activity at all.

Mass spectrometry analysis of fecal fluid from Acromyrmex echinatior (Schiøtt et al. 2010; De Fine Licht et al. 2013; Kooij et al. in press) identified a peptide sequence (SYTNLNLNANAQR) that matched the Lgcel12 gene in the genome sequence of Leucoagaricus gongylophorus, which harboured the amino acid sequence SYTNLNLNANLNK. The two sequences deviate in the last three amino acids, but as both combinations gave the exact same peptide mass, this discrepancy can be attributed to difficulties in interpreting the mass spectra. BLAST search allowed us to

Fungal diet Sucrose water

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Figure 1Enzyme activities for endo-1,4-β-glucanase (from two different

assays) and β-glucosidase in fecal fluid of ants provided with

either a normal fungus garden diet or with sucrose water for 20

days. Wilcoxon rank sum test showed a significant reduction in

enzyme activity for endo-1,4-β-glucanase (A: Megazyme assay,

W = 23, p = 0.016; B: CMC-assay, W = 22, p = 0.028) and

β-glucosidase (C: W = 25, p = 0.006) when ants were deprived

of fungus garden material. Enzyme activity levels are given as

milliunits per fecal droplet.

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assign the Lgcel12 gene to glycosyl hydrolase family 12. This protein was the only one in the obtained fecal fluid proteome that could function as a cellulase, so we inferred that this enzyme mediates the cellulose activity measured in the fecal droplets. Relative changes in Lgcel12 gene expression in gongylidia versus normal mycelium (fold-changes: 2-∆∆Ct) showed that the gene was significantly more expressed (fold change = 3.1, min – max SE = 0.33 – 5.82) in the gongylidia than in normal mycelium (Figure 4A, z = -6.358, p < 0.00001), suggesting that the ingestion of this enzyme by the ants for transfer to the fecal fluid represents a mutual adaptation that somehow enhances symbiotic efficiency. In order to correlate the cellulase activity measurements in the fungus garden with the expression of Lgcel12, we measured the normalized expression levels of this gene (Figure 4B) in the same layers of fungus garden that were used for the activity measure-ments (Figure 2). This showed significantly higher normalized gene expression in the middle layer (F3,16 = 3.699, p = 0.0339) compared to the other layers and the debris material, consistent with the middle layer having the largest amount of gongylidia (De Fine Licht et al. 2013).

A

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Figure 2Enzyme activities for endo-1,4-β-glucanase (from two different assays) and β-glucosidase in three layers of fungus gar-

den (Top, Middle and Bottom) and debris material. (A) Endo-1,4-β-glucanase activity (Megazyme assay) was signifi-

cantly higher in the bottom layer of the fungus garden and in the debris material (overall: Kruskal-Wallis χ23 = 85.976,

p < 0.0001). (B) Endo-1,4-β-glucanase activity (CMC-assay) was significantly higher in the debris material compared

to the middle layer (overall: Kruskal-Wallis χ23 = 11.480, p < 0.01). (C) β-Glucosidase activity was significantly higher

in the bottom layer and debris material compared to the top layer (overall: Kruskal-Wallis χ23 = 15.789, p < 0.001). En-

zyme activity levels are given as units per gram fungus garden material. Different letters above bars refer to differences

in post-hoc tests at P < 0.05.

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DiscussionSince Martin and Weber (1969) found similar cellulose degrading capabilities in the fungal sym-biont and fecal fluid of Atta leaf-cutting ants (see also Martin et al. 1975), the joint cellulose degradation capacity of the ant fungus-farming mutualism has remained controversial. Cellulose degrading capabilities were later confirmed for the fungus (Bacci et al. 1995a) and for bacteria in the fungus garden (Bacci et al. 1995b), but the level of cellulose degradation was found to be relatively low (De Siqueira et al. 1998). The debate continued with different techniques giving varying results, some showing that the fungus was incapable of utilizing cellulose as a substrate (Abril and Bucher 2002; Abril and Bucher 2004; Bucher et al. 2004), but most studies indicating mediocre (D’Ettorre et al. 2002; Silva et al. 2006; Erthal et al. 2009; Kooij et al. 2011; Moller et al. 2011) or relatively high cellulose degradation capabilities (Rønhede et al. 2004; Richard et al. 2005; Schiøtt et al. 2008; De Fine Licht et al. 2010; Suen et al. 2010; Nagamoto et al. 2011; Ayl-ward et al. 2013; Grell et al. 2013) without reaching consensus.

