Pleiotropic effects of juvenile hormone in ant queens and the escape from the reproduction-immunocompetence trade-off

The ubiquitous trade-off between survival and costly reproduction is one of the most fundamental constraints governing life-history evolution. In numerous animals, gonadotropic hormones antagonistically suppressing immunocompetence cause this trade-off. The queens of many social insects defy the reproduction-survival trade-off, achieving both an extraordinarily long life and high reproductive output, but how they achieve this is unknown. Here we show experimentally, by integrating quantification of gene expression, physiology and behaviour, that the long-lived queens of the ant Lasius niger have escaped the reproduction-immunocompetence trade-off by decoupling the effects of a key endocrine regulator of fertility and immunocompetence in solitary insects, juvenile hormone (JH). This modification of the regulatory architecture enables queens to sustain a high reproductive output without elevated JH titres and suppressed immunocompetence, providing an escape from the reproduction-immunocompetence trade-off that may contribute to the extraordinary lifespan of many social insect queens.

Hairy and Groucho mediate the action of juvenile hormone receptor Methoprene-tolerant in gene repression

The arthropod-specific juvenile hormone (JH) controls numerous essential functions. Its involvement in gene activation is known to be mediated by the transcription factor Methoprene-tolerant (Met), which turns on JH-controlled genes by directly binding to E-box-like motifs in their regulatory regions. However, it remains unclear how JH represses genes. We used the Aedes aegypti female mosquito, in which JH is necessary for reproductive maturation, to show that a repressor, Hairy, is required for the gene-repressive action of JH and Met. The RNA interference (RNAi) screen for Met and Hairy in the Aedes female fat body revealed a large cohort of Met- and Hairy-corepressed genes. Analysis of selected genes from this cohort demonstrated that they are repressed by JH, but RNAi of either Met or Hairy renders JH ineffective in repressing these genes in an in vitro fat-body culture assay. Moreover, this JH action was prevented by the addition of the translational inhibitor cycloheximide (CHX) to the culture, indicating the existence of an indirect regulatory hierarchy. The lack of Hairy protein in the CHX-treated tissue was verified using immunoblot analysis, and the upstream regions of Met/Hairy-corepressed genes were shown to contain common binding motifs that interact with Hairy. Groucho (gro) RNAi silencing phenocopied the effect of Hairy RNAi knockdown, indicating that it is involved in the JH/Met/Hairy hierarchy. Finally, the requirement of Hairy and Gro for gene repression was confirmed in a cell transfection assay. Thus, our study has established that Hairy and its cofactor Gro mediate the repressive function of JH and Met.

Homeodomain Protein Scr Regulates the Transcription of Genes Involved in Juvenile Hormone Biosynthesis in the Silkworm

The silkworm Dominant trimolting (Moltinism, M³) mutant undergoes three larval molts and exhibits precocious metamorphosis. In this study, we found that compared with the wild-type (WT) that undergoes four larval molts, both the juvenile hormone (JH) concentration and the expression of the JH-responsive gene Krüppel homolog 1 (Kr-h1) began to be greater in the second instar of the M³ mutant. A positional cloning analysis revealed that only the homeodomain transcription factor gene Sex combs reduced (Scr) is located in the genomic region that is tightly linked to the M³ locus. The expression level of the Scr gene in the brain-corpora cardiaca-corpora allata (Br-CC-CA) complex, which controls the synthesis of JH, was very low in the final larval instar of both the M³ and WT larvae, and exhibited a positive correlation with JH titer changes. Importantly, luciferase reporter analysis and electrophoretic mobility shift assay (EMSA) demonstrated that the Scr protein could promote the transcription of genes involved in JH biosynthesis by directly binding to the cis-regulatory elements (CREs) of homeodomain protein on their promoters. These results conclude that the homeodomain protein Scr is transcriptionally involved in the regulation of JH biosynthesis in the silkworm.

