Many animals show intra-specific variation in body size. In some cases, there appear to be benefits of having a larger body size. For example, size is positively associatedwith improved performance,

competitiveness,andcapacitytomonopolizeresourcessuchasterritoryormatesinsomeanimals[1],andhasastrongassociationwithfemalefecundityininsects[2].Largerindividualscanalsobettercontendwith

someabiotic environmental conditions.Forexample, larger-sized individualsarebetterable tomaintain sufficientbody temperatures in colderenvironmentsandare thereforemore likely tobe foundathigher

latitudesandaltitudes, aphenomenonknownasBergmann’srule[3].However,given that large individuals requirea larger investment in termsof energyand resources,parentsarepresentedwitha trade-off

betweeninvestinginfewer,largeroffspringormore,smallerones[4,5,1].Moreover,alargerbodysizeisnotalwaysmoreadvantageous.Forexample,inhotterclimates,smallerbodysizesandlargersurfaceareato

volumeratiospromoteheatlossandcoolingbyevaporation[6].Wherethereisageneticbasistosizevariation,itmayevolvebylocaladaptation,orbyfluctuating(e.g.differentbodysizesarefavoredindifferent

yearsorseasons)orfrequency-dependent(i.e.frequency-dependentmatingsuccessorpredationofindividualsofacertainbodysize)selection.Theultimateforcesinfluencingsize-relatedfitnessintersectwiththe

underlyingproximatemechanisms thatgeneratebody size variation.Theseproximatemechanismsarevaried,butacrossdisparateanimal taxaoften involvediet and theconserved roleofnutritionalpathways,

includingtheinsulinandtargetofrapamycin(TOR)pathways[7,8],amongothermechanisms.Relationshipsbetweensizeandfitnessarethuscomplex,andbodysizedistributionsareshapedbynaturalselectionand

plasticresponsestotheenvironment.

Thebroaderproximate[7]andultimate[9]mechanismsthatshapeinsectbodysizehavebeenextensivelyreviewedelsewhere.Here,wefocusspecificallyonrecentinsightsintodriversandconsequencesof

intraspecificbodysizevariationinbees(Apoidea:Anthophila).Variationinthebodysizeofbees(Figure1)canbefoundatmanylevels:Inbothsocialandsolitaryspecies,considerablevariationisfoundwithinand

betweenpopulations,andinsocialspecies,sizevariationisalsofoundwithinandbetweencolonies[10,11].Sizevariationinbeesmayaffecttheirpollinationservices,andinsocialspeciescontributestotheirsocial

Bodysizevariationinbees:regulation,mechanisms,andrelationshiptosocialorganization

HannaChole1

SHollisWoodard2

GuyBloch1

guy.bloch@mail.huji.ac.il

1DepartmentofEcology,Evolution,andBehavior,TheAlexanderSilbermanInstituteofLifeSciences,TheHebrewUniversityofJerusalem,Jerusalem91904,Israel

2DepartmentofEntomology,UniversityofCalifornia,Riverside,Riverside,CA92521,USA

Sizepolymorphismiscommoninbees,andisdeterminedbyenvironmentalfactorssuchastemperature,broodcellsize,andthedietprovidedtodevelopinglarvae.Insocialbees,thesefactorsarefurtherinfluencedby

intricateinteractionsbetweenthequeen,workers,andthedevelopingbroodwhicheventuallydeterminethefinalsizeandcasteofdevelopinglarvae.Environmentalandsocialfactorsactinpartonjuvenilehormoneand

ecdysteroids,whicharekeyhormonal regulatorsofbodysizeandcastedetermination. In somesocialbees,bodysizevariation iscentral for socialorganizationbecause it structures reproductivedivisionof labor, task

allocationamongworkers,orboth.Atecologicalscales,bodysizealsoimpactsbee-mediatedpollinationservicesinsolitaryandsocialspeciesbyinfluencingfloralvisitationandpollinationefficacy.

CurrentOpinioninInsectScience2019,35:xx–yy

ThisreviewcomesfromathemedissueonMolecularphysiology

EditedbyXXXX

https://doi.org/10.1016/j.cois.2019.07.006

2214-5745/©2018ElsevierLtd.Allrightsreserved9PublishedbyElsevierInc.

organization.

EcologicaldriversofsizevariabilityinbeesAsinotherholometabolousinsects,beebodysizeisdeterminedduringthelarvalstage,andisstronglyinfluencedbytheenvironment.Forexample,experimentalstudieshavedemonstratedthatnestmicroclimateduringearly

developmentinfluencesadultbodysizeinthemanagedsolitaryspeciesOsmiabicornis[12]andMegachilerotundata[13].Atbroaderecologicalandevolutionaryscales,thereareadditionalassociationsbetweenabioticfactorssuchas

temperatureandbeebodysizes.Mostbeespeciesstudiedsofarshowincreasesinsizewithincreasinglatitudeoraltitude.ThispatternfollowsBergmann’srule,whichiscommonbutnotuniversalininsects,andislikelyshapedby

evolutionaryprocesses[10,14••,15,9].Seasonalchangesintemperatureanddaylengthalsodrivedifferencesininsectbodysizes,includinginbees[12,9,16–18].