The degradation of plant cell wall cellulose is a tedious process, which involves at least three types of cellulases in order to degrade this polysaccharide to single glucose molecules: 1. en-do-cellulases, which open up the matrix of the cellulose fibres and cleave the fibres at random places, 2. exo-cellulases, which cut off short cellulose fragments mainly from the ends of the cellulose chain, and 3. β-glucosidases that break down these smaller chains into single glucose molecules (Béguin 1990; Martin et al. 1991). We show here that both endo-1,4-β-glucanase and β-glucosidase activity increase towards the bottom layer of the fungus garden and in the debris

0.0

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Figure 3Enzyme activity of fecal fluid for six different enzyme classes

expressed as the mean area (cm2 ± SE) of coloured halos on

AZCL plates. Differences in activity between the enzyme class-

es were significant (F5,70 = 68.108, p < 0.0001), with pectinas-

es being the most active, followed by amylases and proteases

and cellulases and hemicellulases being least active (different

letters indicate differences in post-hoc tests at P < 0.05). The

substrates chitosan, dextran and pullulan were grouped together

in the class “other”, as no activity was observed for any of these.

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material (Figure 2), which is consistent with earlier findings (Suen et al. 2010; Moller et al. 2011; Aylward et al. 2013; Grell et al. 2013). Although explicit data on exocellulase activity are lacking, this suggest that the fungus garden has the ability to convert cellulose into glucose molecules that can be taken up as nutrients by fungal cells, but that this activity is mostly expressed towards the end of the substrate degradation process when other more easily accessible nutrients have already been decomposed. Although many studies have pointed out that high cellulolytic activity in the bottom of fungus gardens may also be due to bacteria (Bacci et al. 1995b; Suen et al. 2010; Ayl-ward et al. 2012) or yeasts (Carreiro et al. 1997; Middelhoven et al. 2003; Mendes et al. 2012), the present study and recent results by Aylward et al. (2013) and Grell et al. (2013) are consistent in suggesting that a significant fraction of cellulose decomposition in the bottom of gardens is indeed performed by the fungal symbiont. However, this appears to be less likely for the debris material discarded by the ants, as most of the fungal tissue should have died when the ants stop maintaining it, so only yeasts and bacteria are left.

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-ΔCt

)

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B B

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A B

Figure 4Gene expression in gongylidia relative to mycelium and normalized gene expression after scaling

for the expression of housekeeping genes in fungus garden layers and debris piles. (A) The Lgcel12

gene is upregulated in gongylidia compared to mycelium with an average 3.1x fold change (0.330 -

5.819 min - max SE, z = -6.358, p < 0.00001). (B) Normalized gene expression of the Lgcel12 gene

was significantly higher in the middle layer of the fungus garden compared to any other part of the

fungus garden (F3,16 = 3.699, p = 0.034). Different letters above bars refer to differences in post-hoc

tests at P < 0.05.

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In addition to confirming cellulolytic activity towards the end of the decomposition process, our present study shows that early stages of cellulose decomposition are an explicit target in the top of fungus gardens. A single putative endo-cellulase enzyme (LgCel12) of fungal origin apparently escapes degradation in the ant gut to end up in the fecal fluid where it remains active (Figure 1 and 3). Although our earlier mass spectrometry results (Schiøtt et al. 2010) did not indicate the pres-ence of any β-glucosidases we nevertheless found these enzymes to be active in the fecal fluid and to originate from the fungal symbiont as well (Figure 1). The expression of the Lgcel12 gene was also upregulated in the gongylidia relative to the normal mycelium, indicating that this enzyme has been selected to be ingested by the ants to function either in the ant gut or in the fecal fluid, as has also been shown for many other fecal fluid enzymes (Schiøtt et al. 2010; De Fine Licht et al. 2013; Kooij et al. in press). Overall fungal expression of the Lgcel12 gene was highest in the middle section of the gardens (Figure 4), consistent with gardens producing most of the gongylid-ia when fungal biomass is abundant and substrate not yet depleted (De Fine Licht et al. 2013).