The development of adultoid reproductives and brachypterous neotenic reproductives from the last instar nymphs in Reticulitermes labralis (Isoptera: Rhinotermitidae): a comparative study

Secondary reproductives develop primarily from nymphs. However, they have been rarely studied; in particular, the development of adultoid reproductives (AR) with floppy wings is still unclear. In this study, the change in juvenile hormone (JH) levels, vitellogenin gene expression, and oogenesis during the development of AR and brachypterous neotenic reproductives (BN) from the last instar nymphs of Reticulitermes labralis are investigated and compared. The results showed that the AR derived from the last instar nymphs by molting, and they were more similar to neotenic reproductives in morphology. In addition, the paired AR were not able to survive in the absence of workers. In R. labralis, the process of the last instar nymphs developing into AR and BN took an increase in JH level as a starting point. The JH level of the last instar nymphs molting into BN was approximately 1.5-fold higher than that of the AR. Additionally, The JHIII level of BN peaked on day 5, and that of AR peaked on day 10, which induced the onset of vitellogenesis in BN and AR, respectively. After molting, the vitellogenin gene expression levels of both BN and AR initially increased and then declined, and the expression levels in the BN were significantly higher than those in the AR. In addition, the oocytes of BN matured earlier than those of the AR, and the number of eggs laid by the BN was higher than the number laid by the AR. Our results demonstrate that, in R. labralis, the last instar nymphs can develop into AR, which are significantly different from BN in their development.

High juvenile hormone titre and abdominal activation of JH signalling may induce reproduction of termite neotenics

Termite castes are a key example of polyphenism, in which reproductive division of labour is clearly seen in colonies. The reproductive castes in termites include primary and neotenic reproductives; primary reproductives found a new colony whereas neotenics succeed them in the reproductive role when the primary reproductives die or become senescent. Neotenics usually differentiate from nymphs or workers by developing functional gonads while retaining juvenile characteristics; however, the developmental mechanism during neotenic differentiation remains poorly understood. Juvenile hormone (JH) mediates a number of aspects of developmental regulation in caste differentiation in termites. In the present study we quantified JH titres in neotenic reproductives of Reticulitermes speratus, and compared these with other developmental stages. In addition, expression changes in JH signalling gene homologues (Methoprene-tolerant [Met], Krüppel-homolog1, Broad-Complex) in the head, thorax and abdomen were investigated during neotenic differentiation. Finally, we examined the function of Met in reproduction of neotenics by RNA interference (RNAi). Our results showed that the JH titres of neotenics were significantly higher than those of nymphs and workers. JH signalling genes were highly expressed in neotenic abdomens, compared with those in workers and nymphs. Met RNAi resulted in the inhibition of vitellogenin gene expression in newly moulted neotenics. These results suggest that the fertility of neotenics might be controlled by a large increase of JH titres and body-part-specific activation of JH signalling pathways.

Knockout silkworms reveal a dispensable role for juvenile hormones in holometabolous life cycle

Insect juvenile hormones (JHs) prevent precocious metamorphosis and allow larvae to undergo multiple rounds of status quo molts. However, the roles of JHs during the embryonic and very early larval stages have not been fully understood. We generated and characterized knockout silkworms (Bombyx mori) with null mutations in JH biosynthesis or JH receptor genes using genome-editing tools. We found that embryonic growth and morphogenesis are largely independent of JHs in Bombyx and that, even in the absence of JHs or JH signaling, pupal characters are not formed in first- or second-instar larvae, and precocious metamorphosis is induced after the second instar at the earliest. We also show by mosaic analysis that a pupal specifier gene broad, which is dramatically up-regulated in the late stage of the last larval instar, is essential for pupal commitment in the epidermis. Importantly, the mRNA expression level of broad, which is thought to be repressed by JHs, remained at very low basal levels during the early larval instars of JH-deficient or JH signaling-deficient knockouts. Therefore, our study suggests that the long-accepted paradigm that JHs maintain the juvenile status throughout larval life should be revised because the larval status can be maintained by a JH-independent mechanism in very early larval instars. We propose that the lack of competence for metamorphosis during the early larval stages may result from the absence of an unidentified broad-inducing factor, i.e., a competence factor.