Bioticfactorsalsocontributetointraspecificbodysizevariation.Beesdependoverwhelminglyonfloralresourcesforfood,andtheabundanceandcompositionoftheseresources,whichchangeacrosshabitatsandseasons,are

importantdeterminantsoflarvaldiet.Inbees,asinotherinsects,dietregulatesdevelopmentandterminalbodysize,withthegeneralrulethatsmallerlarvalprovisionsgeneratesmaller-bodiedbees(describedingreaterdetailin

SectionII.‘Proximatemechanismsunderlyingbodysizevariation’).Therearealsoconsequencesofbodysizethatinfluencehowbeesextractfoodresourcesfromtheirenvironments.Bodysizeistypicallypositivelyassociatedwithforaging

range[19]andtheamountofpollenandnectar loadthebeecancarry [20,21,22••].Therefore,smaller-sized individualsmightbemoreprevalent inareaswith fewer foodresources,whereas largersizesmaybeadvantageous in

habitatswhere there is a greater need to forage at greater distances, ormaximize individual foraging loads [23]. However, being large is not always advantageous. Larger individualsmay suffer higher parasitism rates or be

preferentiallytargetedbysomepredators,forexampleinthecaseofconopidflyhostselection[24].Thereisalsoevidencesuggestingthatsmaller-sizedbumblebeesexhibithighersurvivalundersomeconditions[22••,25].

Giventheimportanceofecologicalfactorsinshapingbodysize,itisperhapsnotsurprisingthatrecentstudieshavedemonstratedthatglobalchanges,suchashabitatdegradationandlandusechange,maybedrivingchanges

inbeebodysizedistributionstowardssmallersizes.Forexample,inastudyofagroundnestingandrenid(Andrenanasonii),Renauldetal.(2006)etal.[26]foundmoresmaller-sizedbeesinareasofgreateragriculturalintensification.

Thereisalsoevidencethatoverthelastcenturyandahalf,somebeespecieshavebecomesmaller[27]andthatlarger-bodiedbeespeciesaremorelikelytohaveexhibitdeclinesinrelativeabundance[28].Theunderlyingmechanistic

driversofthesedirectionalshiftsinbeebodysizedistributionshavenotbeenexplicitlystudied,butarelikelyrelatedtothesameaforementionedenvironmentaldriversthatgeneratefluctuationsinbodysizevariation.Forexample,in

degradedorrapidlychanginghabitatsthereareoftentrendstowardsfewerfloralresources,whichmightleadtosmallerpollenmassesprovisionedinlarvalcells,whichisdirectlyrelatedtofinalbodysizeinbees(seebelowSection

II‘Proximatemechanismsunderlyingbodysizevariation’).Thisiscorroboratedbyrecentstudiesonfloweringhabitatenhancementasameanthodforbufferingthisbodysizedecline[29•].

ProximatemechanismsunderlyingbodysizevariationSolitarybees

Mostoftheworld’sbeespeciesaresolitaryandtypicallyhavelimitedabilitytoactivelyregulatetheirnestenvironment.Likeotherholometabolousinsects,allbeesgothroughseverallarvalmolts,withthefinaloneresultingin

developmentintoapupa.Giventhatpupaedonotfeed,thefinalmassofthelastlarvalinstarusuallydeterminesadultsize.Research,mostlywithfruitflies(i.e.Drosophilamelanogaster)andmoths(Manducasexta),indicatesthatbody

sizeislargelydeterminedbygrowthrateandthedurationofsensitiveperiodsduringlarvalgrowth,althoughinsomespeciesvariationinthenumberofinstarsmayalsoplayarole.Thesekeydevelopmentalfeaturesareorchestrated

bymultipleregulatoryprocessesaffectingcellgrowthandproliferation(reviewedinRef.[9]).Beeshavebeenrelativelylittleexploredinthisregard,butgiventheoverallsimilaritybetweenmothsandflies,thereiscurrentlynoreason

Figure1Bodysizevariationinbees.

A(a)BodysizevariationinthefacultativelysocialsmallcarpenterbeeCeratinacalcarata,betweenasmallworker-likesubordinatedaughter(left)andadominantreproductivemother(right)(photocreditSandraRehan).B(b)Sizepolymorphismwithin

abumblebee(Bombusterrestris)colony.Thequeenisthelargestindividualtotheleft,andallotherbeesaresisterworkersthatmayvaryinbodymassbyupto10-fold(photocreditShaiYerushalmi).C(c)Workerforager(Left)andguard(Right)ofthe

stinglessbeeTetragonisca. (undo-pleaseleavethefullnameasprovidedforthespeciesin(a)and(b))angustula.Guardsare30%heavierthanforagers(fromRef.[30••]).

alt-text:Figure1

toassumethattheorganizationprinciplesdeterminingbodysizeinbeesaresignificantlydifferent,althoughthedetailsmayvary.

Foodamount andquality are consideredamong themost important factors controllingbody size inboth solitary and social bees [20,31–34]. In insects, diet impacts growth rate,which in turn affects instar size [9]. The

microenvironment,andspecificallythetemperatureduringlarvalgrowth,isalsoimportant[35–37];Figure2).Studieswithmothsandotherinsectshaveshownthattemperaturehasapositiveeffectongrowthrate,andontherateof

biochemicalreactions,suchasJHclearing,whichaffectsthetimeofpupation[9].Inmassprovisioningsolitaryspecies,broodsizeislimitedbythesizeofthecellandthequalityorquantityofthefoodthemotherdepositsinthecell

beforeshe lays theegg.Thismaternalcontrol isperhapsbestexemplified inrecentstudiesshowing that insolitarymasonbees (Osmiaspp.spp.),pupation is triggeredwhen larvaeconsumeallpollen in thematernally-provided

provision,ratherthanwhentheyreachacriticalweight[38••,39].Insmallcarpenterbees(genusCeratina),thefirstdaughterisfedwithlessfoodandadistinctdietcomposition,andissmaller-bodiedrelativetoherlatersisters[40••].