Endo-cellulase enzyme (LgCel12) activity in the top of gardens would likely serve the purpose of loosening up the cell walls in the leaf pulp macerated by the ants to allow hyphal access to the cell interior with proteins and starch (Schiøtt et al. 2010; Moller et al. 2011; Grell et al. 2013; Kooij et al. in press). However, we also obtained evidence for β-glucosidase activity in the fecal fluid, which would produce free glucose from oligosaccharides that could be absorbed by the myceli-um. Recently, more advanced mass spectrometry (Schiøtt et al., unpublished) has now also shown exocellulases in the fecal fluid of A. echinatior, indicating that all three steps of cellulose degra-dation take place in the top of fungus gardens. However, compared to the activity of other fungal enzymes such as proteinases, pectinases and amylase, fecal fluid cellulolytic activity appears to remain relatively low in all three steps (endocellulases, exocellulases, β-glucosidases)(Figure 3).

Overall, our results suggest a scenario in which the fungal symbiont is capable to degrade cellu-lose but only at a relatively slow rate and not primarily for maximizing glucose production for the symbiosis. As the bottom of fungus gardens hardly produces gongylidia (De Fine Licht et al. 2013), any glucose produced here remains unavailable to the ants and is thus unlikely to benefit the symbiosis. As suggested by Grell et al. (2013), the main function of glucose production at this stage may be to prevent structural fungus-garden collapse and pathogen invasion until the ants discard this otherwise depleted material. At the top of gardens modest endocellulase activity may be required to supplement xylanase breakdown (Schiøtt et al. 2008) to open up plant cells. Proportional production of exocellulases and β-glucosidases at that stage would then merely make sure that complex sugars can benefit fungal growth rather than imposing risks of bacterial growth

148

that may damage symbiotic efficiency. This hypothetical framework would remain consistent with fungus-garden activity of leaf-cutting ants being ultimately focused on maximizing protein acqui-sition per unit of foraging and processing effort by the ants in spite of this leaving behind consid-erable amount of non-degraded cellulose material that is discarded by the ants from the bottom layers of fungus gardens (Schiøtt et al. 2008; De Fine Licht et al. 2010; Moller et al. 2011; Grell et al. 2013; Kooij et al. in press).

This interpretation follows the general idea that decomposition of organic matter takes place in a stoichiometric manner, which keeps ratios of liberated carbon, nitrogen and phosphorus relatively constant and releases any surplus (non-limiting) nutrients into the external environment (Man-zoni et al. 2010; Hartman and Richardson 2013; Sinsabaugh et al. 2013). It is well documented that plant diets offer an excess of carbon relative to nitrogen and phosphorus (Manzoni et al. 2010; Bell et al. 2014), so that decomposers (in this case the symbiotic collaboration between leaf-cutting ants and fungal symbiont) should be under selection to invest primarily in enzymes for acquiring the most limiting nutrients (nitrogen and possibly phosphorus), and to adjust carbon decomposition/liberation to the level needed for achieving stoichiometric equilibrium. Indications of the leaf-cutting ant symbiosis to be nitrogen-limited have emanated from the demonstration of nitrogen-fixing bacteria in the fungus gardens of Atta leaf-cutting ants (Pinto-Tomás et al. 2009) and other nitrogen conserving bacteria in the guts of both Atta and Acromyrmex leaf-cutting ants (Sapountzis et al. in review). Depending on the efficiency of these additional nitrogen preserving bacterial symbionts, it may thus turn out that the availability of phosphorus in the plant substrate is ultimately the most limiting factor for the leaf-cutting ant symbiosis, but much further work will be needed to substantiate these hypotheses.


in Gamboa, and the Autoridad Nacional del Ambiente y el Mar (ANAM) for permission to sample ant colonies in Pan-

ama, the Danish National Research Foundation (DNRF57) and the ERC (323085) for funding.

