Spam and the evolution ofthe fly’s eyeDaniel Osorio

SummaryThe open rhabdoms of the fly’s eye enhance absolutesensitivity but to avoid compromising spatial acuity theyrequire precise optical geometry and neural connec-tions.(1) This neural superposition system evolved fromthe ancestral insect eye, which has fused rhabdoms. Arecent paper by Zelhof and co-workers(2) shows that theDrosophila gene spacemaker (spam) is necessary for de-velopment of open rhabdoms, and suggests thatmutantsrevert to an ancestral state. Here I outline how openrhabdoms and neural superposition may have evolvedvia nocturnal intermediates, and discuss the implicationsfor the role of spam in insect phylogeny. BioEssays29:111–115, 2007. � 2007 Wiley Periodicals, Inc.

Introduction

The ommatidium, which is the basic modular unit of insect

compound eyes (Fig. 1), normally contains a set of eight

photoreceptor cells. Themembrane on one side of the cell is a

specialised microvillar structure known as a rhabdomere. A

high density of photopigment protein (rhodopsin) gives the

rhabdomere a higher refractive index than that of normal

cytosol. Optical waveguide effectsmean that light focussed on

the rhabdomere tends to be trapped within it. Most insect

lineages have a fused rhabdom, where the rhabdomeres of all

photoreceptors form a single waveguide but, on at least five

occasions in evolutionary history, the rhabdomeres have

separated to form what is is usually called a open rhabdom

(Figs. 1–3).

The best-known open rhabdom eyes are those the higher

flies (Brachycera), which include Drosophila. Here the seven

separate rhabdomeres form an open rhabdom, with a

distinctive trapezoidal form. Spam is a secreted protein that

acts with two additional genes chaoptin and prominin to

prevent the rhabdomeres from adhering to one another.2 In

Spammutants, the rhabdomeresmakedirect contact, as is the

case in many other insects (Figs. 1 and 2). When the study2

was extended to other insect lineages, it was found that the

diurnal mosquito Toxorhynchites, which has open rhabdoms,

expresses spam, whereas the mosquito Anopheles gambiae,

which is described as a ‘closed system’, does not. Neither do

unrelated insects where rhabdomeres touch, namely the

beetle Tribolium and the honeybee Apis. Finally, expressing

spam in the Drosophila ocellus causes rhabdomeres there

to separate. The study by Zelhof and co-workers (Ref. 2)

illustrates nicely how expression of a single structural protein

can determine morphology of a complex organ. A possible

implication is that it may be relatively straightforward for the

rhabdom morphology to evolve adaptively according to

functional requirements of the visual system. If spam acts a

switch, one might expect two main types of rhabdom

morphology (Fig. 1) according to whether or not it is expressed

in the compound eye. How then do these observations fit into

the larger picture of compound eye evolution?

Spatial acuity versus absolute sensitivity

The requirement that photoreceptors should operate as

optical waveguides limits the minimum diameter of photo-

receptive structures to about 1.5 mm.(3) In compound eyes,

larger diameter rhabdoms increase absolute sensitivity

(i.e. photon catch) at the expense of spatial acuity. Thus the

grasshoppers Locusta and Valanga vary the diameter of their

fused rhabdom with a pronounced circadian rhythm, from

about 1.5mm in daytime to 5 mm at night.(4,5)

Controlling rhabdom diameter is not the only means of

modifying optics according to the light level. Four insect groups

have annular rhabdoms, where the rhabdomeres of the six

short visual fibres (R1-6) forma ring around the two long visual

fibres (R7,8; Figs. 1, 2). The isopod Ligia (Crustacea,

Malacostraca) has a similar arrangement, but without the

central rhabdomeres.(6) In the primitive insect open rhabdoms,

light is focussed on all receptors in dim conditions whereas, in

bright light, the outer ring of receptors is covered by pupil

pigment (Fig. 3).(7–10) This arrangement gives a high spatial

acuity photopic system based on the long visual fibres, and a

low acuity scotopic system based on R1-6. In higher flies

(Brachycera), the division of labour between receptors is

different, in that the short visual fibres are used at all light

levels.(11)

A possible advantageof an annular over a fused rhabdom is

thatmakesefficient useof a givenvolumeof photopigment and

photoreceptive membrane. Waveguide effects mean that light

does not travel in the central lumen, but is concentrated in the

rhabdomeres. This concentration could be beneficial because

photoreceptive membrane is metabolically costly,(12) and also

School of Life Sciences, University of Sussex, Brighton BN1 9QG, UK.

