insect juvenile hormone: from “status quo” to high society · bumble bees, jh integrates...

157

Braz J Med Biol Res 33(2) 2000

Insect juvenile hormoneBrazilian Journal of Medical and Biological Research (2000) 33: 157-177ISSN 0100-879X

Insect juvenile hormone: from“status quo” to high society

Departamento de Biologia, Faculdade de Filosofia,Ciências e Letras de Ribeirão Preto, Universidade de São Paulo,Ribeirão Preto, SP, Brasil

K. Hartfelder

Abstract

Juvenile hormone (JH) exerts pleiotropic functions during insect lifecycles. The regulation of JH biosynthesis by neuropeptides and bio-genic amines, as well as the transport of JH by specific bindingproteins is now well understood. In contrast, comprehending its modeof action on target organs is still hampered by the difficulties inisolating specific receptors. In concert with ecdysteroids, JH orches-trates molting and metamorphosis, and its modulatory function inmolting processes has gained it the attribute �status quo� hormone.Whereas the metamorphic role of JH appears to have been widelyconserved, its role in reproduction has been subject to many modifica-tions. In many species, JH stimulates vitellogenin synthesis anduptake. In mosquitoes, however, this function has been transferred toecdysteroids, and JH primes the ecdysteroid response of developingfollicles. As reproduction includes a variety of specific behaviors,including migration and diapause, JH has come to function as a masterregulator in insect reproduction. The peak of pleiotropy was definitelyreached in insects exhibiting facultative polymorphisms. In wing-dimorphic crickets, differential activation of JH esterase determineswing length. The evolution of sociality in Isoptera and Hymenopterahas also extensively relied on JH. In primitively social wasps andbumble bees, JH integrates dominance position with reproductivestatus. In highly social insects, such as the honey bee, JH has lost itsgonadotropic role and now regulates division of labor in the workercaste. Its metamorphic role has been extensively explored in themorphological differentiation of queens and workers, and in thegeneration of worker polymorphism, such as observed in ants.

CorrespondenceK. Hartfelder

Departamento de Biologia

FFCLRP, USP

Av. Bandeirantes, 3900

14040-901 Ribeirão Preto, SP

Brasil

E-mail: [email protected]

Presented at the XIII Annual Meeting

of the Federação de Sociedades de

Biologia Experimental, Caxambu, MG,

Brasil, August 26-29, 1998.

Research supported in part by a

DFG grant (Ha 1625/3-1) and a

CAPES-DAAD fellowship.

Publication supported by FAPESP.

Received July 7, 1999

Accepted November 19, 1999

Key words· Juvenile hormone· Corpora allata· Metamorphosis· Insect reproduction· Polymorphism· Honey bee

Introduction

Few hormones have puzzled and fasci-nated entomologists more than juvenile hor-mone (JH). This is obviously due to thehormone�s pleiotropic functions, from a) or-chestrating metamorphosis in concert withthe molt-inducing ecdysteroid hormones, b)

regulating female fertility by stimulatingvitellogenin synthesis in the fat body and itsuptake by the growing oocytes, up to the c)generation of sophisticated polymorphismsin aphids and social insects. In this sense, JHis clearly a pleiotropic master hormone ofinsects which governs most aspects of theirintegration with the ecosystem and affects

158


K. Hartfelder

decisive life history parameters during theirentire life cycles. JH has been considered tobe an exclusive insect hormone and thus hasattracted much attention also in plant andgrain protection-oriented research. Recentstudies, however, provide evidence that ma-rine annelids can detect compounds withjuvenoid activity and can use these as meta-morphosis signals (1). In crustaceans, the JHprecursor methyl farnesoate, which is pro-duced in the mandibular organ, assumes manyroles attributed to JH in insects (2). If com-parative endocrinology and neuroendocri-nology is understood as being more than justthe compilation of endocrine physiology ofnon-mammalian animals, i.e., of approxi-mately 99% of the animal kingdom, then acloser look at such master regulators shouldcontribute to our understanding of the evolu-tion of hormones and of their receptors.Furthermore, we may come to a broadercomprehension of how each class of animalshas dealt during its evolution with such basicproblems as homeostasis, as well as coordi-nated development and reproduction in vari-able environmental conditions.

The purpose of the present review is not togo into all the details and intricacies of JHfunction(s). Excellent recent reviews (3,4) havecovered this field extensively. Instead, the in-tention of this review is to present, as briefly aspossible, the basic paradigms of this hormone�sfunction and physiology in insects, and toconcentrate more on its wide-ranging ecologi-cal implications, i.e., to show how profoundlyit can shape and even modify life histories.The most dramatic changes in life historiescan be encountered in social insects, wheremost individuals of a colony bestow most oftheir reproductive potential upon a dominantqueen (in Hymenoptera) or a royal pair (intermites). Emerging views from studies onsocial insects now clearly indicate that JHalso is deeply entrenched in practically allaspects of social life, affecting the genera-tion not only of different morphs, but also ofneural and resulting behavioral plasticity.

Juvenile hormones: chemistry,biosynthesis, and modes of action

By grafting different life stages of theblood-sucking bug, Rhodnius prolixus,Wigglesworth (see Refs. 3,4) discovered in1936 that JH has two major roles in aninsect�s physiology and life history, namelyto prevent metamorphosis and to regulatereproduction. Its function to prevent the pre-cocious metamorphosis of an insect larvainto an adult when passing through frequentmolts, gained it the title �status quo� hor-mone. The small quantities of JH presenteither in the site of synthesis, the corporaallata (CA) in the retrocerebral endocrinecomplex, or circulating in insect hemolymph,however, considerably delayed the elucida-tion of the chemical structure of JH.

The serendipitious discovery of a copi-ous source of JH in abdomens of male satur-niid silkmoths, Hyalophora cecropia madepossible the chemical identification of JH(then designated as JH I) as a sesquiterpenoidepoxide methyl ester. Subsequently, a smallnumber of closely related compounds wasisolated and identified (for structures, seeRef. 3). JH I and occasionally also JH II areprincipally found in lepidopterans, whereasthe JH homolog detected throughout all in-sect orders is JH III, the trimethyl homolog.JH 0, the triethyl homolog, has only beenfound in lepidopteran embryos. JH acids, thenon-methylated derivatives of JH, also con-stitute a relevant fraction of JH compoundscirculating in hemolymph of moths and mayhave hormonal functions that cannot be per-formed by JH itself. The cyclorraphan dipter-ans and some hemipterans possess a specialtype of JH, namely a bisepoxide version ofJH III. A new class of juvenile hormones, thehydroxy juvenile hormones, have recentlybeen identified as biosynthesis products oflocust corpora allata (5), apparently as by-products of JH III synthesis.

The practically exclusive sites of JH syn-thesis are the CA, a distinct pair of glands in

159


Insect juvenile hormone

the retrocerebral complex (Figure 1) attachedto the brain via two or three pairs of nerves.De novo synthesis of JH starts from acetylCoA and/or propionyl CoA subunits andfollows the mevalonate pathway to farnesoicacid (for a review of JH biosynthesis, seeRef. 6). The last two steps are the methylesterformation, leading to methyl farnesoate, andthe introduction of an epoxide group in posi-tion 10,11 of the carbon skeleton. Tobe andPratt (7) took advantage of the exclusiveutilization of S-adenosyl-methionine as do-nor of the methyl group in the esterificationstep in JH biosynthesis and developed ahighly sensitive radiochemical in vitro assayfor the determination of rates of JH synthe-sis. This assay was crucial to the elucidationof regulatory pathways and for the identifi-cation of neurohormones stimulating or in-hibiting CA activity.

An inhibitory regulation of the CA viathe descending nerves has been proposedseveral decades ago, but the principal play-ers, the allatostatic neuropeptides, have only

been identified ten years ago in the vivipa-rous cockroach, Diploptera punctata. TheDiploptera allatostatins are a family ofamidated peptides with a conserved C-ter-minus (FGLamide), which differ from knownpeptides in other organisms. TheseFGLamides are a widespread class of pep-tides in insects and include the allatostatinsof cockroaches and crickets, as well as struc-turally related peptides of locusts and blow-flies. Regulation of JH synthesis appears tobe only one of the many functions of thesepeptides. As expected, allatostatin immu-noreactive neurons could be detected in lat-eral neurosecretory cells innervating the CAof D. punctata (8), but were also found inother regions throughout the central nervoussystem, including thoracic motoneurons andnerves innervating the gut. In the tobaccohornworm, Manduca sexta, they appear tohave neuromodulatory and myotropic func-tions, but do not exhibit allatoregulatory ac-tivity. The authentic allatostatin of Manducasexta, however, does not resemble the

brain

NCC I

CC

aorta

CA

NCC II

NCC III

SEG

esophagus

PG

Figure 1 - The retrocerebral endocrine complex of insects as exemplified by the honey bee. SEM micrograph (left) of the corpora cardiaca and thejuvenile hormone-producing corpora allata. The NCC is seen to cross the corpora cardiaca which are tightly connected to the aorta on top of theesophagus. Schematic drawing (right) of the brain and associated endocrine system of a honey bee pupa. Three pairs of nerves descend from differentbrain regions forming the brain-corpora cardiaca-corpora allata neuroendocrine axis. CA, Corpora allata; CC, corpora cardiaca; NCC I-III, nervi corporiscardiaci I-III, respectively; PG, string of prothoracic gland cells; SEG, subesophageal ganglion.

160


K. Hartfelder

Diploptera allatostatins. The only peptidestimulating JH synthesis has been identifiedin M. sexta (9). It stimulates JH synthesis inCA of adults only, and appears to be re-stricted to lepidopterans in its activity.