However,itisunknownwhetherthemotherexplicitlycontrolsthedietprovisionforthefirstcell,orwhetheritsdistinctprovisionsimplyreflectsseasonalchangesinfloralresourceavailability.Studieswithhoneybeesinwhichthe

effectofcell sizeand foodprovisionedwereuncoupledsuggest thatboth factorsare important in thisspecies,but it isyetnotclearhow informationaboutcell-sizearedetectedandprocessedtoaffect the larvaldevelopmental

program[37].

Inprogressivelyprovisioningspecies,nursingadultsdynamicallycontrolthedietofdevelopinglarvaeandcanrespondtohungersignalsfromthebrood(seebelowSection II.b.1‘Thecontrolof larvaldietbytendingadults’).

Interactionsbetweentendingadultsandlarvaehavebeenstudiedmostlyinsocialinsects(discussedbelow,SectionII.b‘Socialbees’),butsimilarmechanismsarealsofoundinsomesolitarywaspsandmayalsoapplyforsolitarybees

[41].

SocialbeesThecontroloflarvaldietbytendingadults

Insocialbees,complex interactionsbetween thequeen,workers,anddevelopingbrooddetermine larvaldietandenvironment,which in turn influencedevelopmentand final size (Figure2, right). Inprogressiveprovisioners, tendingadults

dynamicallycontrollarvaldietandfeedingschedule.Themosticonicmanifestationofdiet-relatedcontroloverdevelopmentinsocial insectsisinthecaseofroyaljellyofhoneybees(Apismellifera),whichdetermineswhethera larvadevelops intoa

(larger)queenoraworker[42].Royaljellycontainsproteinsknownas‘'MajorRoyalJellyProteins'’(MRJPs),aswellasadditionalbioactivemoleculesthatareproducedintheheadglandsofnurseworkersandaffectregulatoryprocessescontrollinglarval

development[37,43–46].Forexample,aremarkablerecentfindingisthatMRJP-3actsasanextracellularRNAbindingproteinthatfacilitatesthetransferofRNAsfromworkerstolarvae[47].Thereisalsoevidencesuggestingthatphytochemicalsand

Figure2Asummaryofthefactorsaffectingbodysizeinsolitaryandsocialbees.

Bodysizevariationdependsonenvironmentalconditionsthataffectthebroodambientcondition(e.g.temperature)andfoodamountandquality.Inbothsolitaryandsocialbees,resourceavailabilityandthesizeandpositionofthelarvalcellmayinfluenceoffspringsize.In

socialbees,acomplexinterplaybetweenthequeen,theworkers,andthebrood("‘thesocialenvironment"’)determinetheambientconditions,location,cellsize,andthedietprovisionedtothedevelopinglarva,whichtogetherdetermineitsfinalbodysizeandcastefate.Notall

arrowsareapplicabletoallspeciesandtheirrelativeimportancemayvarywithspeciesecologyandlifehistorytraits.

alt-text:Figure2

smallRNAspresentinpollen,whichisonlyfedtoworker-destinedlarvae,alsoinfluencedevelopmentandcastefateinhoneybees[48,49•].ThenineMRJPsofthehoneybeeevolvedfromasingleprogenitorgeneencodingamemberoftheancientYellow

proteinfamily.ThetwosequencedbumblebeesgenomesencodeonlyasingleMRJP,suggestingthattheexpansionofthisproteinfamilyinbeesisspecifictohoneybees[50].Thereiscurrentlynoevidenceforqualitativedifferencesbetweenregurgitate

fedtoworker-destinedversusqueen-destined larvae inbumblebees [51].However, there isevidence thatdirectcontactwith thequeen impacts larvaldevelopmentalduration inB.ombusterrestris,which is consistentwith thenotion that thequeen

transferssomecurrentlyundescribedpheromonesand/orothernutritionalorbioactivemoleculestolarvae(Shpigleretal.(2013)etal.[52••],Figure3).Interestingly,thereisevidencethatdietaryhoneybeeroyaljellycanaffectdevelopmentandbodysizein

non-Apisinsects,suchasDrosophila[43]andbumblebees(Bloch,unpublishedresults).Thissuggeststhatsomeaspectsoftheregulatoryarchitectureofbodysizedeterminationaresharedbetweentheselineages,evenifupstreamdrivers(e.g.royaljelly)

differ.