How Did Arthropod Sesquiterpenoids and Ecdysteroids Arise? Comparison of Hormonal Pathway Genes in Noninsect Arthropod Genomes

The phylum Arthropoda contains the largest number of described living animal species, with insects and crustaceans dominating the terrestrial and aquatic environments, respectively. Their successful radiations have long been linked to their rigid exoskeleton in conjunction with their specialized endocrine systems. In order to understand how hormones can contribute to the evolution of these animals, here, we have categorized the sesquiterpenoid and ecdysteroid pathway genes in the noninsect arthropod genomes, which are known to play important roles in the regulation of molting and metamorphosis in insects. In our analyses, the majority of gene homologs involved in the biosynthetic, degradative, and signaling pathways of sesquiterpenoids and ecdysteroids can be identified, implying these two hormonal systems were present in the last common ancestor of arthropods. Moreover, we found that the “Broad-Complex” was specifically gained in the Pancrustacea, and the innovation of juvenile hormone (JH) in the insect linage correlates with the gain of the JH epoxidase (CYP15A1/C1) and the key residue changes in the binding domain of JH receptor (“Methoprene-tolerant”). Furthermore, the gain of “Phantom” differentiates chelicerates from the other arthropods in using ponasterone A rather than 20-hydroxyecdysone as molting hormone. This study establishes a comprehensive framework for interpreting the evolution of these vital hormonal pathways in these most successful animals, the arthropods, for the first time.

NMDA receptor activation upstream of methyl farnesoate signaling for short day-induced male offspring production in the water flea, Daphnia pulex

BACKGROUND

The cladoceran crustacean Daphnia pulex produces female offspring by parthenogenesis under favorable conditions, but in response to various unfavorable external stimuli, it produces male offspring (environmental sex determination: ESD). We recently established an innovative system for ESD studies using D. pulex WTN6 strain, in which the sex of the offspring can be controlled simply by changes in the photoperiod: the long-day and short-day conditions can induce female and male offspring, respectively. Taking advantage of this system, we demonstrated that de novo methyl farnesoate (MF) synthesis is necessary for male offspring production. These results indicate the key role of innate MF signaling as a conductor between external environmental stimuli and the endogenous male developmental pathway. Despite these findings, the molecular mechanisms underlying up- and downstream signaling of MF have not yet been well elucidated in D. pulex.

RESULTS

To elucidate up- and downstream events of MF signaling during sex determination processes, we compared the transcriptomes of daphnids reared under the long-day (female) condition with short-day (male) and MF-treated (male) conditions. We found that genes involved in ionotropic glutamate receptors, known to mediate the vast majority of excitatory neurotransmitting processes in various organisms, were significantly activated in daphnids by the short-day condition but not by MF treatment. Administration of specific agonists and antagonists, especially for the N-methyl-D-aspartic acid (NMDA) receptor, strongly increased or decreased, respectively, the proportion of male-producing mothers. Moreover, we also identified genes responsible for male production (e.g., protein kinase C pathway-related genes). Such genes were generally shared between the short-day reared and MF-treated daphnids.

CONCLUSIONS

We identified several candidate genes regulating ESD which strongly suggests that these genes may be essential factors for male offspring production as an upstream regulator of MF signaling in D. pulex. This study provides new insight into the fundamental mechanisms underlying how living organisms alter their phenotypes in response to various external environments.

Molecular cloning and characterization of the allatotropin precursor and receptor in the desert locust, Schistocerca gregaria

