Pleiotropic Effects of methoprene-tolerant(Met), a Gene Involved in Juvenile Hormone Metabolism, on Life History Traits in Drosophila melanogaster

Life history theory assumes that there are alleles with pleiotropic effects on fitness components. Although quantitative genetic data are often consistent with pleiotropy, there are few explicit examples of pleiotropic loci. The Drosophila melanogaster gene Methoprene-tolerant (Met) may be such a locus. The Met gene product, a putative juvenile hormone receptor, facilitates the action of juvenile hormone (JH) and JH analogs; JH affects many life history traits in arthropods. Here we use quantitative complementation to investigate effects of Met mutant and wildtype alleles on female developmental time, onset of reproduction, and fecundity. Whereas the alleles did not differ in their effects on developmental time, we detected allelic variation for the onset of reproduction and for age-specific fecundity. Alleles influenced phenotypic co-variances among traits (developmental time and onset of reproduction; onset of reproduction and both early and late fecundity; early and late fecundity), suggesting that alleles of Met vary in their pleiotropic effects upon life history. Furthermore, the genetic covariance between developmental time and early fecundity attributed to alleles of Met was negative, indicating consistent pleiotropic effects among alleles on these traits. The allelic effects of Met support genetic models where pleiotropy at genes associated with hormone regulation can contribute to the evolution of life history traits.     

Comparing thyroid and insect hormone signaling

Transitions between different states of development, physiology,and life history are typically mediated by hormones. In insects,metamorphosis and reproductive maturation are regulated by aninteraction between the sesquiterpenoid juvenile hormone (JH)and the steroid 20-hydroxy-ecdysone (20E). In vertebrates andsome marine invertebrates, the lipophilic thyroid hormones (THs)affect metamorphosis and other life history transitions. Interestingly,when applied to insects, THs can physiologically mimic manyfacets of JH action, suggesting that the molecular actions ofTHs and JH/20E might be similar. Here we discuss functionalparallels between TH and JH/20E signaling in insects, with aparticular focus on the fruit fly, Drosophila melanogaster,a genetically and physiologically tractable model system. Comparingthe effects of THs with the well defined physiological rolesof insect hormones such as JH and 20E in Drosophila might provideimportant insights into hormone function and the evolution ofendocrine signaling.     

Variation in endocrine signaling underlies variation in social life history

Variation in endocrine pathways can be a major mechanism underlying life-history evolution. Yet it is unclear whether this insight, derived primarily from solitary species, explains the origins of complex life-history traits in highly social taxa. Thus, we here document and study variation in social life-history syndromes of female fecundity, behavior, and life span in selectively bred honeybee (Apis mellifera) strains. Associated variation in endocrine signaling was uncovered by RNA interference (RNAi) silencing of the juvenile hormone (JH) suppressor gene vitellogenin. High versus low endocrine reactivity in response to vitellogenin knockdown consistently correlated with rapid social behavioral ontogeny and short life span versus slow social behavioral ontogeny and long life span. Variation in JH reactivity, furthermore, was a function of variation in fecundity (ovary size and follicle development). A JH-mediated pleiotropy of female life-history traits, including fecundity, behavior, and life span, characterizes the distantly related solitary insect Drosophila. For the first time, we document a similar regulatory principle in a highly social species where most females are alloparental helpers (workers) that seldom reproduce. We conclude that variation in endocrine pathways of solitary origin can underlie variation and evolvability of complex social life-history traits.     

Diversity of juvenile Chinook salmon life history pathways

Life history variability includes phenotypic variation in morphology, age, and size at key stage transitions and arises from genotypic, environmental, and genotype-by-environment effects. Life history variation contributes to population abundance, productivity, and resilience, and management units often reflect life history classes. Recent evidence suggests that past Chinook salmon (Oncorhynchus tshawytscha) classifications (e.g., ‘stream’ and ‘ocean’ types) are not distinct evolutionary lineages, do not capture the phenotypic variation present within or among populations, and are poorly aligned with underlying ecological and developmental processes. Here we review recently reported variation in juvenile Chinook salmon life history traits and provide a refined conceptual framework for understanding the causes and consequences of the observed variability. The review reveals a broad continuum of individual juvenile life history pathways, defined primarily by transitions among developmental stages and habitat types used during freshwater rearing and emigration. Life history types emerge from discontinuities in expressed pathways when viewed at the population scale. We synthesize recent research that examines how genetic, conditional, and environmental mechanisms likely influence Chinook salmon life history pathways. We suggest that threshold models hold promise for understanding how genetic and environmental factors influence juvenile salmon life history transitions. Operational life history classifications will likely differ regionally, but should benefit from an expanded lexicon that captures the temporally variable, multi-stage life history pathways that occur in many Chinook salmon populations. An increased mechanistic awareness of life history diversity, and how it affects population fitness and resilience, should improve management, conservation, and restoration of this iconic species.     