RearingenvironmentSimilartosolitarybeesandotherinsects,larvaldevelopmentandtheterminalbodysizeofsocialbeesissensitivetotheenvironment.Insocialbees,thesefactorsarefurtherinfluencedbyinteractionsbetweenthequeen,workers,anddeveloping

brood.Thesumofthesefactorsisoftenreferredtoasthe‘'socialenvironment'socialenvironment’whichalsoincludescolonyage//size.Socialbeescantightlyregulatemanyaspectsofthenestmicroenvironmentthatimpactbrooddevelopmentandbodysize

(Figure2,rightpanel).Forexample,honeybeesthermoregulatetheirneststo35 ± 0.5 °CandhavestrongcontroloverotherfactorssuchashumidityandCO2levels[42].Thelocationoflarvaeinthenesthasbeenshowntoaffectbodysizeinthebumble

beeB.ombusimpatiens[53]andinthestinglessbeeTetragoniscaangustula[54]andcanbeanimportantsourceforwithin-colonyvariationinbodysize.Larvaeinthecenterofthebroodcombarefedmorefrequently,buttheymayalsobenefitfrom

improvedthermoregulationcomparedwithbrooddevelopingatthecombperiphery(butseeKelemenandDornhaus(2018)[16]forastudysuggestingthatthermoregulationissimilarinthecenterandperipheryofcoloniesofthebumblebeeB.impatiens).

Insomebumblebees,averageworkersizeincreaseswithcolonyageinawaythatisconsistentwithincreasingfoodavailabilityorbroodcarewithcolonygrowth(Fig.3)[5556,–57,52••].Across-fosteringexperimentinB.terrestris,exchanginglarvae

betweenyoungandoldcolonies,demonstratedthattheseeffectsaredrivenbytherearingcolonyenvironment,ratherthanfactorsintheegg[52••].Thus,bodysizevariationcanoriginatefromspatialortemporalvariationinresourcedistributionand,

more generally, brood care among the larvae. In addition to the location of the larvae, this distribution can also be affected by the ratio of nurse bees to larvae, and according to the task repartitionwith other duties needed, such as foraging or

thermoregulation,dependingoncolonygrowthandgeophysicalconditions[36,35,12,37,31,16,17].

InteractionswithtendingadultsthataffectbodysizeBoththequeenandtheworkersmaycontrolthelarvaldietandqualityofcare,andthiscanbeinfluencedbyinteractionsbetweenthesepartiesandwiththebrood.Forexample,inhoneybeecolonies,groupsizeandqueenpheromonesaffectthe

nurseworkerdecisionofwhethertofeedfemalelarvaewithqueenorworkerjelly[42].Thereisalsoevidencethatlarvaeinsocialspeciescommunicatetheirneedstotendingadults.Forexample,inbumblebees,cuticularpheromonescaninformabout

Figure3Aschematicself-organizedmodelfortheincreaseinworkersizeasafunctionofcolonyageinthebumblebeeBombusterrestris..terrestris.

Themodelisbasedonempiricalevidenceshowingthatlarvaethatcontactthequeenpupateearliercomparedtolarvaethataretendedbyonlyworkers,andthatthenumberoftendingworkerspositivelyaffectsthefinalsizeofthedevelopinglarva.A.(a)Schematicgraphical

modelforhowchangesinlarvaltendingbythequeenandworkersthroughoutcolonydevelopmentcandeterminebodysize.Doubledashedline:estimatedpercentageofbroodcareprovidedbythequeen.Dottedline:numberofworkersnursingthebrood.Thebluediamonds

(workers)andcircles(gynes,toprightoftheplot),showthesizeoffemalesemerginginarepresentativecolony,andthelinearregressionmodelfortheinfluenceofcolonyageonbodysize.(basedonShpigleretal2013[52]).B.(basedonShpigleretal.[52••]).(b)Summaryofknown

interactionsbetweenthequeen,theworkers,andthedevelopinglarvae.Lettersreferstothefollowingreferences:a:Blochetal.1996etal.[65],b:Bortolottietal.2001etal.[66••],c:Pereboometal.2003etal.[67],d:SmeetsandDuchateau2001[59],e:BoerandDuchateau2006[58],f:

Alauxetal.2006[68],g:Alauxetal.2007[69],h:Shpigleretal.2013[58••],f:Alauxetal.[68],g:Alauxetal.[69],h:Shpigleretal.[52••].

alt-text:Figure3

larvalhungerstate[58••],althoughtendingworkerscanvaryintheirresponsethresholdormotivationtorespondtothissignal[59].ThevolatilehoneybeebroodpheromoneE-β-ocimenealsoappearstofunctionasahungersignal,inducingworkersto

tendbroodcellsandfeedlarvae[60••].However,theinterestsofdevelopingbroodandadultsmaydiffer.Nursingadultsallocatetheirlimitedresourcesbetweenmultipledevelopingorfuturebrood,ratherthanoptimizingfoodavailabilityforasinglelarva.

Inaddition,adultsmayact inspecificwaysthat limitthesizeofthedevelopinglarvae.AninterestingexamplecomesfromPolistespaperwasps, inwhichadultsproducemechanicalsignalsthatmodulatelarvaldevelopmentandreducepropensityto

developintolargergynes[61,62].SocialinhibitionseemstoalsobeimportantinthebumblebeeB.terrestris.Inthisspecieslarvaetendedbythequeenpupateearlier,arecommonlysmaller,andarelesslikelytodevelopintogynes,comparedwithlarvae

rearedbyonlyworkers[52••,63].Queeninhibitionismediated,atleastpartially,bydirectcontactwiththequeen,andiscompromisedinlarvaeseparatedfromherbyaqueenexcluder[52••].Ascolonygroupsizesgrowlarger,moreworkersareavailable

tocareforthelarvae,andtheprobabilitythatlarvaewillbetendedbythequeendecreases(Figure3Aa).Theincreaseinworkernumberfurtheraffectsqueenbehavior,asshereducesbroodfeedingandincreasesegglaying,whichmayfurtherdiminish

herinfluenceonlarvaldevelopmentrelativetothatofworkers[64••].Theincreaseinnumberofbroodtendingbeesfurtherleadstoanincreaseinfinalbroodsize,probablybecausemoretendingworkersprovidebettercareormorefoodtothedeveloping

larvae.Figure3BbsummarizesadditionalevidenceforsocialinteractionsaffectingthedevelopmentofB.terrestrislarvae.