Allatotropins (ATs) are pleiotropic neuropeptides initially isolated from the tobacco hornworm, Manduca sexta. In 2008, the first receptor for AT-like peptides (ATR) was characterized in Bombyx mori. Since then, ATRs have also been characterized in M. sexta, Tribolium castaneum, Aedes aegypti and Bombus terrestris. These receptors show sequence similarity to vertebrate orexin (ORX) receptors. When generating an EST-database of the desert locust (Schistocerca gregaria) central nervous system, we found cDNA sequences encoding the Schgr-AT precursor and a fragment of its putative receptor. This receptor cDNA has now been completed and functionally expressed in mammalian cell lines. Activation of this receptor, designated as Schgr-ATR, by Schgr-AT caused an increase in intracellular calcium ions, as well as cyclic AMP (cAMP), with an EC₅₀ value in the nanomolar range. In addition, the transcript distribution of both the Schgr-AT precursor and Schgr-ATR was investigated by means of quantitative real-time PCR. Moreover, we found more evidence for the myotropic and allatostimulatory actions of Schgr-AT in the desert locust. These data are discussed and situated in a broader context by comparison with literature data on AT and ATR in insects.

MiR-2 family regulates insect metamorphosis by controlling the juvenile hormone signaling pathway

In 2009 we reported that depletion of Dicer-1, the enzyme that catalyzes the final step of miRNA biosynthesis, prevents metamorphosis in Blattella germanica. However, the precise regulatory roles of miRNAs in the process have remained elusive. In the present work, we have observed that Dicer-1 depletion results in an increase of mRNA levels of Krüppel homolog 1 (Kr-h1), a juvenile hormone-dependent transcription factor that represses metamorphosis, and that depletion of Kr-h1 expression in Dicer-1 knockdown individuals rescues metamorphosis. We have also found that the 3'UTR of Kr-h1 mRNA contains a functional binding site for miR-2 family miRNAs (for miR-2, miR-13a, and miR-13b). These data suggest that metamorphosis impairment caused by Dicer-1 and miRNA depletion is due to a deregulation of Kr-h1 expression and that this deregulation is derived from a deficiency of miR-2 miRNAs. We corroborated this by treating the last nymphal instar of B. germanica with an miR-2 inhibitor, which impaired metamorphosis, and by treating Dicer-1-depleted individuals with an miR-2 mimic to allow nymphal-to-adult metamorphosis to proceed. Taken together, the data indicate that miR-2 miRNAs scavenge Kr-h1 transcripts when the transition from nymph to adult should be taking place, thus crucially contributing to the correct culmination of metamorphosis.

Identification of plant compounds that disrupt the insect juvenile hormone receptor complex

Insects impact human health through vector-borne diseases and cause major economic losses by damaging crops and stored agricultural products. Insect-specific growth regulators represent attractive control agents because of their safety to the environment and humans. We identified plant compounds that serve as juvenile hormone antagonists (PJHANs). Using the yeast two-hybrid system transformed with the mosquito JH receptor as a reporter system, we demonstrate that PJHANs affect the JH receptor, methoprene-tolerant (Met), by disrupting its complex with CYCLE or FISC, formation of which is required for mediating JH action. We isolated five diterpene secondary metabolites with JH antagonist activity from two plants: Lindera erythrocarpa and Solidago serotina. They are effective in causing mortality of mosquito larvae at relatively low LD50 values. Topical application of two diterpenes caused reduction in the expression of Met target genes and retardation of follicle development in mosquito ovaries. Hence, the newly discovered PJHANs may lead to development of a new class of safe and effective pesticides.

Exaggerated trait growth in insects

Animal structures occasionally attain extreme proportions, eclipsing in size the surrounding body parts. We review insect examples of exaggerated traits, such as the mandibles of stag beetles (Lucanidae), the claspers of praying mantids (Mantidae), the elongated hindlimbs of grasshoppers (Orthoptera: Caelifera), and the giant heads of soldier ants (Formicidae) and termites (Isoptera). Developmentally, disproportionate growth can arise through trait-specific modifications to the activity of at least four pathways: the sex determination pathway, the appendage patterning pathway, the insulin/IGF signaling pathway, and the juvenile hormone/ecdysteroid pathway. Although most exaggerated traits have not been studied mechanistically, it is already apparent that distinct developmental mechanisms underlie the evolution of the different types of exaggerated traits. We suggest this reflects the nature of selection in each instance, revealing an exciting link between mechanism, form, and function. We use this information to make explicit predictions for the types of regulatory pathways likely to underlie each type of exaggerated trait.