Juvenile hormone mediates developmental integration between exaggerated traits and supportive traits in the horned flour beetle Gnatocerus cornutus

Sexually selected exaggerated traits are often coupled with modifications in other nontarget traits. In insects with weapons, enlargements of nontarget characters that functionally support the weapon often occur (i.e. supportive traits). The support of sexual traits requires developmental coordination among functionally related multiple traits—an explicit example of morphological integration. The genetic theory predicts that developmental integration among different body modules, for which development is regulated via different sets of genes, is likely to be coordinated by pleiotropic factors. However, the developmental backgrounds of morphological integrations are largely unknown. We tested the hypothesis that the juvenile hormone (JH), as a pleiotropic factor, mediates the integration between exaggerated and supportive traits in an armed beetle Gnatocerus cornutus. During combat, males of this beetle use exaggerated mandibles to lift up their opponents with the supportive traits, that is, the head and prothoracic body parts. Application of methoprene, a JH analog (JHA), during the larval to prepupal period, induced the formation of large mandibles relative to the body sizes in males. Morphometric examination of nontarget traits elucidated an increase in the relative sizes of supportive traits, including the head and prothoracic body parts. In addition, reductions in the hind wing area and elytra length, which correspond to flight and reproductive abilities, were detected. Our findings are consistent with the genetic theory and support the idea that JH is a key pleiotropic factor that coordinates the developmental integration of exaggerated traits and supportive characters, as well as resource allocation trade‐offs.     

Disruption of insulin pathways alters trehalose level and abolishes sexual dimorphism in locomotor activity in Drosophila

Insulin signaling pathways are implicated in several physiological processes in invertebrates, including the control of growth and life span; the latter of these has also been correlated with juvenile hormone (JH) deficiency. In turn, JH levels have been correlated with sex-specific differences in locomotor activity. Here, the involvement of the insulin signaling pathway in sex-specific differences in locomotor activity was investigated in Drosophila. Ablation of insulin-producing neurons in the adult pars-intercerebralis was found to increase trehalosemia and to abolish sexual dimorphism relevant to locomotion. Conversely, hyper-insulinemia induced by insulin injection or by over-expression of an insulin-like peptide decreases trehalosemia but does not affect locomotive behavior. Moreover, we also show that in the head of adult flies, the insulin receptor (InR) is expressed only in the fat body surrounding the brain. While both male and female InR mutants are hyper-trehalosemic, they exhibit similar patterns of locomotor activity. Our results indicate that first, insulin controls trehalosemia in adults, and second, like JH, it controls sex-specific differences in the locomotor activity of adult Drosophila in a manner independent of its effect on trehalose metabolism.     

A search for trade-offs among life history traits inDrosophila melanogaster

Summary Are there underlying developmental and physiological properties of organisms that can be used to build a general theory of life history evolution? Much of the theoretical work on the evolution of life histories is based on the premise of negative developmental and genetic correlations among life history traits. If negative correlations do not exist as a general rule then no general theory taking them into account is possible. Negative genetic correlations among life history traits can come about by antagonistic pleiotropy. One cause of antagonistic pleiotropy is cost allocation trade-offs. Since cost allocation trade-offs are due to underlying physiological constraints they are expected to be common to closely related groups. A second form of antagonistic pleiotropy is specialization of genotypes to different niches. This type of antagonistic pleiotropy is expected to be specific to each population. We looked for trade-offs in life history traits of longevity and fecundity inDrosophila melanogaster. We used a half-sib mating design and raised the offspring at two temperatures, 19°C and 25°C. Correlations between longevity and fecundity showed some evidence of antagonistic pleiotropy at high temperature with no evidence of any trade-offs at low temperature. Correlations of early and late fecundity traits did show evidence of cost allocation trade-offs at both temperatures. Antagonistic pleiotropy was also found for cross-environmental correlations of fecundity traits. We conclude that, although life history trade-offs can not be generally assumed, they are frequently found among functionally related traits. Thus, we provide guidelines for the development of general theories of life history evolution.     