EndocrineandmolecularmechanismsOurunderstandingoftheendocrineandmolecularmechanismsdeterminingbodysizeinholometabolousinsectsisbasedmostlyonexperimentswithmodelspecies,suchasManducasextaandDrosophilamelanogasterM.sextaandD.melanogaster

[9,70,71].ThesestudieshaveidentifiedthecriticalroleofhormonessuchasJuvenileHormone(JH),prothoracicotropichormone(PTTH),andecdysteroids,whichtogetherorchestratethetimingofmetamorphosis,whichinturnsdictateslarvalmassat

eachmolt.ThemassofthelastlarvalinstaratwhichJHsecretionceasesanditsenzymaticbreakdownisupregulated(termedthe"criticalweight"‘criticalweight’)isthemostcriticalimportantprocessesdeterminingthetimeofpupationandthesizeofthe

emergingadult[72].Althoughtheredoesnotappeartobeacriticalweightinsomesolitarybees[38••],itiscurrentlyunknownwhetherthisdoesexistinsocialbeespecies,andhowthismightoperateinasystemwithhighlyvariableworkersizes,suchas

bumblebees.AtleastpartoftheinfluenceoftemperatureonbodysizedeterminationoccursviaitsregulationontherateofenzymaticbreakdownofJH.BodysizeincreaseswithdecreasingtemperaturebecauseofaslowrateofJHclearance,which

inducesalongerdevelopmenttimeandlargerbodysizeatpupation[12,73].Theinsulin/insulin-likegrowthfactorsignaling(IIS)pathwayisthekeypathwayslinkingnutritiontolarvalgrowthandisthereforealsoparamountfortheregulationoflarval

growthandterminalsize.

Insolitaryandsomesocialbees,sizevariationiscontinuous,whereasintheeusocialbeespecies,queensandworkerstypicallyhavenon-overlappingsizedistributions.Castedetermination,specificallyinthehighlysocialhoneybeesandstingless

bees,providessomeofthemostremarkableevidenceforpolyphenisms, theproductionof twoormoredistinctphenotypes fromthesamegenome. In insectpolyphenism,upstreamenvironmentalconditionsare integratedbythebrain,which in turn

modulatespatternsofsecretionofdevelopmentalhormonessuchasJH,ecdysteroids,andIIS.Theresultantendocrinesignalsdirectalternativedevelopmentaltrajectoriesthatleadtotheproductionofdistinctphenotypes[74•].Thesesignalingpathways

havebeenalsoimplicatedintheregulationofcastedifferentiationinthehoneybeeApis.mellifera,thebeespeciesinwhichthemechanismsunderlyingcaste-differentiationarebeststudied[75,76].Gyne-destinedhoneybeelarvaearerearedinspecial

queencellsandarefedwithadistinctiveroyaljellydiet(seeaboveSectionII.b.1.‘Thecontroloflarvaldietbytendingadults’).Subsequently,theinsulinpathwaymediatesadiet-dependentelevationinthebiosynthesisrateandcirculatinghemolymphtitersof

JH in queen-destined larvae. JH activates the expression of JH signaling genes such as Kruppel-homolog 1 (Kr-h1) andMethoprene tolerance (Met) to regulate downstream differential gene expression patterns underlying caste determination and

differentiation[75–77].JHmayalsoaffectgeneexpressionbyregulatingepigeneticprocessessuchasDNAmethylationandexpressionofmicroRNAsthathavebeenimplicatedincastedetermination[37,44–46].Castedeterminationinthehoneybee

involvesintricateprocessesotherthanthosecontrollingbodysize,andthesetopicsarebeyondthescopeofthecurrentpaper.JHandecdysteroidtitersarealsohigherinqueen-versusworker-destinedversusworker-destinedlarvae,inthebumblebeeB.

terrestris[63,78,66••](reviewedin[79]).Bortolottietal.(2001)Ref.[79]).Bortolottietal.[66••]furtherreportedthatJHtreatmentaffectslarvalgrowthandfinalbodysizeinadose-dependentandinstar-dependentmanner,withthehigherdosesappliedto

early instars resulting in development into gynes. Little is knownabout themolecular processesunderlying castedifferentiation in bumblebees, but as in honeybees, there is evidence for caste-specific expression ofmicroRNAs, suggesting their

involvementincastedifferentiation[80•].Takentogether,studiesofinsectmetamorphosis,sizedetermination,andcastedifferentiationinbeessuggestthatalloftheseprocessesareorchestratedbyinterplaybetweenJH,ecdysteroids,andIIS.Amajor

challengeforfutureresearchistounderstandhowthefine-scaletemporalmodulationofthesesameendocrinesignalingpathways(anddownstreammolecularprocesses)regulatesallthesethreepivotalprocesses.