The mode of action of allatostatins onCA cells is principally mediated via calciumsignalling involving protein kinase C (10).The enzymes affected by allatostatins ap-pear to be situated at both ends of the biosyn-thesis pathway. Sutherland and Feyereisen(11) propose an inhibitory action on thetransfer of the initial C2 units from mito-chondria to the cytoplasm, but, interestingly,not on the much speculated HMG-CoA re-ductase, which is a rate-limiting enzyme invertebrate steroid synthesis. In some insects,such as D. punctata, allatostatins also affectthe terminal steps of JH biosynthesis in theCA (12). In addition to these specific neu-ropeptides, some biogenic amines, octopa-mine, serotonin, and dopamine, also turnedout to be potent modulators of CA activity,apparently being coupled to cAMP as sec-ond messenger system (13,14). Their activ-ity, however, strongly varies between spe-cies, and even between different develop-mental stages in the same species.

Being highly lipophilic, the insect JHsare transported from their site of synthesis totarget tissues via specific high affinity bind-ing proteins that aid their transport in thehemolymph. Most insects use lipophorins orhexameric proteins (Orthoptera) as high-molecular-weight high-affinity JH-transportproteins, except for the lepidopterans whichhave specific low-molecular-weight JH car-riers (15). Detailed simultaneous analyses ofJH titer and of JH-binding proteins in hemo-lymph revealed that practically all JH is boundto these carriers, which appear to be alwayspresent in excess. In conjunction with theirtransport role, these proteins also protect JHfrom degradation by general esterases. Meta-bolic inactivation of JH occurs by two majorpathways, by specific JH esterases in thehemolymph and by tissue bound epoxide

hydrolases. JH esterases clear JH from thehemolymph during critical developmentalphases (16), apparently acting in synergismwith JH-binding proteins.

Crucial to the isolation and identificationof these JH-binding and -metabolizing pro-teins was the synthesis of highly specific JHanalogs for photoaffinity labeling (17). Thisstrategy was also employed in the search forthe most interesting of all JH-binding pro-teins, the JH receptor(s). Based on the chem-ical nature of JH and on analogy to ecdyster-oids, most research concentrated on nuclearreceptors. The most promising candidate,however, a 29-kDa protein isolated fromlarval epidermis of M. sexta, has since turnedout to be artifactual. Putative JH receptorsisolated from the cockroach Leucophaeamaderae and from Locusta migratoria alsoappear to be cellular JH-binding proteins(18), possibly involved in the transport ofthis hydrophobic ligand to its actual recep-tor. These proteins are present far in excessof the 104 copies/cell estimated for steroidand thyroid hormone receptors. Due to thestructural similarity of JH to retinoids, ho-mologs of RXR have been proposed as pos-sible candidates for a JH receptor. The RXRhomolog of insects, however, turned out tobe the product of the ultraspiracle gene,which is the heterodimer partner of the ecdy-sone receptor.

In addition to a nuclear JH receptor, theaction of JH on a Na+/K+-ATPase in theovary of Locusta migratoria has stimulatedresearch on a JH-binding protein in the mem-brane of the follicular epithelium (19), whichhas been proposed as a putative membranereceptor for JH. So, the somewhat enigmaticsituation is whether the elusive insect-JHreceptor not only exists as different isoforms,just as the ecdysone receptor, but possiblyalso as distinct nuclear and transmembranemoieties. Even more than in the search forthe ecdysone receptor most attempts to iden-tify the JH receptor have been plagued by thepropensity of JH to bind nonspecifically to

161



many substrates and by the apparent abun-dance and variety of specific JH-bindingproteins. Taking a genetic approach, such asthe methoprene selection screen in Drosoph-ila, has led to the identification of themethoprene-tolerant (Met) mutation. Clon-ing and sequencing of the Met(+)-geneshowed homology to the basic helix-loop-helix-PAS family of transcription factors,implicating Met protein in a gene regulatoryfunction of JH (20). Met(+) could now belinked to the ecdysone-inducible Broad-Com-plex transcription factors (21), suggestingthat it is an early downstream element in theJH response cascade. Looking for gene prod-ucts that interact with Met(+) may turn out tobe an interesting alternative in the searchstrategies for the JH receptor.

Juvenile hormone: the “status quo”factor in molting and metamorphosis

During larval development, the presenceof JH in the hemolymph prevents an insectfrom precociously turning into an adult, thusexplaining the nomenclature �juvenile hor-mone� and �status quo� factor. Early modelsproposed a gradual decline of JH titers asso-ciated with cyclic surges of ecdysone, themolt-inducing hormone. Actual quantifica-tions of JH and ecdysteroid titers, however,revealed a much more complicated and intri-cate regulation of the premetamorphic andmetamorphic molts, particularly in holo-metabolous insects which have to passthrough a complete reorganization of bodystructure during the pupal stage. Meticuloustiter analyses, together with surgical experi-ments and application of synthetic hormones,were carried out mainly on the tobacco horn-worm, Manduca sexta, making this moth thepreferred model system for studies on insectmetamorphosis. In each larval stage, a JH-sensitive period was detected which usuallyoccurs concomitantly with the molt-induc-ing ecdysone peak. In the last (fifth) larvalinstar of M. sexta, two such JH-sensitive

periods could be found, together with markeddifferences in hormone titer profiles whencompared to the earlier instars (for review,see Ref. 3). In the fifth instar, the JH titerdeclines when the larva has reached a criti-cal weight. This decline stimulates the lat-eral neurosecretory cells of the brain to re-lease prothoracicotropic hormone (PTTH),which in turn elicits ecdysone synthesis andrelease from the prothoracic glands. Thus,for the first time in the life cycle, a surge ofecdysone occurs in the absence of JH. Thissurge of ecdysone has been termed the �com-mitment� peak since it has a profound effecton the larval epidermis, essentially erasingthe program of larval cuticle synthesis. Twodays later, another surge of PTTH, now ac-companied by a high JH titer, results in amuch higher ecdysone peak, which elicitsthe next molt. The presence of JH during theJH-sensitive period early in this second ec-dysone peak ensures that the epidermis syn-thesizes a pupal cuticle and not precociouslyan adult cuticle. Thus, JH preserves the �sta-tus quo� in terms of the program triggered bythe ecdysteroid peak, and it does so, appar-ently universally, in all insects. The onlydifference between hemimetabolous and hol-ometabolous insects seems to lie in the com-plexity of interactions between the molt-inducing ecdysteroids, in particular 20-hydroxyecdysone (20E), and the molt-di-recting JH.

As stated above, JH is a pleiotropic mas-ter regulator acting both at the central levelin the neuroendocrine axis and on peripheraltarget tissues like the epidermis and fat body.At the central level, JH inhibits the release ofthe ecdysiotropic brain hormone PTTH earlyin the fifth larval instar of M. sexta, but thentwo days later it looses this repressor effect.It is not yet clear how JH exerts, and espe-cially reverses, its effect on the PTTH-pro-ducing lateral neurosecretory cells. Appar-ently, the actual regulatory pathways withinthe neuroendocrine axis are not only com-plex and distinct within different life cycle

162


K. Hartfelder

stages of a species, but may also be quitedifferent between insect orders. Whereas inlepidopterans JH affects prothoracic glandactivity primarily through the PTTH path-way (22), incubation of isolated prothoracicglands of honey bee larvae in the presence ofthe JH analog methoprene directly stimulat-ed ecdysteroid production in vitro (23).

In-depth studies on the premetamorphicand metamorphic effects of JH on peripheraltarget tissues have mainly focussed on thelarval epidermis and prolegs of M. sexta.The model of coordinated switches from alarval to a pupal and then to an imaginalprogram of cuticle synthesis, proposed frommacroscopic analyses of cuticle types pro-duced during the molting cycles, however,has not yet been validated at the molecularlevel. Temporal expression patterns of indi-vidual genes coding for different larval cu-ticle proteins of M. sexta, as well as forpigmentation factors (larval insecticyanin andthe melanization-triggering enzyme dopade-carboxylase) were studied extensively (seeRef. 3). These genes exhibit many appar-ently idiosyncratic temporal expression pat-terns, suggesting that JH and probably evenmore directly ecdysteroids interact or com-pete for binding sites in the upstream controlregion of the respective genes. A putativemodel for the interaction of 20E and JHbased on studies of dopadecarboxylase geneexpression proposes that JH in conjunctionwith 20E induces a long-lived positive tran-scription factor A, whereas 20E alone in-duces a short-lived negative-effect factor (3).In this scenario, degradation of the 20E-induced factor is expected to result in a burstof dopadecarboxylase expression whichshould last until the JH/20E-coregulated fac-tor is also degraded.

Whereas most of these studies on JH-regulated gene expression were looking attarget genes at the end of a JH-induced re-sponse cascade, a more recent line of re-search is trying to decipher the putative ele-ments within such a cascade. Just as the

Broad-Complex mentioned above, the ex-pression pattern of a M. sexta homolog of the20E-induced transcription factor E75A ofDrosophila melanogaster exhibited respon-siveness also to JH (24). Drosophila-E75encodes an orphan member of the nuclearhormone receptor superfamily. In Manduca,the rapid, transient induction of E75 mRNAby 20E turned out to be much attenuated inepidermis of fourth-instar larvae, which typi-cally have a high endogenous JH titer. Thesefindings show for the first time that JH playsa role in 20E-induced early gene expressionand suggest that the higher levels of E75Amay be required for maintenance of larvalcommitment of this epidermis. It is not yetclear whether JH acts directly on the E75gene to influence its transcription rate, orwhether it affects the stability of E75 mRNA.