E-mail: d.osorio@sussex.ac.uk

DOI 10.1002/bies.20533

Published online in Wiley InterScience (www.interscience.wiley.com).

BioEssays 29:111–115, � 2007 Wiley Periodicals, Inc. BioEssays 29.2 111

What the papers say

Figure 1. Diagramofommatidiawith fusedandopen rhabdoms,showing radial views (left)andcrosssections through theommatidia.Terms that

are used in the text can be defined as follows: the ommatidium is the modular unit from which the compound eye is constructed. The cellular

organization of the ommatidium is highly conserved amongst the insects andmany crustaceans (Malacostraca).(26,27) Normally each ommatidium

containseightphotoreceptors.Short visual fibres (R1,6)arephotoreceptor cellswhoseaxonsproject to the first optic ganglion.Typically theycontain

a photopigment that ismost sensitive to green light (max sensitivity at about 500nm). Long visual fibres (R7,8) are photoreceptor cellswhose axons

project through the firstopticganglion.Often theycontainaphotopigment that ismost sensitive toUV light (maxsensitivityatabout360nm),but there

isconsiderablevariation. Inarthropodphotoreceptorsvisualphotopigment (rhodopsin) resides inmicrovilli,which liealongonesideof thecell to form

a rhabdomere. Thehighdensityof rhodopsin in the rhabdomeregives thisstructureahigher refractive index thansurroundingcytoplasm,andallows

it toactasanopticalwaveguide.The rhabdom is thecollectivenamefor the rhabdomereswithinanommatidium.The fused rhabdom is theancestral

form, and found inmost insect lineages.Here the rhabdomeres are fused to forma singlewaveguide, so that light focussed on the rhabdom travels

through all eight photoreceptors. In open rhabdoms the rhabdomeres do not form a single waveguide. There are many variants (Figs 2,3), but

typically R1–6 surround R7,8. The version illustrated with R1–6 having fully optically isolated rhabdomeres occurs many Diptera. Note that the

conventional nomenclature differs from that in the recent Nature paper.(2) Conventionally ‘open’ rhabdoms are contrasted with ‘fused’, whereas

the term ‘closed’ isnotused.Given that in flies there isnocleardistinctionbetween fullyopenrhabdoms,where rhabdomeresdonot touch,and those

where they do (Figs 2,3) the conventional distinction seemed logical — at least until the discovery of Spam.

Figure 2.

What the papers say

112 BioEssays 29.2

because noise due to spontaneous isomerization of the

photopigment is proportional to the quantity of pigment.(13)

An open rhabdom, associated with pooling of receptor

signals, almost inevitably sacrifices spatial acuity compared to

the narrowest possible fused rhabdom, but the extent of the

blurring depends on two factors: firstly, the degree of optical

coupling between receptors (Fig. 2), and secondly, the angular

divergence between optical axes of photoreceptors whose

outputs are combined in the optic lobe (Fig. 4). The precise

structure of the fly eye virtually eliminates these acuity losses,

and gains a six-fold increase in quantum catch.(14–17)

The performance of open rhabdom eyes is dependent on

the connectivity of the photoreceptor axons (Fig. 4). In fused

rhabdomcompound eyes, it is usual for all receptor axons from

the ommatidium to project to a single cartridge (i.e. column) of

neurons in the lamina, where they can be pooled. Some eyes

enhance sensitivity at the expense of acuity by pooling signals

via receptor axon collaterals that connect neighbouring

ommatidia, as in the scorpionfly, Panorpa (Mecoptera).(9,18,19)