ConsequencesofsizevariationinbeesReproductivedivisionoflaborinsocialbees

Reproductivedivisionoflaborisahallmarkofinsectsocieties,althoughthedegreeofreproductiveskewanditsrelationtobodysizedifferssignificantlyamongbeespecieswithvaryinglevelsofsocialcomplexity.Inmost

eusocialspecies,theexceedinglyfertilequeenisalsolargerthanthelessfertileworkers[81,82].Castedifferentiationcanalsoproducemorphologicalorphysiologicalvariationthatisbeyondsimplesizevariation(andthusbeyondthe

scopeofthecurrentpaper,seeSectionII.b.4‘Endocrineandmolecularmechanisms’);thesehavebeenbestexploredinthehoneybeeApis.mellifera[83],andtoalesserextentinstinglessbees[84]andbumblebees[85].

Intherelativelysmallandsimplesocietiesofsomesweatbeesandcarpenterbees,therearenodistinctfemalecastes,butthefirstfemalestoemergecanbesmallerthanlater-emergingindividuals,andonlythelatterwill

becomethefuturereproductives[36,86,87]. InthesubsocialandfacultativelysocialsmallcarpenterbeeCeratinacalcarata, largerfemalesaremorelikelytoestablishsocialnests[88•].Thefirstdaughter insocialnests is typically

smaller,non-reproductive,andbehaveslikeaworkerbyhelpingtorearhersisters,whocanpotentiallyreproducethefollowingseason(Figure1Aa)[89].Reproductivedominanceisprimarilydeterminedviaagonisticinteractionsinthe

lesscomplexinsectsocieties,wheresizecommonlycorrelateswithphysicalstrengthforwinningaggressivecontests.Agoodmodelforstudiesondominanceinteractionsisinfoundressesassociations,whichhavebeenbest-studiedin

overwinteredPolistespaperwasps[90–92],butarealsofoundinseveralbees,suchascarpenterbees(subfamilyXylocopini;e.g.,Xylocopasulcatipes,Xylocopapubescens,Ceratina.calcarata),andsweatbees(familyHalictidae;e.g.,Dialictus

lineatulus).Dominancehierarchiesarealsoimportantinqueenlessbumblebeeworkers[93].Inthesesystems,largerfemalesaremorelikelytoreproduceandlesslikelytoforage[40••,94,95].

DivisionoflaboramongstworkersinsocialspeciesMorphologicalpolymorphismispivotaltotheorganizationofworkerdivisionoflabor(knownas'alloethism'‘alloethism’)inmanyhighlysocialspecies[81].Althoughbeststudiedinthemorphologicallydistinctworkercastesof

antsandtermites,recentstudiesreportasimilarmorphologically-specializedsoldiersub-casteinseveralgeneraofstinglessbees(Figure1C;Supp.TableS1)c;SupplementaryTableS1)[30••,54].Othersocialbeespecieshaveoverall

morphologicallysimilarworkersthatvaryprimarilyinbodysize.Bumblebeesfollowthispattern,andshowthemostnotablebodysizepolymorphismamongworkers,withuptotenfoldvariabilityinmassinB.ombusterrestrisandB.

lucorum[96]andninefoldinB.impatiens[97],(Figure1B;Supp.TableS1b;SupplementaryTableS1).Largebumblebeeworkersaremorelikelytoforage,fan,andguard,thansmallerbees,whichtypicallyperformin-nestactivitiessuchas

broodtending[21,96,98–102].Larger-bodiedworkersmayalsoforageatanearlierage[98,99,102,103].Even incoloniesartificiallycomposedofonlymediumsizeworkers,andthusshowing littlesizevariability, foragerswerestill

significantlylargerthannurses,indicatingthatevensmalldifferencesinsizecanaffectthepropensitytoundertakecertaintasks[HollandandBloch,BEinrevision].Thereisevidencesuggestingthateveninhoneybees,inwhichthe

divisionoflaborrelatestoage[82],thelittlesizevariabilityamongtheworkersstillaffectstheageatwhichworkersswitchfromin-nesttoforagingactivities[104].Newgenomicstudiesinbumblebeesarebeginningtoshedlighton

underlyingdriversofthephysiologicalandbehavioraldifferencesassociatedwithworkersizepolymorphism.Forexample,thereisevidencethatsmallandlargebeesdifferinthepatternsofgeneexpression[105]andRNAediting

[106].

Beesofcertainbodysizesmaybebetterinperformingsometasks,whichcanimprovedivisionoflaborandoverallcolonyperformance.Inbumblebees,largeworkersaremoreefficientinbringingpollenandnectarbackto

thecolony[21,22••,100],andalsoappeartohavesuperiorsensorycapacities.Theyhavemoreantennalsensillaandarebetterindetectingfloralodors[107].Theircompoundeyesarealsolarger,containmoreommatidiawithalarger

facet,andhave improvedvisualdiscrimination [108].Theirocelliarealso larger,and theycan flyunder lower light intensitiescompared tosmallerworkers [109].The largereyesandocellimayalsocontribute to their stronger

phototacticresponse[110].Largebumblebeeworkersarealsobetterinsomelearningparadigms[97,111,112],havemorecircadianclockcellsintheirbrains[113],andexhibitstrongercircadianrhythms[102].Thelattermaybe

importantforsun-compassorientationandtimingvisitstoflowersattheirpeaknectarproduction.