Evidence for a modulatory role of JH inthe expression of 20E-regulated genes comesfrom studies on larval storage proteins syn-thesized in the fat body of the cabbage looper,Trichoplusia ni, strongly supporting the ideathat JH affects both transcription rates andmRNA stability. With the exception ofarylphorin, most of the larval storage pro-teins of T. ni turned out to be regulated(suppressed) by JH at the transcriptionallevel. DNase-I hypersensitive site mappingsuggested conformational changes in a re-gion 50-100 bp 5� upstream of the TATAbox (25). Nuclear run-on assays presentedevidence that poly(A) tail length of storageprotein mRNAs was modulated by exposureto JH. These storage proteins belong to theclass of hexamerins which are synthesizedby the larval fat body. They serve as reservesfor the reconstruction processes occurringduring the pupal phase. Additionally, theymay serve as metabolic reserves for insectsthat are not feeding in the adult stages.

The gonadotropic roles of juvenilehormone

JH acts on so many aspects of insect

163



reproduction that it can justly be called amaster regulator of the �female reproductionsyndrome�. In addition to its central role inoogenesis, JH affects dispersal and flightactivity, calling behavior, postcopulatorychanges in female behavior, and ovipositionbehavior. The diversity of life history strate-gies in insects and a myriad of adaptationsthat enabled them to colonize practicallyevery terrestrial habitat, however, generateda plethora of twists and turns that precludethe proposal of a unifying JH hypothesis forhormonal regulation of insect reproduction.Quite different from the more conserved roleof JH as a modulator of ecdysteroid action inlarval-pupal development, the roles of JH inadult insects are, at first sight, rather confus-ing. However, once the roles and implica-tions of JH action in the different aspects ofthe reproduction syndrome are carefullyevaluated with respect to timing of repro-duction and trade-offs between different re-productive strategies, some general principlesmay emerge. In many aspects, the role of JHin insect reproduction - schematically com-piled in Figure 2 - could, therefore, be con-sidered as that of a conserved integratorwhich provides a platform for flexibility inlife history strategies.

Vitellogenesis

The gonadotropic role of JH was estab-lished by ingenuous transplantation andligation experiments carried out by Wiggles-worth on females and males of the blood-sucking bug, Rhodnius prolixus (see Ref.4). Vitellogenesis in females and syntheticactivity of the male accessory gland wereblocked by ligation and could be reinstalledin this species by transfer of active CA. Dueto their larger size and ease of rearing, mostsubsequent experiments on vitellogeninsynthesis by the female fat body and uptakeof this protein by the growing oocytes wereperformed on cockroaches and locustsas model systems. Insect vitellogenins

are members of a large family of lipid-bind-ing proteins that exhibit conserved domainsbetween protostome and deuterostome phyla(26). The titer of these proteins secreted bythe fat body of adult female insects canaccount for up to 50% of their total hemo-lymph proteins. Particularly in long-livedadult insects with distinct reproductive cycles,synthesis of this major hemolymph proteinmust be correlated with oocyte development.The regulation of vitellogenin synthesis, itsdependence on JH, and the mode of action ofJH in vitellogenin gene expression has been,and continues to be, most extensively stud-ied in the migratory locust, Locustamigratoria, and in the viviparous cockroachLeucophaea maderae (for review, see Ref.4). Vitellogenin transcription rates and JHIII titers are fairly well correlated throughout

Figure 2 - Schematic representation of the principal functions of juvenile hormone (JH) inreproduction of adult insects. JH released by the corpora allata (CA) modulates behaviorassociated with dispersal and female reproduction. In the female fat body, JH stimulatesvitellogenin synthesis, and in the ovary it facilitates vitellogenin uptake by increasingpatency of the follicular epithelium. In males, JH stimulates protein synthesis in the maleaccessory glands (mag). In some species, a sex peptide produced by these glands istransmitted to the female during mating. This peptide subsequently acts on the CA andmodifies postcopulatory behavior in the females and stimulates oogenesis. Obviously,many of these aspects are species-specific and subject to considerable variation.

oviposition

sex peptide

migration

mating

vitellogenin

patencymag

JH

164


K. Hartfelder

the oogenesis cycles, and vitellogenin tran-scription could be selectively activated afterapplication of JH analogs to chemicallyallatectomized (precocene-treated) locusts.The regulation of such gene expression iscurrently best understood for a low MWprotein (JHP21) synthesized and secreted bythe locust fat body in close coordination withthe vitellogenin gene products VgA and VgB.The upstream control region of jhp21 hasbeen shown to contain a palindromic 15-nucleotide motif (27) which resembles acanonical element recognized by the ecdy-sone receptor.

Regulation of vitellogenin gene transcrip-tion is clearly one of the prominent rolesexerted by JH in reproductive cycles offemale insects, and in this context the modeof action can be expected to involve a nuclearreceptor. The second and not less importantrole of JH in oogenesis, however, appearsto rely on an entirely different mechanism.In most insects, vitellogenin circulating inthe hemolymph reaches the oocyte surfaceby passing through large spaces that appearbetween follicular epithelial cells in compe-tent follicles. In Rhodnius prolixus, enlarge-ment of these intercellular spaces in a pro-cess termed patency is entirely dependenton JH and is the result of a reduction involume of the follicular epithelium cells. Invitro studies of this process in follicularepithelium membranes of R. prolixus andphotoaffinity labeling with JH analogs ofsuch membrane preparations in L. migratoria(19) have provided evidence for a trans-membrane JH receptor. Binding of JH tothis putative transmembrane receptor resultsin protein kinase C-mediated phosphoryla-tion of a 100-kDa protein which is thoughtto represent the a-subunit of Na+/K+

ATPase.But to what extent can these aspects of

the gonadotropic role of JH be generalized?The early and convincing results on the cen-tral role of JH in the control of female repro-ductive cycles in dictyopterans, hemipterans

and orthopterans established the JH para-digm in insect reproduction, but also clearlyimpaired the interpretation of negative evi-dence obtained in other insect species andorders. In the autogenous mosquito Aedesaegypti, induction of vitellogenin synthesisin the fat body requires 20E (28), which issynthesized by the ovary in response to ablood meal-dependent release of ovarianecdysteroidogenic hormone (OEH, alsotermed EDNH) from the brain (29). In addi-tion to ecdysteroids, which regulate fat bodyactivity in mosquito reproduction, JH alsoplays an important but different role in mos-quito oogenesis by preparing resting stagefollicles to become competent for vitello-genesis after a blood meal. Furthermore, JHprimes the fat body of newly emerged fe-male mosquitoes to respond to 20E. Thispriming function of JH in mosquitoes issimilar to the one observed in newly emergedlocusts and cockroaches, where it also con-fers competence on the fat body to respondwith vitellogenin synthesis at full blast tosubsequent peaks of the JH titer. A minimalmodel suggests that JH acts on the fat bodyof adult insects in two steps, requiring induc-tion of one or more transcription factorswhich then induce enhanced transcription ofvitellogenin genes (30).

Male reproduction and effects of male sexpeptides on female reproduction

In contrast to the well established andcomplex role of JH in female reproductivephysiology, there is little evidence for a sig-nificant role of JH in spermatogenesis. Inmany insect species, spermatogenesis ini-tiates well before adult eclosion and maycontinue throughout adult life. It thus ap-pears to occur equally well under quite dif-ferent hormonal milieus, and only in a fewspecies JH has been reported to affect sper-matogenesis duration or spermatogonial mi-totic cycles. In contrast to the gonads, theaccessory glands of the male reproductive

165



tract of many insect species turned out to betargets for JH (4). Synthesis of specific pro-teins in the male accessory gland of themigratory grasshopper, Melanoplussanguinipes, is highly JH sensitive. A high-affinity binding protein for JH has been iden-tified from nuclear extracts of this gland, andlevels of this JH-binding protein increaseduring sexual maturation in close correla-tion with general protein synthesis in themale accessory glands of this grasshopper(31).

The products of the male accessory glandsare not merely physical constituents of sper-matophores and seminal fluid, but some oftheir products turned out to have pronouncedeffects on female reproduction. Less than 1picomol of a 36-amino acid peptide, a so-called �sex peptide� produced by the acces-sory gland of Drosophila melanogastermales, has been shown to elicit characteris-tic postmating behavior in females, i.e.,loss of receptivity for males and inductionof oviposition behavior (32). Interestingly,though not totally surprising in the contextof regulatory circuits in female reproduc-tion, the female CA turned out to be targetsof this sex peptide after its transfer to thefemale genital tract during copulation. Invitro assays with CA dissected from Dro-sophila virgin females showed thatfemtomolar concentrations of synthetic sexpeptide resulted in strongly enhanced ratesof JH biosynthesis (33). Interestingly, theDrosophila sex peptide is not a species-specific bioactive compound, but exhibitsactivity beyond species and even genus level.Recently, the synthetic Drosophila peptidehas been shown to stimulate JH synthesisand to depress pheromone production in thenoctuid moth Helicoverpa armigera (34).Male influence on female reproductive be-havior by sperm fluid substances that modu-late JH synthesis is probably the most re-cently discovered facet of JH as an endo-crine master regulator of female reproduc-tion.

JH pleiotropy in female reproductivebehavior, migration, and reproductivediapause

In its simplest and clearest form, the roleof JH as a pleiotropic master regulator offemale reproductive physiology and behav-ior is expressed in cockroaches, where vitel-logenesis and cyclic maturation of oocytes isdependent on JH and occurs in coordinationwith sexual behavior. This has led to theproposal that JH is the common regulator ofreproduction. Sexual behavior, however, ismore variable than reproductive physiology,and doubts have been raised whether JH actsas a true regulator for all aspects of femalesexual behavior, or whether, at least in somecases, it merely plays a permissive role. InBlattella germanica, production and release(calling) of sex pheromone by the femalesoccurs in close correlation with an increasein JH synthesis (35). However, a furtherincrease in CA activity, as was observedafter transfer of male factors during copula-tion, curtailed calling behavior and sexualreceptivity. Other aspects of reproduction infemale cockroaches, such as changes in feed-ing activity in relation to gonocycle and fastersexual maturation of females kept undercrowded conditions in comparison to soli-tary females, are less well understood. A rolefor the CA has been proposed for both theseaspects (35) but signal transduction path-ways for social effects and feeding stimulihave not been worked out definitively.