Translating environmental gradients into discontinuous reaction norms via hormone signalling in a polyphenic butterfly

Polyphenisms—the expression of discrete phenotypic morphs in response to environmental variation—are examples of phenotypic plasticity that may potentially be adaptive in the face of predictable environmental heterogeneity. In the butterfly Bicyclus anynana, we examine the hormonal regulation of phenotypic plasticity that involves divergent developmental trajectories into distinct adult morphs for a suite of traits as an adaptation to contrasting seasonal environments. This polyphenism is induced by temperature during development and mediated by ecdysteroid hormones. We reared larvae at separate temperatures spanning the natural range of seasonal environments and measured reaction norms for ecdysteroids, juvenile hormones (JHs) and adult fitness traits. Timing of peak ecdysteroid, but not JH titres, showed a binary response to the linear temperature gradient. Several adult traits (e.g. relative abdomen mass) responded in a similar, dimorphic manner, while others (e.g. wing pattern) showed a linear response. This study demonstrates that hormone dynamics can translate a linear environmental gradient into a discrete signal and, thus, that polyphenic differences between adult morphs can already be programmed at the stage of hormone signalling during development. The range of phenotypic responses observed within the suite of traits indicates both shared regulation and independent, trait-specific sensitivity to the hormone signal.     

Juvenile hormone as a regulator of the trade-off between reproduction and life span in Drosophila melanogaster

Trade-offs between reproduction and life span are ubiquitous, but little is known about their underlying mechanisms. Here we combine treatment with the juvenile hormone analog (JHa) methoprene and experimental evolution in Drosophila melanogaster to study the potential role of juvenile hormone (JH) in mediating such trade-offs at both the physiological and evolutionary level. Exposure to JHa in the larval medium (and up to 24 h posteclosion) increased early life fecundity but reduced life span of normal (unselected) flies, supporting the physiological role of JH in mediating the trade-off. This effect was much smaller for life span, and not detectable for fecundity, in fly lines previously bred for 19 generations on a medium containing JHa. Furthermore, these selection lines lived longer than unselected controls even in the absence of JHa treatment, without a detectable reduction in early life fecundity. Thus, selection for resistance to JHa apparently induced some evolutionary changes in JH metabolism or signaling, which led to longer life span as a correlated response. This supports the hypothesis that JH may mediate evolution of longer life span, but--contrary to our expectation-this apparently does not need to trade--off with fecundity.     

Hmgcr in the corpus allatum controls sexual dimorphism of locomotor activity and body size via the insulin pathway in Drosophila

The insulin signaling pathway has been implicated in several physiological and developmental processes. In mammals, it controls expression of 3-Hydroxy-3-Methylglutaryl CoA Reductase (HMGCR), a key enzyme in cholesterol biosynthesis. In insects, which can not synthesize cholesterol de novo, the HMGCR is implicated in the biosynthesis of juvenile hormone (JH). However, the link between the insulin pathway and JH has not been established. In Drosophila, mutations in the insulin receptor (InR) decrease the rate of JH synthesis. It is also known that both the insulin pathway and JH play a role in the control of sexual dimorphism in locomotor activity. In studies here, to demonstrate that the insulin pathway and HMGCR are functionally linked in Drosophila, we first show that hmgcr mutation also disrupts the sexual dimorphism. Similarly to the InR, HMGCR is expressed in the corpus allatum (ca), which is the gland where JH biosynthesis occurs. Two p[hmgcr-GAL4] lines were therefore generated where RNAi was targeted specifically against the HMGCR or the InR in the ca. We found that RNAi-HMGCR blocked HMGCR expression, while the RNAi-InR blocked both InR and HMGCR expression. Each RNAi caused disruption of sexual dimorphism and produced dwarf flies at specific rearing temperatures. These results provide evidence: (i) that HMGCR expression is controlled by the InR and (ii) that InR and HMGCR specifically in the ca, are involved in the control of body size and sexual dimorphism of locomotor activity.     