SizepolymorphismasastrategyforoptimalresourceallocationInspiteoftheevidenceabovesuggestingthatundersomeconditionssmallbeesoutperformlargerones,theprimaryvalueofsmallerindividualsmayactuallybethereducedcostoftheirproduction.Thetrade-offbetween

offspringsizeandnumberisacommonandwell-studiedlife-historyoptimizationstrategy[4,5],andthereisevidencethatsolitarybeescanadjusttheinvestmentinoffspringtooptimizetheirfitness[39].Abumblebeecolonycan

theoretically substitute investment ina fewhighlycapable largeworkerswitha largernumberof smaller,albeit lesscompetent, individuals,whosegreaternumbersprovide insuranceagainst the lossofworkersdue todisease,

predation,orotherthreats.Resourceinvestmentstrategyisespeciallycrucialduringcolonyfoundation,whenthequeenisalone,herresourcesarestrictlylimited,andwronginvestmentdecisionscanbefatal.Ifsmallerworkersare

moreresilientagainststarvation,assuggestedbyCouvillon&Dornhaus(2010)andDornhaus[25],thentheirproductioninsuresagainstthehighresourceunpredictabilityduringcolonyfoundation.Thisearly-stagebethedgingstrategy

mayhelp inachieving the laterproductionof largerworkers,males,andgynes. Inaddition, small individuals should take less time todevelopandallow thequeen to switchearlier to investment inegg-laying rather thanbrood

provisioning[64••].Theevidencethatinmanyannualspeciesofbeesthefirstbroodofworkersiscomposedofsmallerindividualsisconsistentwiththisidea[20,32,40••,52••,88•].

SizepolymorphismandpollinationservicesBodysizemayenablebetterexploitationofavailablefloralresources.Bothacross[19]andwithin[109,114]beespecies,bodysize ispositivelyrelatedtoforagingrange,andisalsorelatedtothecapacitytocarrygreater

amountsofpollenandnectar[22••].Sizecanalsoaffectthetemporalnicheofbeesbecauseitaffectthetemperaturesinwhichbeescanbeactive,andthesizeofthecompoundeyescanaffecttheminimallightintensityandtimeof

dayunderwhichtheycanforagesuccessfully[115•].Someflowersarebetterhandledbybeesofaparticularbodysize.Forexample,tonguelengthaccountsforsomeofthevariationinflowersvisitedbylargeandsmallbumblebees,

andimpactstheefficiencyofforaginginlongversusshortcorollaflowers[11,116].However,tonguelengthsdonotnecessarilyscalewithoverallbodysizewithinbeespecies[117].Thegreaterphysicalstrengthoflargebeesmakes

thembetterinhandlingsomeflowers.Forexample,in‘tripping’,theinsect'slegsandthoraxexposetheanthersandstigmaoftheflower[10].Inbuzzpollination,beesholdaflower'’santhersandvibrateatacertainfrequencyinorder

toextractthepollen[118•],andheretoo,bodysizeappearstobeimportantinrespecttoamplitude[119•]orfrequency[120•].Thereisalsoevidencethatsomeflowersarebetterhandledbysmallbees[121].Insomestinglessbees,

suchasMeliponaquadrifasciata[20]andM.eliponaflavolineata[122],smallerforagershavelargercorbiculaerelativetotheirbodysizeandareabletocarryheavierloadsofpollenperunitofbodycomparedwithlargerindividuals[20].In

socialbees,size-variableindividualsofthesamespeciescanpollinateabroaderrangeofflowerspecieswithinthesamehabitat[10,11,123],makingthemkeystonepollinatorsinmanypollinationnetworks.

SummaryandfuturedirectionsBodysizevariationiscommoninbees,andcanbefunctionallysignificantforbothsolitaryandsocialspecies.Giventhesometimesconflictinganddynamiccostsandbenefitsofbodysize,itisnotsurprisingthatbodysizeisnot

alwayspositivelycorrelatedwithhighfitnessinbees[124–128].Asinotheranimals,itisassumedthattheinvolvementofmultiplefactorsonbodysizeleadstostabilizingselection,whereselectivepressuresandtrade-offsfavoring

largerversussmallersizesarebalancedwithinpopulations [1]. Insocialbeessuchasbumblebees,bodysizepolymorphismmaybe important forsocialorganizationbecause it influences twoof themost importantorganization

principlesofinsectsocieties:reproductivedivisionoflaborbetweenqueensandworkers,andvariationintaskperformanceamongworkers.Thus,notsurprisingly,insomesocialinsectstheregulationofbodysizeisunderstrong

social influence, inways thatwe are only beginning to understand. In both solitary and social bees, there are significantmaternal influences on brood development through control of diet composition, the brood cell, and the

microenvironment(Figure2),),andalsofeedingrateintheprogressively-provisioningspecies.Thesituationismorecomplexinsocialbees,inwhichbrooddevelopmentisinfluencedbyintricateinteractionsbetweenthebrood,the

motherqueen,andtheworkers.Animportantvenueforfutureresearchistounderstandhowthedifferentpartiesacttomeettheirinterests,specificallyincasesofconflictrelatedtobodysize.Tounderstandthesecomplexsystems,

wewillneedtobetterunderstandcommunicationbetweentheseinteractingparties,includingthechemicalandfunctionalcharacterizationofhungersignals,andtheways,inwhichtendingadultsresponsedtothem.Muchresearchis

alsoneededtoidentifythespecificnutrientsorbioactivemoleculesthatplayaroleinsizedetermination.Identificationofthesesignalswillallowustounderstandhownutritionalandsocialfactorsactonthemolecularpathwaysthat

regulatelarvaldevelopmentandultimatebodysize.Fromabroaderevolutionaryperspective,comparativestudiesonthesepathwaysacrossdifferentbeelineagesandacrosslevelsofsocialcomplexitywillshedlightonhowbodysize

evolvesasafunctionofsociality,whichisanoverlookedaspectoftheevolutionofinsectsociality.