In response to environmental factors (e.g.,pronounced seasonality especially in coldertemperate climates, decline in habitat qual-ity, etc.), elements such as migration or dia-pause are introduced into insect life cyclesand, consequently, regulation of reproduc-tion becomes more complex. These elementscan be integrated with reproduction in avariety of ways. The observation that migra-tory flights in search for more suitable sitesfor reproduction are undertaken by sexuallyimmature adults in many species has led to

166


K. Hartfelder

the proposal of an �oogenesis-flight syn-drome�, meaning that migration and repro-duction should be mutually exclusive andcould not occur at the same time. Even thoughthis formula is probably oversimplifying andmay not fit all systems it has a considerableheuristic value. Strong evidence for a role ofJH as a master regulator for the oogenesis-migration syndrome comes from studies onthe true armyworm, Pseudaletia unipunc-tata, a noctuid moth which exhibits migra-tory behavior in Northern populations and isnon-migratory in the South. Comparison ofonset of female calling behavior, progress inoogenesis, and rates of JH biosynthesis byCA of females during the first days afteremergence revealed a time shift for thesetwo populations, with the non-migratorySouthern population being more advanced(36). In this context, JH has been proposed toregulate the synthesis of a pheromonotropicbrain factor (PBAN-like factor) in specificpeptidergic neurons, but in this respect P.unipunctata appears to be a rather excep-tional case. Studies on another noctuid moth,Agrotis ipsilon, have shown that JH does notmodulate PBAN synthesis but rather stimu-lates its release from the CNS (37).

The hormonal regulation of diapause,which is a physiological and not a behavioralresponse to unfavorable environmental con-ditions, is much better understood (for re-view, see Ref. 38). According to the posi-tioning of diapause in an insect�s life cycle,JH acts as a regulator in two distinct phases,i.e., in the larva and the adult. High JH titers,in some cases in conjunction with low ecdy-steroid titers, prevent the pupal molt. In adults,low JH titers prevent reproduction and mayinduce searching behavior for suitable over-wintering sites. Low JH titers during dia-pause are maintained either by repression ofCA activity or by elevated levels of JH es-terase. Diapause in other developmentalstages is generally under the control of ecdy-steroids, or, as in bivoltine strains of thesilkworm, Bombyx mori, is induced by a

maternal peptide that is released from thesubesophageal ganglion during oogenesis andblocks embryonic development after ovipo-sition.

Timing and frequency of mating are im-portant life history parameters which arecontrolled by the endocrine system and whichare integrated with the above describedoogenesis-migration/diapause syndrome.Compiling datasets on egg development,timing of mating, and on number of matings,i.e., whether a female mates only once(monandry) or several times (polyandry),Ramaswamy et al. (39) distinguished fourdifferent strategies in moths and butterflies.In the first group (exemplified by Bombyxmori, Lymantria dispar and some other largemoths), egg development starts as early asthe larval stages and is completed in earlypupae. In these species, vitellogenesis is de-pendent on ecdysteroids. The adult femalesdo not feed and have a very short life span;they mate only once immediately after emer-gence and oviposit. In the second group,which includes the pyralid moths, egg devel-opment and vitellogenesis are shifted to-wards the end of pupal development and areassociated with declining ecdysteroid titers.With some exceptions, females of this groupalso do not feed as adults and are monandrous.In the third group, exemplified by the to-bacco hornworm, M. sexta, oogenesis startsin the pupal phase but is not regulated by themetamorphosis-triggering ecdysteroid titer.Choriogenesis is completed only in the newlyemerged adults and is dependent on JH.Females copulate several times shortly afteremergence and exhibit stimulation of oogen-esis after each mating. In the fourth group,which includes papilionid butterflies andnoctuid moths, egg development initiates inthe adult phase and JH is necessary forvitellogenin synthesis in the fat body,vitellogenin uptake by the oocytes, and alsofor choriogenesis. As described above, in anumber of these latter species, the adults aresexually immature when they initiate migra-

167



tory flights, and reproductive activities suchas pheromone synthesis, calling, and oocytematuration are closely linked and are JHdependent. Most of these species are polyan-drous, and every additional mating results infurther stimulation of egg production.

Juvenile hormone anddevelopmental polymorphism

Upholding the option to migrate or toreproduce right away clearly implies costsfor building and maintaining a flight appara-tus, and natural selection should act to re-duce the cost of such flight-reproductivetrade-offs. Strong support for a fitness trade-off between flight capability and reproduc-tion is provided by comparative studies acrossa wide range of wing-polymorphic insects(40). Primary evidence for such a trade-offcomes from a strong negative correlationbetween flight-muscle mass and ovarianmass, a relationship which suggests that con-struction and maintenance of the insect flightapparatus competes with egg production fora limited general pool of available nutrients.Various environmental cues such as crowd-ing, host plant condition, temperature, andphotoperiod have an influence on wing form.Wing dimorphism is well exemplified ingryllids and is even more pronounced inaphids. It is only in these two groups thatquestions of hormonal control of wing poly-morphism have been pursued persistently(41).

Wing dimorphism in crickets

Early proposals have suggested that JHmight play a prominent role in such develop-mental polymorphisms, i.e., in the evolutionof correlated traits of flight and reproductivepotential, but obviously, such traits are de-termined in different phases of the insect lifecycle. In most of the above discussion Iaddressed the role of JH in metamorphosisas rather distinct from its later role in female

reproduction. The fact that it is a pleiotropicmaster regulator for both programs, how-ever, makes it at the same time a primecandidate to act as an integrative element inthe evolution of wing and consequent repro-ductive polymorphic systems. In the cricketGryllus rubens, application of JH III to lar-vae destined to mature into long-winged in-sects resulted in the formation of brachypter-ous adults, corresponding to the higher JHtiter in the last larval instars of the brachyp-terous as compared to the macropterousmorph. Interestingly, it is not an enhancedrate of JH biosynthesis which maintains theelevated JH titer in brachypters but ratherdecreased rates of JH metabolism due to alower activity of JH esterase (41). Whenelevated JH titers persist in the last larvalinstar they are thought to inhibit metamor-phosis of the wing anlagen and developmentof flight muscles.

Phase polymorphism in aphids

The evolution of flight-reproductivetrade-offs probably has reached its apex inthe aphids where apterous/alate wing dimor-phism can occur in combination with switchesbetween parthenogenetic and sexual modesof reproduction, and also with seasonal os-cillation between host plant species. Theprimary mode of aphid reproduction is vi-viparous parthenogenesis. This has led to aneven further abbreviation of life cycles bytelescoping generations within a female�sovary. Virginoparae (parthenogenetic fe-males which give birth to parthenogeneticprogeny) contain embryos in different de-velopmental stages within their ovaries. Inthe ovaries of the furthest developed em-bryos, oogenesis has already initiated and anext generation is being produced. Thus, theproblem of how environmental cues are trans-lated into endocrine signals to switch be-tween the different modes of reproductionbecomes very complex (42). The small sizeof aphids has furthermore precluded many

168


K. Hartfelder

surgical experiments and has also hamperedthe determination of morphogenetic hormonetiters. The most convincing evidence for arole of JH in morph determination has beenobtained for photoperiod-mediated wingpolymorphism in Aphis fabae where JH ap-plication during a critical phase sensitive tophotoperiod strongly redirected developmentfrom the winged (alate) to the flightless (apter-ous) form. Numerous aphid species, how-ever, are not apterized by JH applicationduring critical phases of morph determina-tion. Instead, products of neurosecretory cellsof the brain have been suggested as potentialcandidates for inducing aphid wing poly-morphism (42). The pest status of manyaphids has promoted intense research on theeffects of precocenes which act as cytotox-ins in CA cells of many insect species. Inaphids these compounds should have theadvantage of destroying not only the CA ofadult females but also of their nascent prog-eny telescoped within the ovaries. Screeningthe activity of different precocene compoundsin the pea aphid, Aphis pisum, has shownthat some compounds inhibit wing produc-tion, whereas others promote wing forma-tion (43). Such equivocal results for just onespecies, together with equally diverse resultson effects of application of JH or JH analogsobtained in other aphid species, indicate thatJH is only one amongst several factors in thedetermination of aphid wing polymorphism.

The overestimated role of JH in locust phasepolymorphism

Another characteristic developmental poly-morphism related to migration and reproduc-tion is the phase polymorphism of migratorylocusts, Locusta migratoria and Schistocercagregaria. Depending on population densityand habitat quality, locusts appear in two forms,termed phases, with a continuous range ofintermediates between the extreme phases.These morphs differ in behavior, coloration(solitary green versus gregarious brown), mor-

phometric traits, metabolism related to mobili-zation of energy resources for flight activity,and fertility (for review, see Ref. 44). In thegregarious phase, females tend to producefewer eggs than in solitary generations. Thefact that the green coloration typical for thesolitary form could be induced by CA implan-tation into nymphs kept under crowded condi-tions, and the demonstration that the JH titerand CA activity during critical phases of de-velopment were indeed higher in isolated thanin crowded locusts (45) suggested that JHcould, also in this case, control morph charac-teristics and their integration with reproduc-tion. Numerous apparently contradictory re-sults obtained during screening tests for JHanalogs as locust control agents, however,showed that the green coloration, which isconsidered typical for solitary locusts, and itsinducibility by JH was strongly dependent onhumidity. Another coloration characteristic,the gregarious yellow color, also turned out tobe inducible by JH treatment. JH thus appearsto be capable of shifting coloration character-istics quite independently of phase (46), and itappears to do so in synergism with peptidehormones released from the corpora cardiaca.