This type of pooling may have been associated with the initial

appearance of open rhabdoms. Such an eye has poor

angular resolution. From this nocturnal system, acuity can be

improved by optical isolation of rhabdomeres, and by refine-

ment of their projections to the lamina, first by so-called

asymmetric pooling, and then by true neural superposition

(Figs. 1 and 3).(9,18,20,21)

Evolution of Dipteran neural

superposition eyes

Although one can readily envisage how the fly’s eye evolved,

this does not mean that the intermediate stages were

necessarily inferior. These stages probably represented

solutions to the problem of optimising the compromise

between absolute sensitivity and spatial acuity, which vary

according to lifestyle. It is therefore no surprise that open

rhabdoms have evolved repeatedly (Fig. 2), and that, within

this basic scheme, there are many variants.(7,8,21) All known

Diptera have open (as opposed to fused) rhabdoms, and all

higher flies (Brachycera) have neural superposition essentially

similar to that of Drosophila.(21,22) Thus the ‘lower’ Diptera

(Nematocera) are of most potential interest in relating the

expression of spam to morphological diversity.

Nematocerans are often relatively sluggish and/or

noctural, and rhabdoms of such flies have been described

from several families including: mosquitoes (Culicidae:

Anopheles), midges (Simulidae; Wilhelmia), fungus midges

Figure 2. A: Rhabdoms from compound eyes of various insects. In most orders, photoreceptor rhabdomeres fuse to form a single light

guide, typically 2—5mm across. Scorpionflies (Mecoptera), which are a sister taxon to the Diptera, have fused rhabdoms. Open rhabdoms

evolved independently in at least four insect groups: earwigs (Dermaptera), Heteropteran bugs (Heteroptera), a major group of beetles

(Coleoptra: Cucujiformes), which includes Tribolium, and Diptera. The extent of contact between rhabdomeres differs according to the

species, the light adaptation state, and, amongst some flies (Diptera: Culicidae, Tipulidae) and other groups location along the rhabdom

(Fig. 3).(6,7,20,21,25) A trapezoidal open rhabdom isa commoncharacter sharedby, andunique to, higher flies (Brachycera).Otherwise thesix

short visual fibres photoreceptors (R1–6) are arranged in a roughly hexagonal pattern around the long visual fibres (R7,8). In some cases

(Heteroptera, Tipula), light adaptation causes pupil pigment to block light from the outer rhabdomeres, which suggests that R1–6 are high

sensitivity low resolution system,with the long visual fibres giving high spatial acuity in bright conditions.(9) The figure is redrawn fromRef. 9,

incorporating data from other sources.(20,21)B:Convergent evolution of open (as opposed to fused) rhabdoms illustrated by a phylogeny of

insects andmalacostracan crustaceans showing the five orders that have open rhabdomsafter. A sister taxon is shown for each that has

retained the fused rhabdom. Whilst there is much variation in the morphology of open rhabdoms, I am unaware of any instance where an

open rhabdom has reverted to the fused form. The phylogeny is based on various sources.

Figure 3. Illustrations open rhabdoms from Grenacher’s

masterpiece on compound eyes.(7) Both show how rhabdo-

meres tend to converge or fuse distally, an arrangement that

benefits the trade-off between absolute and angular sensitivity

in theabsenceof neural superposition.(20)A:Anommatidiumof

a bug, the backswimmer (Notonecta, Heteroptera), in the light

adapted state, showing pigment covering the rhabdomeres of

R1–6.(9,10) B: Three ommatidia of a tipuld fly (Tipula sp;

Diptera, Nematocera).(9,12,25)

What the papers say

BioEssays 29.2 113

(Mycetophilidae: Arachnocampa), ghost midges (Chao-

boridae: Chaoborus) and craneflies (Tipulidae: Tipula;