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First of all I want thank the appr. 7000 (six-legged) girls who’s butt I was allowed to pinch, with-out them this thesis would not exist.

Special thanks go out for my two supervisors, Koos Boomsma and Morten Schiøtt, for all their support during the last three and a half years. You both have been great teachers and showed me how to see results in a bigger picture. Our discussions on various subjects have inspired me to think beyond the results as they were and reflect them to the greater aspects of the science of mu-tualisms.

Duur Aanen, without whom I would not have found the Centre of Social Evolution. My Masters project with you on fungus-growing termites led me to a great interest in fungus-growing insect systems and mutualisms in general. Also thank you for our work together on entangling the won-ders of the multiple nuclei in the ant fungi.

Michael Poulsen, for your expert help on incompatibility in ant-fungi, the long hours of squeezing ants, staring at fungi, and analyzing data. And off course thank you for the interesting discussions we had about fungus-growing ants, fungus-growing termites, and other topics, scientific or not.

My other co-authors, Adelina Rogowska-Wrzesinska, Peter Roepstorff and Daniel Hoffmann, for your help and expertise on various aspects of the proteases produced by the fungus and transport to the right place by the ants.

The Smithsonian Tropical Research Institute, Panama, for allowing me to do field work at the Gamboa field station and the scientific support there.

The Department of Biology, University of Copenhagen, for giving me this opportunity to perform and defend my PhD.

The Danish National Research Foundation for providing part of the money for this PhD.

I want to thank Henrik, for his great supervision during my internship at the Centre, which insti-gated me to apply for a PhD position here. Also thanks for the various discussions we had on the multitude of topics of fungus-growing ants.

My office mate, Luigi. I had a great time sharing my office with you these three and a half years,

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from interesting discussions on politics, our both research topics or when to put up our Christmas decorations.

Bernhardt and Luigi, for the long evenings of shouting at each other while playing FIFA, and still find time together to discuss science.

Panos and Saria, for the after work beers at the end of these three years as a nice break in between the writing, and reminding me there are other topics in science then the ones described in this thesis.

And of course, Sylvia and Louise, the best lab-techs in the world! You both deserve a special place here. You help has been fantastic! Especially to Sylvia, for listening to my whining about dirty labs over and over and for the delicious banana-chocolate cakes.

My parents, Jos and José, for making it possible for me to study and make it possible for me to stand where I stand today. And my parents as well as my brothers and sisters, Taco, Eelke, Sanne and Jojanneke (and Jalke and Avine) for their support during these years.

My new family in Colombia, Edgar, Gladys, Paula and Juan, for welcoming me in their midst and their support.

My friends who I left in the Netherlands or who moved away themselves, Remco&Dorine, Roos&Freek, Silvan&Zuuz, Marijke, Jan-Willem, Ezra, Fam&Chris, Liset and others I might have forgot. As well as all the new friends I found, Kostis&Laetitia, Claudia&Francesco, Danka, Lilia&Martin, Rasmus, Marlene, Jesper, Caya, Vale, Lina&Lina, Alex, Vicky, and everyone else.

And everyone else at the Centre for Social Evolution, and the Section for Ecology and Evolution in Copenhagen, Denmark, and the Laboratory of Genetics in Wageningen, The Netherlands.

Poes, welcoming me every morning with a happy meow and for keeping my lap warm while writing.

And last but not least, Andrea, mi amor. Without this PhD we would probably never have met, and without you this PhD would have been the most boring ever. You bring the light in my life, and I will always be grateful for all your support and love during this years! I love you.

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