Behavioral phase characteristics couldalso be steared into opposite directions byJH treatment. Typical solitary behavior, suchas aggression towards conspecifics, was en-hanced in JH-treated crowded locusts andaggregation behavior was notably reduced.However, the marching activity of crowdedhoppers, a gregarious phase characteristic ofimmatures, was intensified by JH treatment.Also with respect to reproduction, JH hadphase-independent effects. It increased fe-cundity in crowded adult females to levelstypical for the solitary phase, but at the sametime it also accelerated oocyte maturation,which is not a phase characteristic. Thus, JHclearly is not the master regulator of locustphase polymorphism. Rather, it seems toplay the same roles typically observed inother insects as well. Firstly, cuticle colora-tion may be varied by the well-known effects

169



of JH on uptake and incorporation of hemo-lymph insecticyanins during the molting cycleand by interference with the melanizationprogram during preimaginal development (3).Secondly, JH may be stimulating oogenesisin general and vitellogenesis in particular inadult females. The shifts between solitaryand gregarious phase characteristics, on theother hand, appear to depend on a variety ofother factors (46), including corpora car-diaca peptides, which may or may not ex-hibit interaction with JH-regulated programs.Synergism and antagonism of such factorswith JH-regulated programs and the timingof such interactions may well be one of thereasons why locusts exhibit a continuousphase polymorphism with shifts extendingover generations.

JH and high society: castepolymorphism and division of laborin social insects

The ecological significance of social in-sects is reflected in their shear biomass inwhich they are represented in the most var-ied ecosystems, from boreal climates wheresingle ant species often represent the largestshare of the biomass of soil arthropods, tothe Amazonian rain forest, where social in-sects (bees, wasp, ants and termites) canaccount for up to 30% of the entire animalbiomass. The answer to why they occupysuch a dominant position is thought to residein their social mode of life. Insect societieshave frequently been compared to vertebrateand especially human societies, and theremay even be lots of parallels regarding ulti-mate reasons. The proximate mechanismsgenerating the systems of division of laborobserved in social insects, however, are ob-viously completely different from those invertebrates, and in this regard, social insectsare insects. The haplodiploid genetic systemof sex determination in the Hymenoptera(including wasps, bees, and ants), and highgenetic relatedness combined with depend-

ence on endosymbionts in termites are con-sidered to constitute the driving forces ofsocial evolution in these groups which re-sulted in complex division of labor based oncaste polymorphism and elaborate commu-nication systems.

Members of the worker caste(s) havemore or less given up on reproduction andhave developed special structures makingthem more efficient foragers and defendersof their colonies, whereas reproduction hasbecome the monopoly of one or few indi-viduals, generally referred to as queens. Assuch, the system of division of labor is pri-marily a decision about who is to reproduceand who is not, and ultimately this is reducedto the problem of developing a functionalreproductive tract. In addition, the imaginaldisks for wings degenerate in the late larvalstages in ant workers, and in termites, wingdevelopment in the workers is either blockedor does not start. Development of the gonadsand of the ducts of the reproductive system,as well as wing differentiation obviously areaspects of metamorphosis. The differentia-tion of polymorphic castes can thus be ex-pected to be controlled by the endocrinesystem, which itself responds to externalcues. Such external cues are differential feed-ing of the larvae and/or inhibitory signalsreleased by the functional reproductives.With few exceptions, caste determinationdoes not result from a genetic bias.

The key role of JH in honey bee castedifferentiation

Much groundwork elucidating the promi-nent role of JH in caste differentiation hasbeen laid in the laboratory of Lüscher inBern working on different termite species(for review, see Ref. 47). Size of individualsand colonies, and facility of rearing andmanipulating colonies, however, have nowestablished the honey bee, Apis mellifera, asthe most advanced model for caste develop-ment and social regulation of division of

170


K. Hartfelder

labor (for a recent review, see Ref. 23).Detailed analyses of JH titers during embry-onic, larval and pupal development of queenand worker honey bees have revealed dra-matic differences in the third to early fifthlarval instars, concurrently with a switch inthe feeding program for queen and workerlarvae. While nurse workers continuouslyfeed copious amounts of royal jelly to queenlarvae, worker larvae are reared on a mixeddiet (royal jelly diluted with pollen and nec-tar). It is not yet clear how this trophic infor-mation is processed, but it eventually resultsin high rates of JH synthesis by the CA ofqueen larvae and low rates of JH synthesis inworkers. One of the possibilities is that in-formation on food quality and quantity istransmitted via the stomatogastric nervoussystem of honey bee larvae (48) which di-rectly connects to the retrocerebral endo-crine system via a hypocerebral nerve tract.The fact that a) this pathway ends near sero-tonergic cells located in the proximity of theCA, and b) serotonin and octopamine stimu-late CA activity in honey bee larvae makesthis pathway a plausible route for transmis-sion of a nutritional switch signal.

The mode of action of JH in honey beecaste differentiation is highly pleiotropic. Inthe developing ovaries, JH affects cell pro-liferation during a critical phase in the lastlarval instar, and coincidently inhibits in-duction of programmed cell death in germcells and somatic cells in the developingfemale gonad (49). Slightly later in the fifthinstar, JH appears to stimulate the protho-racic glands, resulting in an earlier increasein ecdysteroid titers in queen larvae. Ecdy-steroids in turn direct the caste-specific pro-gram of protein synthesis in the larval ova-ries.

Honey bee reproduction: JH is not a regula-tor of reproductive physiology but ofreproductive behavior

The early recognized prominent integra-

tive role of JH in the morphological differen-tiation of the queen and worker caste and theparadigm of JH as a regulator of femalefertility in insects in general has stronglyinfluenced the direction of research activi-ties on reproduction and division of labor insocial insects, and particularly in honey bees.The reproductive physiology of adult honeybee queens differs from that of workers intwo important respects, the rate and amountof vitellogenin synthesis and the productionof queen pheromone. At egg laying rates upto 2,000 eggs per day it is not surprising thatvitellogenin synthesis is consistently highand that its titer represents more than 50% ofthe hemolymph proteins of adult queens.Vitellogenin titers in workers reach elevatedlevels (25-30% of total hemolymph proteins)only during the nurse bee phase. Workers arethus principally capable of producing andlaying eggs (haploid eggs giving rise to males)as well, but are repressed to do so in thepresence of a dominant queen. The queeninhibits ovary activation and follicle devel-opment in workers by her pheromones (prin-cipally (E)-9-oxo-2-decenoic acid). Underqueenless conditions and in the absence ofyoung larvae, however, egg laying by work-ers can frequently be observed.

Indeed, JH plays only a very subtle rolein the reproductive physiology of femalehoney bees, and it clearly does not stimulatevitellogenin synthesis. Rather it appears toserve as an integrative element in socialbehavior and colony function (for review,see Ref. 23). Elevated JH levels are associ-ated with periods of flight activity in queensand workers, and also in drones (50), theonly difference being that the flights of queensand drones occur early in adult life and areassociated with mating activities, whereasflight activities in workers initiate much laterand serve to collect nectar, pollen, and wa-ter. JH application or allatectomy affectedthe onset of flight activities, thus demon-strating a general requirement for JH in flightactivities in all three honey bee morphs

171



(51,52). In the case of workers, the onset offlight activity for foraging is controlled byage and colony conditions, and stands withinthe context of division of labor and age-dependent performance of different tasks(age polyethism).

Division of labor in highly social insects: anew role for an old hormone

Recent studies have shown that the tran-sition from intranidal activities to foraging isaccompanied by a significant increase in CAactivity, and that this modulation of CA ac-tivity depends on the age structure of a colony,i.e., the frequency at which a young beeencounters older bees affects its CA activity(53). Modulation of CA activity appears torequire full physical contact and the possi-bility for exchange of age-characteristic sig-nals in such worker-worker encounters (54).This JH-mediated transition to foraging ac-tivity does not only involve behavioralchanges, but rather has, once more, pleiotro-pic effects. It results in degeneration andfunctional changes in the larval food-pro-ducing hypopharyngeal glands, a decrease invitellogenin synthesis, and in restructuringof the protocerebral mushroom bodies (re-viewed in Ref. 23). These latter changes area prime example of JH-mediated neuronalplasticity occurring in concert with changesin behavior. The transition from a nurse beeto a forager, thus evidently involves struc-tural changes as the bee expands its learningcapacity (55). When foraging, a bee has toknow the location of the hive, learn thelocation of the food source, and has to com-municate the latter via an elaborate dancelanguage.

Behavioral integration of a honey beecolony does not only depend on encountersof workers of different age cohorts, but alsoinvolves genetic predisposition to certaintasks (56) and the signal of presence of afunctional queen. As mentioned above, thequeen suppresses worker fertility via a queen

pheromone, but this queen pheromone isalso a relevant indicator that colony repro-duction is running at full speed. In this sense,the repressor action of queen pheromonecomponents on CA activity in young beescan be interpreted as a regulator of colonydemography guaranteeing an adequate co-hort of nurse bees in colony reproduction.This primer effect of queen pheromone onworkers apparently delays their behavioralontogeny without affecting typical behav-ioral responses of workers to queen phero-mone, such as retinue formation (57). Inter-estingly, biogenic amines appear to play atransmitter role in this context. Octopaminestimulates JH biosynthesis (58) and seroto-nin injections reduce the vitellogenin titer innurse bees (Simões ZLP, personal informa-tion). These results are in accordance withthe general view that external and colonyconditions affect the levels of biogenicamines in the CNS of honey bee workers,and that these levels, in turn, may adjust taskperformance of individual workers by mod-ulating their JH titers.