Ptilogyna).(18,20–22) Faster flying and diurnal nematoceran

flies include the large mosquito Toxorhynchites, and the

bibionid fly, Bibio marci, whose males have a specialised high

acuity dorsal eye for finding females.(2,20,23) The underlying

connection pattern of receptor axons reflects this diversity, and

neural superpositionappears to haveevolved independently in

Brachycera, in Bibionidae and in Culicidae (Fig. 4).(16,23–25)

As we have said, it is no surprise that the nocturnal forms

have more spatial pooling, and diurnal forms have fully

developed neural superposition (Figs. 1 and 3). With respect

to the role of spam, the subtleties of rhabdom structure in

nematocerans are of interest. In particular, there is no clear

distinction between eyes where rhabdomeres do not touch

(and are termed open by Zelhof and co-workers(2)), and those

where they do (and are termed closed by Zelhof and co-

workers(2)). For example, when its eye is dark-adapted, the

rhabdomeres of the tipulid Ptilogyna fuse in the distal (outer)

part of the rhabdom, where the image is focussed, but theyare

separated proximally. Ptilogyna’s rhabdomeres become fully

separate on light adaptation (Fig. 2).(21,22) The isopod

crustacean Ligia has a similar arrangement to Ptilogyna

(Fig. 2)(6) whereas, in Anopheles (Culicidae), rhabdomeres

never separate on light adaptation.20 In the absence of

neural superposition, a fused distal rhabdom that opens

proximally allows high sensitivity with relatively high acuity

(Fig. 3).(20)

Conclusion

To conclude, several insect and crustacean lineages have

open rhabdoms, probably to enhance vision in dim light. Open

rhabdom eyes with neural superposition are derived by

refinement of rhabdom structure and of neural connections

in three dipteran lineages that moved from nocturnal to

daytime activity (Fig. 4).(9,18,21) The range of rhabdom

structures reflects the specific demands of visual acuity and

sensitivity, aswell as the insect’s phylogenetic history. The role

of Spacemaker in preventing rhabdomeres from touching one

another(2) shows how a relatively simple genetic mechanism

could underlie this adaptive radiation. However, the evidence

fromnematoceran flies suggests that continuous variation is to

Figure 4. A rangeof neural connectionpatterns beneath open rhabdoms that could account for the evolution of neural superposition from

nocturnal eyes.(9,18) Normally in fused rhabdom apposition eyes axons of all eight photoreceptors project to a single coaxial lamina

cartridge. Short visual fibre (SVF; R1–6) axons terminate in the first optic ganglion the lamina. Long visual fibre axons (LVF; R7,8, not

illustrated) project through the lamina to the second optic ganglion. LVFs may however synapse in the lamina. A: Symmetrical pooling

probably occurs in some fused rhabdom eyes such as that of the scorpionfly, Panorpa where SVF axons connect to outputs from

neighbouring ommatidia.(9,18) This pooling reduces spatial resolution, and may favour evolution of open rhabdoms. Resolution can be

improved by screening the SVFs, as in Heteroptera (Fig. 2).B:Asymmetric pooling occurs in certain nematoceran flies such asChaoborus

and Tipula. Here spatial acuity is improved because axons from the outer rhabdomeres (typically R1–6) divide to project asymmetrically to

connect to lamina cartridges that share inputs with overlapping receptive fields.(18) C: Neural superposition. A set of six SVF axons with

coincident receptive fields project to a single laminacartridge. InBrachyceran flies, the projection is to aneighbouringcartridge,(16) but it is to

the second nearest in the bibionid Bibio.(23)

What the papers say

114 BioEssays 29.2

beexpectedandnot simplyaswitch fromopen toclosed forms.

It will be interesting to see how its patterns of expression of

spam match the diversity of rhabdom forms, and diurnal

changes that occur in the nematoceran flies, and perhaps

other insect lineages.

Acknowledgments

I thank M.F. Land, R Melzer and D.-E. Nilsson for comments

and advice.

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