Constraint and developmental dissociation of phenotypic integration in a genetically accommodated trait

SUMMARY The genetic accommodation of novel adaptive traits may be accompanied by the evolution of correlated traits that constrain adaptive evolution. Very little is known about the removal of maladaptive correlated traits. In the present study, body size was found to have evolved as a correlated trait during the artificial selection for a polyphenism and a monophenism, and the developmental basis underlying this correlated trait was investigated. The body size and coloration were found to be developmentally integrated by titers of the insect developmental hormone, juvenile hormone (JH). Attempts to uncouple the two traits resulted in the evolution of one of the body size determinants—the critical weight—but not the delay period whose evolution is constrained by JH titers. Thus, maladaptive correlated traits can be removed when multiple developmental modules exist, and the evolution of one or more of these modules is not constrained.     

Juvenile Hormone and Insulin Regulate Trehalose Homeostasis in the Red Flour Beetle,Tribolium castaneum

Insulin/IGF-1 signaling (IIS) has been well studied for its role in the control of life span extension and resistance to a variety of stresses. The Drosophila melanogaster insulin-like receptor (InR) mutant showed extended life span due to reduced juvenile hormone (JH) levels. However, little is known about the mechanism of cross talk between IIS and JH in regulation of life span extension and resistance to starvation. In the current study, we investigated the role of IIS and JH signaling in regulation of resistance to starvation. Reduction in JH biosynthesis, JH action, or insulin-like peptide 2 (ILP2) syntheses by RNA interference (RNAi)-aided knockdown in the expression of genes coding for juvenile hormone acid methyltransferase (JHAMT), methoprene-tolerant (Met), or ILP2 respectively decreased lipid and carbohydrate metabolism and extended the survival of starved beetles. Interestingly, the extension of life span could be restored by injection of bovine insulin into JHAMT RNAi beetles but not by application of JH III to ILP2 RNAi beetles. These data suggest that JH controls starvation resistance by regulating synthesis of ILP2. More importantly, JH regulates trehalose homeostasis, including trehalose transport and metabolism, and controls utilization of stored nutrients in starved adults.     

Juvenile hormone regulates extreme mandible growth in male stag beetles

The morphological diversity of insects is one of the most striking phenomena in biology. Evolutionary modifications to the relative sizes of body parts, including the evolution of traits with exaggerated proportions, are responsible for a vast range of body forms. Remarkable examples of an insect trait with exaggerated proportions are the mandibular weapons of stag beetles. Male stag beetles possess extremely enlarged mandibles which they use in combat with rival males over females. As with other sexually selected traits, stag beetle mandibles vary widely in size among males, and this variable growth results from differential larval nutrition. However, the mechanisms responsible for coupling nutrition with growth of stag beetle mandibles (or indeed any insect structure) remain largely unknown. Here, we demonstrate that during the development of male stag beetles (Cyclommatus metallifer), juvenile hormone (JH) titers are correlated with the extreme growth of an exaggerated weapon of sexual selection. We then investigate the putative role of JH in the development of the nutritionally-dependent, phenotypically plastic mandibles, by increasing hemolymph titers of JH with application of the JH analog fenoxycarb during larval and prepupal developmental periods. Increased JH signaling during the early prepupal period increased the proportional size of body parts, and this was especially pronounced in male mandibles, enhancing the exaggerated size of this trait. The direction of this response is consistent with the measured JH titers during this same period. Combined, our results support a role for JH in the nutrition-dependent regulation of extreme mandible growth in this species. In addition, they illuminate mechanisms underlying the evolution of trait proportion, the most salient feature of the evolutionary diversification of the insects.     