The changing roles of JH in socioevolution

It is interesting to note that in this highlysocial context, as exemplified by behavioralintegration in a honey bee colony, JH hasacquired two new functions. Firstly, elevatedJH titers have become associated with flightactivity and JH has apparently lost its gona-dotropic role. This represents a dissociationof the �oogenesis-flight syndrome� as de-scribed above. Secondly, JH has become akey element in social integration. I decidedto describe the dissociation of JH from re-productive functions other than mating flightsas a loss of function because in bumble beesand polistine wasps, JH still retains such afunction. In contrast to the highly socialspecies where reproductive dominance is byand large a matter of �chemical control�, thehierarchies of reproductive dominance incolonies of primitively social species are

172


K. Hartfelder

strongly based on and established by aggres-sive behavioral interactions, especiallyoophagy.

In the subfamily Polistini, colonies arefounded by single females which are soonjoined by cofoundresses. Caste differencesare virtually absent in the paper wasps (ge-nus Polistes) of temperate climates. All fe-males present in such incipient colonies aremated and capable of egg laying. A singlefemale, however, will eventually becomedominant and monopolize reproduction. Shesignals her status of dominance to subordi-nate females by antennal tapping, and when-ever she encounters eggs laid by other fe-males on the comb she removes them. At theendocrine level, this hierarchy is reflectedby elevated JH titers in the dominant fe-males, and by low titers and low CA activityin the subordinate females (for review, seeRef. 59). High JH titers in these wasps areassociated with activated ovaries and pro-gressing oogenesis, and ovariectomy oftenleads to a reduction in JH titer and domi-nance status. Colony organization in the sub-tropical and tropical subfamilies Ropalidiniand Polybiini is more complex. Incipientmorphological caste characteristics have beenpostulated for Ropalidia marginata. Hor-monal control mechanisms, however, havenot yet been investigated. The advanced so-cial polybiines already exhibit a system ofage polyethism in their large colonies. InPolybia occidentalis, this age polyethismcould be accelerated by JH application witheffects strongly reminiscent of JH effects inhoney bees (60).

The interactions of JH titers, progressingfollicle development, and reproductive domi-nance appear to have been extensively ex-ploited in social evolution, as exemplifiedby the bumble bees. The colony cycle inthese predominantly temperate climate spe-cies initiates with colony foundation by asingle queen. Once the first brood is rearedand workers emerge, these aid the queen inthe next brood cycles until the queen starts to

lay haploid eggs giving rise to males (switchpoint). Subsequently, queens are frequentlychallenged by workers over the productionof unfertilized, male-producing eggs (com-petition point), and soon afterwards the colo-nies disintegrate. Thus, at the end of thecolony cycle the dominant queen appears tolose her dominant status which she had pre-viously maintained by dominant behaviorand a pheromone, and some of the workersof an �elite� group become egg layers as well(for review, see Ref. 61). Egg laying and theestablishment of dominance hierarchies arealso observed under conditions of queenloss. Thus, in the bumble bees, oogenesisand egg laying have apparently become dis-sociated within the context of social organi-zation. Follicle development initiates in ova-ries of workers, both in the presence andabsence of the queen, but in the presence ofthe queen and dominant workers, these fol-licles are resorbed in the subdominant work-ers. Follicle development in workers gener-ally starts 5 days after emergence and may beobserved during much of their life span,quite different from the narrow temporalwindow for egg development in honey beeworkers. Late stages of follicle development,however, were mainly observed in bumblebee workers belonging to the �elite� groupafter the competition point of the colonycycle. Ovary activation was also observed inexperimental groups of queenless workerswhich were put together before their respec-tive colonies had reached the competitionpoint (62).

JH enters as a key player at several pointsin the regulation of dominance and egg de-velopment in bumble bees. JH titers werefound to be high in egg-laying queens, andthese high levels are considered to be re-quired for vitellogenesis (61,63). ElevatedCA activity and corresponding JH titers werealso observed in queenless workers whichexhibited significantly more advanced lev-els of oogenesis than queenright workers(63,64). Ovariectomy, however, did not af-

173



fect CA activity in dominant workers, indi-cating that the dominance status, which ap-pears to be established by aggressive inter-actions with fellow workers and possiblyalso by the production of a dominance-indi-cating pheromone in these workers at thecompetition point of the colony cycle, can bedissociated from their egg-laying capacity(61). The existence of a worker-produceddominance pheromone was also hypoth-esized from analyses of JH synthesis in youngworkers introduced into queenless groupsestablished from colonies before and afterthe competition point (64).

The role of JH in colony organization ofthe more primitively social bumble bees ex-hibits striking differences from the role itplays in the highly social honey bees: a) JH isassociated with dominance and reproduc-tion (vitellogenesis) in the former but not thelatter, b) age polyethism and division oflabor among workers is not affected by JH inbumble bees (65), c) foraging workers inbumble bee colonies have lower JH titersthan workers that perform tasks within thenest, and d) mating flight activity of virginqueens is not affected by JH in bumble bees(61). An interesting parallel for JH functionin primitively and highly social bees, though,becomes apparent when JH titers during thelarval stages of prospective queens and work-ers are compared. Even though bumble beesdo not have morphologically but only physi-ologically differentiated castes, CA activityin penultimate-instar queen larvae of Bombusterrestris is much higher than that detectedin prospective workers (66). These incipientdifferences in levels of hormone synthesis,however, appear to be suppressed again dur-ing the prepupal phase (67), and thus appar-ently do not permit the development of mor-phological caste differences.

From these observations we can proposea dichotomy for the role of JH in socialorganization of hymenopteran colonies. Inthe primitively social wasps and bumble bees,JH still exerts a gonadotropic function and,

in addition, becomes associated with thedominance status of individual females. Thedominance status evidently is still tightlyconnected with reproduction. In the highlysocial honey bees, and possibly also in thestingless bees and in highly social wasps, thefixation of morphological caste differenceshas liberated JH from these functions. Eggproduction by queens in such coloniesreaches levels far beyond the capacities of abumble bee queen and requires very highand constant levels of vitellogenin synthesisby the fat body. This eliminates the need forcyclic regulation of oogenesis by JH, so thategg production can now also occur at low JHtiters. Evidently, in at least some of the highlysocial Hymenoptera this �old� hormone hasbeen coopted for a fundamentally new func-tion, which can only become apparent in ahighly social context, namely division oflabor based on age polyethism. In this con-text, JH even affects the organization ofelementary brain structures. In highly socialinsects, the originally gonadotropic functionof JH has thus been transformed into anintegrator of colony function (colony repro-duction), whereas its metamorphic functionhas been preserved and even been expandedto generate polymorphic castes.

This metamorphic function of JH hasreached its summit in the ants, where JHorchestrates not only the differentiation ofthe queen and worker castes, but also guidesthe formation of morphologically distinctworker morphs. This role of JH has beeninvestigated in detail in the genus Pheidolewhere production of the strongly dimorphicsoldiers and workers depends on differencesin JH levels during the late larval stages,whereas the queen-worker decision is mademuch earlier in larval development (68).Developmental rules underlying the produc-tion of these secondary castes have beenoutlined as JH-mediated modulation of allo-metric growth parameters in the differentimaginal discs (69). Only few ant specieshave been studied with regard to a putative

174


K. Hartfelder

gonadotropic role of JH, and rather equivo-cal results were obtained. Whereas JH treat-ment stimulates shedding of the wings andoogenesis in newly emerged virgin queensof the fire ant, Solenopsis invicta (70), JHtiters in reproductives of the ponerine antDiacamma sp were consistently low (71). InDiacamma, JH titers were found to be posi-tively correlated with worker age, suggest-ing a role for JH in division of labor, muchlike the picture emerging in the honey bee.

The evolution of insect polymorphisms,including the complex caste systems of socialinsects, has been reviewed under premises ofphenotypic plasticity (23,72). The drivingforces towards sociality in the Hymenopteraappear to have been a) the haplodiploid systemof sex determination generating asymmetriesin the relatedness among offspring, and b) thetendency of females to return and reuse theparental nest. The subsequent chain of effectsprobably did not require genetic change. Itincluded the formation of groups, establish-ment of reproductive dominance by aggres-sive interactions between nestmates, and carefor larvae by non-ovipositing females. All thesetraits can be seen as derived from species thatdo not nest socially and are interpretable asbehavioral trait amplification. Adaptive phe-notypes could then have originated by minorgenetic changes via correlated shifts in theexpression of phenotypically plastic traits. Thepleiotropic functions of JH in insect develop-ment and reproductive physiology in generalmade this hormone system an ideal integratorfor caste development and social organization.During social evolution, the gonadotropic roleof JH has apparently suffered a striking trans-formation from its original function in eggproduction and dominance rank to a behavior-al pacemaker in division of labor among work-ers (70).

Concluding remarks

Methylfarnesoate derivatives made theirappearance in arthropod evolution as �sup-

porting actors� in the ecdysteroid-regulatedmolting process. In the larval and pupal stagesof insect life cycles, JH is now known tomodify the expression patterns of ecdyster-oid-regulated genes, creating the possibilityfor an insect to express different types ofcuticles, and in particular to develop from alarval form to an adult form through a com-plex metamorphosis. Complete metamorpho-sis, as observed in the holometabolous in-sects, is the key to the intraspecific separa-tion of ecological niches occupied by larvaeand adults, and undoubtedly has played amajor role in the tremendous ecological andevolutionary radiation of holometabolousinsects.