ReferencesandrecommendedreadingPapersofparticularinterest,publishedwithintheperiodofreview,havebeenhighlightedas:

•ofspecialinterest

••ofoutstandinginterest

AcknowledgementsThisworkwassupportedbyresearchgrantsfromtheUS–IsraelBinationalScienceFoundation(BSFBSF;toG.B.andS.H.W.),theUS–IsraelBinationalAgriculturalResearchandDevelopmentFund(BARDBARD;

toG.B.andS.H.W.),TheNationalAcademiesKeckFuturesInitiative(NAKFI;toG.B),andUSDAUSDANIFA(toS.H.W.).

AppendixA.SupplementarydataSupplementarymaterialrelatedtothisarticlecanbefound,intheonlineversion,atdoi:https://doi.org/10.1016/j.cois.2019.07.006.

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QueriesandAnswersQuery:Theauthornameshavebeentaggedasgivennamesandsurnames(surnamesarehighlightedintealcolor).Pleaseconfirmiftheyhavebeenidentifiedcorrectly.Answer:CholeWoodardBloch

Query:Pleasecheckwhetherthedesignatedcorrespondingauthoriscorrect,andamendifnecessary.Answer:GuyBlochisthecorrespondingauthor,hise-mailiscorrect

Query:Fullnames(includingfullfirstname,noinitials)aremandatoryforallauthors.Pleaseprovidethefullfirstnamefortheauthor‘SHollisWoodard’.Answer:SarahHollisWoodard

Query:Pleasecheckthee-mailaddressforthecorrespondingauthorthathasbeenaddedhere,andcorrectifnecessary.Answer:Itiscorrect

Query:Highlightsmustbeprovidedas3–5bulletpoints,eachbulletpointhavingamaximumof125characters.Pleaserephrasethefollowinghighlightsorprovidenewhighlights.Answer:Insocialbees,larvaldevelopmentandfinalsizearealsodeterminedbyintricateinteractionsbetweenthequeen,workers,andbrood

453–462.

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128.J.Alcock,L.W.SimmonsandM.Beveridge,DoesvariationinfemalebodysizeaffectnestingsuccessinDawson’sburrowingbee,AmegilladawsoniAmegilladawsoni(Apidae:Anthophorini)?,EcolEntomol31,2006,

352–357.

AppendixA.SupplementarydataThefollowingisSupplementarydatatothisarticle:

MultimediaComponent1

Highlights

• Bodysizevariationiscommoninsolitaryandsocialbees.

• Insolitaryspecies,larvalgrowthiscontrolledbydietandtheabioticenvironment.

• Insocialbees,larvaldevelopmentandfinalsizeisarealsodeterminedbyintricateinteractionsbetweenthequeen,workers,andthedevelopingbrood.

• Bodysizestructuresdivisionoflaborinsomebees.

• Bodysizemayinfluenceforaging,floralhandling,andpollinationservicesatecosystemscales.

Query:Thischeckthe4thhighlightpointforclarityandamendifnecessary.Answer:Bodysizeinfluencesdivisionoflaborinsomesocialbees.

Query:Pleasesupplyashorttitleof80charactersorfewer(includingspaces)andthatdoesnotcontainanyabbreviations.Answer:Bodysizevariationinbees

Query:Pleaseprovidetheexpansionforgenusname‘B.lucorum’.Answer:weexpandthefirstB.toBombus,itisclearthatthefollowingonesareofthesamegenus

Query:Asperjournalstylesheet’ConflictofIntereststatement’shouldbepublishedbothinPDFandScienceDirect.KindlydownloadandfilltheCOIformfromhttps://www.elsevier.com/declaration-of-competing-interestsandsubmititalongwiththecorrections.Answer:Done.Theauthorsdeclarethattheyhavenoknowncompetingfinancialinterestsorpersonalrelationshipsthatcouldhaveappearedtoinfluencetheworkreportedinthispaper.Attachments:declaration-of-competing-interestsGB.docx

Query:Havewecorrectlyinterpretedthefollowingfundingsource(s)andcountrynamesyoucitedinyourarticle:BSF,BARD,NationalAcademiesKeckFuturesInitiative,USDA?Answer:Yes

Query:PleaseprovidethevolumenumberandpagerangeforthebibliographyinRefs.29•,70,81,86,110,119•,120•?Answer:Ref29:inpress?Ref70:Pages207-218inL.B.Martin,C.K.Ghalambor,andH.A.Woods,eds.Intergrativeorganismalbiology.Wiley,Hoboken,NJ.Ref81562pages.Ref86:volume46pages359-423.Ref110:bioRxiv.Ref119:2019inpress.Ref120:9(8):4875-4887.

Query:PleasechecktheeditsmadeinRef.103andamendifnecessary.Answer:Fine