Mediation of vertebrate life histories via insulin‐like growth factor‐1

Life‐history traits describe parameters associated with growth, size, survival, and reproduction. Life‐history variation is a hallmark of biological diversity, yet researchers commonly observe that one of the major axes of life‐history variation after controlling for body size involves trade‐offs among growth, reproduction, and longevity. This persistent pattern of covariation among these specific traits has engendered a search for shared mechanisms that could constrain or facilitate production of variation in life‐history strategies. Endocrine traits are one candidate mechanism that may underlie the integration of life history and other phenotypic traits. However, the vast majority of this research has been on the effects of steroid hormones such as glucocorticoids and androgens on life‐history trade‐offs. Here we propose an expansion of the focus on glucocorticoids and gonadal hormones and review the potential role of insulin‐like growth factor‐1 (IGF‐1) in shaping the adaptive integration of multiple life‐history traits. IGF‐1 is a polypeptide metabolic hormone largely produced by the liver. We summarize a vast array of research demonstrating that IGF‐1 levels are susceptible to environmental variation and that IGF‐1 can have potent stimulatory effects on somatic growth and reproduction but decrease lifespan. We review the few studies in natural populations that have measured plasma IGF‐1 concentrations and its associations with life‐history traits or other characteristics of the organism or its environment. We focus on two case studies that found support for the hypothesis that IGF‐1 mediates adaptive divergence in suites of life‐history traits in response to varying ecological conditions or artificial selection. We also examine what we view as potentially fruitful avenues of research on this topic, which until now has been rarely investigated by evolutionary ecologists. We discuss how IGF‐1 may facilitate adaptive plasticity in life‐history strategies in response to early environmental conditions and also how selection on loci controlling IGF‐1 signaling may mediate population divergence and eventual speciation. After consideration of the interactions among androgens, glucocorticoids, and IGF‐1 we suggest that IGF‐1 be considered a suitable candidate mechanism for mediating life‐history traits. Finally, we discuss what we can learn about IGF‐1 from studies in free‐ranging animals. The voluminous literature in laboratory and domesticated animals documenting relationships among IGF‐1, growth, reproduction, and lifespan demonstrates the potential for a number of new research questions to be asked in free‐ranging animals. Examining how IGF‐1 mediates life‐history traits in free‐ranging animals could lead to great insight into the mechanisms that influence life‐history variation.     

The Drosophila juvenile hormone receptor candidates methoprene-tolerant (MET) and germ cell-expressed (GCE) utilize a conserved LIXXL motif to bind the FTZ-F1 nuclear receptor

Juvenile hormone (JH) has been implicated in many developmental processes in holometabolous insects, but its mechanism of signaling remains controversial. We previously found that in Drosophila Schneider 2 cells, the nuclear receptor FTZ-F1 is required for activation of the E75A gene by JH. Here, we utilized insect two-hybrid assays to show that FTZ-F1 interacts with two JH receptor candidates, the bHLH-PAS paralogs MET and GCE, in a JH-dependent manner. These interactions are severely reduced when helix 12 of the FTZ-F1 activation function 2 (AF2) is removed, implicating AF2 as an interacting site. Through homology modeling, we found that MET and GCE possess a C-terminal α-helix featuring a conserved motif LIXXL that represents a novel nuclear receptor (NR) box. Docking simulations supported by two-hybrid experiments revealed that FTZ-F1·MET and FTZ-F1·GCE heterodimer formation involves a typical NR box-AF2 interaction but does not require the canonical charge clamp residues of FTZ-F1 and relies primarily on hydrophobic contacts, including a unique interaction with helix 4. Moreover, we identified paralog-specific features, including a secondary interaction site found only in MET. Our findings suggest that a novel NR box enables MET and GCE to interact JH-dependently with the AF2 of FTZ-F1.     

Study of Natural Genetic Variation in Early Fitness Traits Reveals Decoupling Between Larval and Pupal Developmental Time in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

Characterizing the relationships between genotype and phenotype for developmental adaptive traits is essential to understand the evolutionary dynamics underlying biodiversity. In holometabolous insects, the time to reach the reproductive stage and pupation site preference are two such traits. Here we characterize aspects of the genetic architecture for Developmental Time (decomposed in Larval and Pupal components) and Pupation Height using lines derived from three natural populations of Drosophila melanogaster raised at two temperatures. For all traits, phenotypic differences and variation in plasticity between populations suggest adaptation to the original thermal regimes. However, high variability within populations shows that selection does not exhaust genetic variance for these traits. This could be partly explained by local adaptation, environmental heterogeneity and modifications in the genetic architecture of traits according to environment and ontogenetic stage. Indeed, our results show that the genetic factors affecting Developmental Time and Pupation Height are temperature-specific. Varying relationships between Larval and Pupal Developmental Time between and within populations also suggest stage-specific modifications of genetic architecture for this trait. This flexibility would allow for a somewhat independent evolution of adaptive traits at different environments and life stages, favoring the maintenance of genetic variability and thus sustaining the traits’ evolvabilities.