The multiple functions of JH duringpreimaginal development may have been apreadaptation for its implication in the regu-lation of reproductive processes, where theprimary functions appear to have been thesynchronization of vitellogenin synthesis bythe fat body and vitellogenin uptake by de-veloping oocytes during distinct reproduc-tive cycles. Diversification of life and repro-ductive cycles, together with the necessity toavoid conditions unfavorable for reproduc-tion, have added ever more facets to JHfunction. One of the most intriguing newfacets is the recent discovery that sex pep-tides produced by the male accessory glandscause a complex behavioral switch in matedfemales, from mate-seeking to mate-rejec-tion and oviposition behavior. A syntheticDrosophila sex peptide has now been shownto modulate JH biosynthesis levels in the CAof different insect species.

Life cycle complexity in insects is furtherincreased in species exhibiting facultativepolymorphism, such as wing dimorphism incrickets, phase polymorphism in desert lo-custs and aphids, and the castes of socialinsects. In social evolution, JH seems tohave first been implicated in the coupling ofoogenesis to status in a dominance hierar-chy. This is a reflection of the early steps ofsocioevolution where colonies are established

175



by the joining of sisters or unrelated females.All of these females are mated and thus canfunction as potential egg layers, but in gen-eral one of these will effectively monopolizereproduction and suppress oviposition bythe others, mainly by antagonistic behavior.In these primitively social insects, high JHtiters generally exhibit a high correlationwith dominance status and follicle develop-ment in the ovaries. In the populous coloniesof highly social insects, one or more repro-ductively dominant females reign by meansof pheromones. These elicit behavioral re-sponses in the workers, guaranteeing colonycohesion and suppressing follicle develop-ment in the ovaries of workers. JH is in-volved in several aspects of the life cycle insuch highly social species, and its multipleroles are currently best understood in thehoney bee. First of all, JH titers exhibit caste-specific modulation during critical develop-mental periods in the larval instars. Thesetiter differences eventually lead to the ex-pression of different (alternative) develop-

mental programs in the ovaries and in othertarget tissues. JH has thus gained a key func-tion in the generation of polymorphic castes.Secondly, JH has lost its association withoogenesis, and thus, could be coopted intoother functions in adult bees. It now coordi-nates mating flight activities in newlyemerged queens and drones, and serves as abehavioral pacemaker in workers. This latterfunction represents a new and fascinatingfacet in JH pleiotropy which allows to inte-grate colony demography and colony needswith the age-related performance of specifictasks by each individual worker bee.

Acknowledgments

I would like to thank Zilá Luz PaulinoSimões (USP, Ribeirão Preto), AnnaRachinsky (Kansas State University, Man-hattan) and Isabel Boleli (UNESP, Jabotica-bal) for comments on a previous version ofthis manuscript and for sharing with meunpublished results.

References

1. Biggers WJ & Laufer H (1996). Detectionof juvenile hormone-active compounds bylarvae of the marine annelid Capitella sp.Archives of Insect Biochemistry and Phys-iology, 32: 475-484.

2. Homola E & Chang ES (1997). Methylfarnesoate: Crustacean juvenile hormonein search of functions. Comparative Bio-chemistry and Physiology. B, Biochemis-try and Molecular Biology, 117: 347-356.

3. Riddiford LM (1994). Cellular and molecu-lar actions of juvenile hormone. I. Generalconsiderations and premetamorphic ac-tions. Advances in Insect Physiology, 24:213-274.

4. Wyatt GR & Davey KG (1996). Cellular andmolecular actions of juvenile hormone. II.Roles of juvenile hormone in adult insects.Advances in Insect Physiology, 26: 1-155.

5. Darrouzet E, Mauchamp B, Prestwich GD,Kerhoas L, Ujvary I & Couillaud F (1997).Hydroxy juvenile hormones: New puta-tive juvenile hormones biosynthesized bylocust corpora allata in vitro. Biochemicaland Biophysical Research Communica-

tions, 240: 752-758.6. Tobe SS & Stay B (1985). Structure and

regulation of the corpus allatum. Ad-vances in Insect Physiology, 18: 305-432.

7. Tobe SS & Pratt GE (1974). The influenceof substrate concentrations on the rate ofjuvenile hormone biosynthesis by corporaallata of the desert locust in vitro. Bio-chemical Journal, 144: 107-113.

8. Stay B, Fairbairn S & Yu CG (1996). Roleof allatostatins in the regulation of juve-nile hormone synthesis. Archives of In-sect Biochemistry and Physiology, 32:287-297.

9. Kataoka H, Toschi A, Li JP, Carney RL,Schooley DL & Kramer S (1989). Identifi-cation of an allatotropin from adult Man-duca sexta. Science, 243: 1481-1483.

10. Rachinsky A & Tobe SS (1996). Role ofsecond messengers in the regulation ofjuvenile hormone production in insects,with particular emphasis on calcium andphosphoinositide signaling. Archives of In-sect Biochemistry and Physiology, 33:259-282.

11. Sutherland TD & Feyereisen R (1996). Tar-get of cockroach allatostatin in the path-way of juvenile hormone biosynthesis.Molecular and Cellular Endocrinology,120: 115-123.

12. Wang ZW, Ding Q, Yagi KJ & Tobe SS(1994). Terminal stages in juvenile hor-mone biosynthesis in corpora allata ofDiploptera punctata - Developmentalchanges in enzyme activity and regulationby allatostatins. Journal of Insect Physiol-ogy, 40: 217-223.

13. Thompson CS, Yagi KJ, Chen ZF & TobeSS (1990). The effects of octopamine onjuvenile hormone biosynthesis, electro-physiology, and cAMP content of the cor-pora allata of the cockroach Diplopterapunctata. Journal of Comparative Physiol-ogy. B, Biochemical Systemic and Envi-ronmental Physiology, 160: 241-249.

14. Rachinsky A (1994). Octopamine and se-rotonin influence on corpora allata activityin honey bee (Apis mellifera) larvae. Jour-nal of Insect Physiology, 40: 549-554.

15. Goodman WG (1990). Biosynthesis, titer

176


K. Hartfelder

regulation, and transport of juvenile hor-mones. In: Gupta AP (Editor), Morphoge-netic Hormones of Arthropods. Vol. 1.Rutgers University Press, New Bruns-wick, 83-124.

16. deKort CAD & Granger NA (1996). Regu-lation of JH titers: The relevance of degra-dative enzymes and binding proteins. Ar-chives of Insect Biochemistry and Physi-ology, 33: 1-26.

17. Prestwich GD, Wojtasek H, Lentz AJ &Rabinovich JM (1996). Biochemistry ofproteins that bind juvenile hormones. Ar-chives of Insect Biochemistry and Physi-ology, 32: 407-419.

18. Wyatt GR (1997). Juvenile hormone ininsect reproduction - a paradox? EuropeanJournal of Entomology, 94: 323-333.

19. Sevala VL, Davey KG & Prestwich GD(1995). Photoaffinity labeling and charac-terization of a juvenile hormone bindingprotein in the membranes of follicle cellsof Locusta migratoria. Insect Biochemis-try and Molecular Biology, 25: 267-273.

20. Ashok M, Turner C & Wilson TG (1998).Insect juvenile hormone resistance genehomology with the bHLH-PAS family oftranscriptional regulators. Proceedings ofthe National Academy of Sciences, USA,95: 2761-2766.

21. Restifo LL & Wilson TG (1998). A juvenilehormone agonist reveals distinct develop-mental pathways mediated by ecdysone-inducible Broad Complex transcription fac-tors. Developmental Genetics, 22: 141-159.

22. Gruetzmacher MC, Gilbert LI, Granger NA,Goodman W & Bollenbacher WE (1984).The effect of juvenile hormone on protho-racic gland function during the larval-pu-pal development of Manduca sexta: An insitu and in vitro analysis. Journal of InsectPhysiology, 30: 331-340.

23. Hartfelder K & Engels W (1998). Socialinsect polymorphism: Hormonal regula-tion of plasticity in development and re-production in the honeybee. Current Top-ics in Developmental Biology, 40: 45-77.

24. Zhou BH, Hiruma K, Jindra M, Shinoda T,Segraves WA, Malone F & Riddiford LM(1998). Regulation of the transcription fac-tor E75 by 20-hydroxyecdysone and juve-nile hormone in the epidermis of the to-bacco hornworm, Manduca sexta, duringlarval molting and metamorphosis. Devel-opmental Biology, 193: 127-138.

25. Jones G, Schelling D & Chokar V (1996).Overview of the regulation of metamor-phosis-associated genes in Trichoplusiani. Archives of Insect Biochemistry andPhysiology, 32: 429-437.

26. Hagedorn HH, Maddison DR & Tu ZJ(1998). The evolution of vitellogenins,cyclorrhaphan yolk proteins and relatedmolecules. Advances in Insect Physiolo-gy, 27: 335-384.

27. Zhang JZ, Saleh DS & Wyatt GR (1996).Juvenile hormone regulation of an insectgene: A specific transcription factor and aDNA response element. Molecular andCellular Endocrinology, 122: 15-20.

28. Hagedorn HH, O’Connor JD, Fuchs MS,Sage B, Schlaeger DA & Bohm MK (1975).The ovary as a source of a-ecdysone in anadult mosquito. Proceedings of the Na-tional Academy of Sciences, USA, 72:3255-3259.

29. Klowden MJ (1997). Endocrine control ofmosquito reproduction. Archives of InsectBiochemistry and Physiology, 35: 491-512.

30. Wyatt GR, Braun RP & Zhang J (1996).Priming effect on gene activation by juve-nile hormone in locust fat body. Archivesof Insect Biochemistry and Physiology,32: 633-640.

31. Ismail SM & Gillot C (1996). Identificationof a nuclear juvenile hormone-binding pro-tein in the long hyaline tube of male Mela-noplus sanguinipes. Archives of InsectBiochemistry and Physiology, 32: 623-631.

32. Schmidt T, Choffat Y, Klauser S & Kubli E(1993). The Drosophila melanogaster sexpeptide - a molecular analysis of struc-ture-function relationships. Journal of In-sect Physiology, 39: 361-368.

33. Moshitzky P, Fleischmann I, Chaimov N,Saudan P, Klauser S, Kubli E & ApplebaumSW (1996). Sex peptide activates juvenilehormone biosynthesis in Drosophila me-lanogaster corpus allatum. Archives of In-sect Biochemistry and Physiology, 32:363-374.

34. Fan YL, Rafaeli A, Gileadi C, Kubli E &Applebaum SW (1999). Drosophila mela-nogaster sex peptide stimulates juvenilehormone synthesis and depresses sexpheromone production in Helicoverpaarmigera. Journal of Insect Physiology, 45:127-133.

35. Schal C, Holbrook GL, Bahmann JAS &Sevala VL (1997). Reproductive biology ofthe German cockroach, Blatella germa-nica: Juvenile hormone as a pleiotropicmaster regulator. Archives of Insect Bio-chemistry and Physiology, 35: 405-426.

36. McNeil JN, Laforge M, Bédard C &Cusson M (1996). Juvenile hormone pro-duction and sexual maturation in true ar-myworm, Pseudaletia unipunctata (HAW.)(Lepidoptera: Noctuidea): A comparison

of migratory and non-migratory popula-tions. Archives of Insect Biochemistry andPhysiology, 32: 575-584.

37. Picimbon JF, Becard JM, Sreng L, Clem-ent JL & Gadenne C (1995). Juvenile hor-mone stimulates pheromonotropic brainfactor release in the female black cut-worm, Agrotis ipsilon. Journal of InsectPhysiology, 41: 377-382.

38. Denlinger DL (1985). Hormonal control ofdiapause. In: Kerkut GA & Gilbert LI (Edi-tors), Comprehensive Insect Physiology,Biochemistry and Pharmacology. Vol. 8.Pergamon Press, Oxford, 353-412.

39. Ramaswamy SB, Shu S, Park YI & Zeng F(1997). Dynamics of juvenile hormone-mediated gonadotropism in the Lepi-doptera. Archives of Insect Biochemistryand Physiology, 35: 539-558.

40. Roff DA (1986). The evolution of wingdimorphism in insects. Evolution, 40: 30-37.

41. Zera AJ & Denno RF (1997). Physiologyand ecology of dispersal polymorphism ininsects. Annual Review of Entomology,42: 207-231.

42. Hardie J & Lees AD (1985). Endocrinecontrol of polymorphism and polyphen-ism. In: Kerkut GA & Gilbert LI (Editors),Comprehensive Insect Physiology, Bio-chemistry and Pharmacology. Vol. 8.Pergamon Press, Oxford, 441-490.

43. Hardie J, Gao N, Timár T, Sebök P &Honda K-I (1996). Precocene derivativesand aphid morphogenesis. Archives of In-sect Biochemistry and Physiology, 32:493-501.

44. Pener MP (1991). Locust phase polymor-phism and its endocrine relations. Ad-vances in Insect Physiology, 23: 1-79.

45. Injeyan HS & Tobe SS (1981). Phase poly-morphism in Schistocerca gregaria: as-sessment of juvenile hormone synthesisin relation to vitellogenesis. Journal of In-sect Physiology, 27: 203-210.

46. Pener MP & Yerushalmi Y (1998). Thephysiology of locust phase polymorphism:An update. Journal of Insect Physiology,44: 365-377.

47. Noirot C & Bordereau C (1991). Termitepolymorphism and morphogenetic hor-mones. In: Gupta AP (Editor), Morphoge-netic Hormones of Arthropods. Vol. 3.Rutgers University Press, New Bruns-wick, 295-324.

48. Boleli IC, Simoes ZLP & Hartfelder K(1998). The stomatogastric nervous sys-tem of the honey bee (Apis mellifera) in acritical phase of caste development. Jour-nal of Morphology, 236: 139-149.

49. Schmidt Capella IC & Hartfelder K (1998).

177



Juvenile hormone effect on DNA synthe-sis and apoptosis in caste-specific differ-entiation of the larval honey bee (Apismellifera L.) ovary. Journal of Insect Phys-iology, 44: 385-391.

50. Robinson GE, Strambi C, Strambi A &Feldlaufer MF (1991). Comparison of ju-venile hormone and ecdysteroid hemo-lymph titers in adult worker and queenhoney bees (Apis mellifera). Journal ofInsect Physiology, 37: 929-935.

51. Giray T & Robinson GE (1996). Commonendocrine and genetic mechanisms ofbehavioral development in male andworker honey bees and the evolution ofdivision of labor. Proceedings of the Na-tional Academy of Sciences, USA, 93:11718-11722.

52. Tozetto SO, Rachinsky A & Engels W(1997). Juvenile hormone promotes flightactivity in drones (Apis mellifera carnica).Apidologie, 28: 77-84.

53. Huang ZY & Robinson GE (1992). Honey-bee colony integration - worker-worker in-teractions mediate hormonally regulatedplasticity in division of labor. Proceedingsof the National Academy of Sciences,USA, 89: 11726-11729.

54. Huang ZY, Plettner E & Robinson GE(1998). Effects of social environment andworker mandibular glands on endocrine-mediated behavioral development inhoney bees. Journal of Comparative Phys-iology. A, Sensory Neural and BehavioralPhysiology, 183: 143-152.

55. Fahrbach SE (1997). Regulation of agepolyethism in bees and wasps by juvenilehormone. Advances in the Study of Be-havior, 26: 285-316.

56. Page RE & Robinson GE (1991). The ge-netics of division of labor in honey beecolonies. Advances in Insect Physiology,23: 117-169.

57. Pankiw T, Huang ZY, Winston ML &Robinson GE (1998). Queen mandibulargland pheromone influences workerhoney bee (Apis mellifera L.) foraging on-togeny and juvenile hormone titers. Jour-nal of Insect Physiology, 44: 685-692.

58. Kaatz H-H, Eichmüller S & Kreissl S (1994).Stimulatory effect of octopamine on juve-nile hormone biosynthesis in honey bees(Apis mellifera): Physiological and immu-nocytochemical evidence. Journal of In-sect Physiology, 40: 865-872.

59. Strambi A (1990). Physiology and repro-duction in social wasps. In: Engels W (Edi-tor), Social Insects - an Evolutionary Ap-proach to Castes and Reproduction.Springer, Berlin, 59-75.

60. O’Donnell S & Jeanne RL (1993).Methoprene accelerates age polyethismin workers of a social wasp (Polybia occi-dentalis). Physiological Entomology, 18:189-194.

61. Röseler P-F & van Honk CGJ (1990).Castes and reproduction in bumblebees.In: Engels W (Editor), Social Insects - anEvolutionary Approach to Castes and Re-production. Springer, Berlin, 147-166.

62. Bloch G & Hefetz A (1999). Regulation ofreproduction by dominant workers inbumblebee (Bombus terrestris) queen-right colonies. Behavioral Ecology and So-ciobiology, 45: 125-135.

63. Bloch G, Borst DW, Huang ZY, RobinsonGE, Cnaani J & Hefetz A (2000). Juvenilehormone titers, juvenile hormone biosyn-thesis, ovarian development and socialenvironment in Bombus terrestris. Jour-nal of Insect Physiology, 46: 47-57.

64. Bloch G, Borst DW, Huang ZY, RobinsonGE & Hefetz A (1996). Effects of socialconditions on juvenile hormone mediatedreproductive development in Bombusterrestris workers. Physiological Entomol-

ogy, 21: 257-267.65. Cameron SA & Robinson GE (1990). Juve-

nile hormone does not affect division oflabor in bumble bee colonies. Annals ofthe Entomological Society of America, 83:626-631.

66. Cnaani J, Borst DW, Huang ZY, RobinsonGE & Hefetz A (1997). Caste determina-tion in Bombus terrestris: Differences indevelopment and rates of JH biosynthe-sis between queen and worker larvae.Journal of Insect Physiology, 43: 373-381.

67. Strambi A, Strambi C, Röseler P-F &Röseler I (1984). Simultaneous determi-nation of juvenile hormone and ecdyster-oid titers in the hemolymph of bumble-bee prepupae (Bombus hypnorum and B.terrestris). General and Comparative En-docrinology, 55: 83-88.

68. Wheeler DE & Nijhout HF (1983). Soldierdetermination in the ant, Pheidole bicari-nata: Hormonal control of caste and sizewithin castes. Journal of Insect Physiolo-gy, 29: 847-854.

69. Nijhout HF & Wheeler DE (1996). Growthmodels of complex allometries in holo-metabolous insects. American Naturalist,148: 40-56.

70. Robinson GE & Vargo EL (1997). Juvenilehormone in adult eusocial Hymenoptera:Gonadotropin and behavioral pacemaker.Archives of Insect Biochemistry and Phys-iology, 35: 559-583.

71. Sommer K, Hölldobler B & Rembold H(1993). Behavioral and physiological as-pects of reproductive control in a Dia-camma species from Malaysia (Formici-dae, Ponerinae). Ethology, 94: 162-170.

72. West-Eberhard MJ (1989). Phenotypicplasticity and the origins of diversity. An-nual Review of Ecology and Systematics,20: 249-278.

insect juvenile hormone: from “status quo” to high society · bumble bees, jh integrates...

Documents