Developmental regulation of ecdysone receptor (EcR) and EcR-controlled gene expression during pharate-adult development of honeybees (Apis mellifera)

Major developmental transitions in multicellular organisms are driven by steroid hormones. In insects, these, together with juvenile hormone (JH), control development, metamorphosis, reproduction and aging, and are also suggested to play an important role in caste differentiation of social insects. Here, we aimed to determine how EcR transcription and ecdysteroid titers are related during honeybee postembryonic development and what may actually be the role of EcR in caste development of this social insect. In addition, we expected that knocking-down EcR gene expression would give us information on the participation of the respective protein in regulating downstream targets of EcR. We found that in Apis mellifera females, EcR-A is the predominantly expressed variant in postembryonic development, while EcR-B transcript levels are higher in embryos, indicating an early developmental switch in EcR function. During larval and pupal stages, EcR-B expression levels are very low, while EcR-A transcripts are more variable and abundant in workers compared to queens. Strikingly, these transcript levels are opposite to the ecdysteroid titer profile. 20-hydroxyecdysone (20E) application experiments revealed that low 20E levels induce EcR expression during development, whereas high ecdysteroid titers seem to be repressive. By means of RNAi-mediated knockdown (KD) of both EcR transcript variants we detected the differential expression of 234 poly-A⁺ transcripts encoding genes such as CYPs, MRJPs and certain hormone response genes (Kr-h1 and ftz-f1). EcR-KD also promoted the differential expression of 70 miRNAs, including highly conserved ones (e.g., miR-133 and miR-375), as well honeybee-specific ones (e.g., miR-3745 and miR-3761). Our results put in evidence a broad spectrum of EcR-controlled gene expression during postembryonic development of honeybees, revealing new facets of EcR biology in this social insect.

The roles of juvenile hormone, insulin/target of rapamycin, and ecydsone signaling in regulating body size in Drosophila

Understanding how organisms regulate their body size has interested biologists for decades. Recent work has shown that both insulin/target of rapamycin (TOR) signaling and the steroid hormone ecdysone act to regulate rates of growth and the duration of the growth period in the fruit fly, Drosophila melanogaster. Our recent work has uncovered a third level of interaction, whereby juvenile hormone (JH) regulates levels of both ecdysone and insulin/TOR signaling to control growth rates. These studies highlight a complex network of interactions involved in regulating body and organ size.

Bemisia tabaci females from the Mediterranean (Q) species detect and avoid laying eggs in the presence of pyriproxyfen, a juvenile hormone analogue

BACKGROUND

Pyriproxyfen, a juvenile hormone analogue, disrupts embryogenesis, metamorphosis and adult formation in Bemisia tabaci, but does not directly affect adult females. The effect of pyriproxyfen on egg-laying preference and performance of B. tabaci females and the influence of resistance to pyriproxyfen on these reproductive behaviours were studied.

RESULTS

Choice experiments utilising cotton plants treated and not treated with pyriproxyfen revealed a significant preference for egg laying on non-treated plants both by resistant and susceptible females. No-choice assays indicated a reduction of ∼60% in the number of eggs laid on pyriproxyfen-treated plants by both resistant and susceptible females. The reduction in oviposition on treated plants was not accompanied with reduced expression of the vitellogenin gene or a delay in oocyte maturation, but significant accumulation of mature oocytes in the ovaries was observed, and could be reversed by transferring the females to non-treated plants.

CONCLUSION

Pyriproxyfen caused reduced oviposition and enhanced mature oocyte accumulation in pyriproxyfen-resistant and pyriproxyfen-susceptible females. These findings can be explained by two alternative mechanisms: pyriproxyfen-regulated physiological arrest of oviposition, involving hormonal regulation of myotrophic factors, or the hierarchy-threshold behavioural theory of host choice, in which pyriproxyfen-treated plants are defined as low-quality hosts. Aspects of application are discussed.

Reproductive Soldier-like Individuals Induced by Juvenile Hormone Analog Treatment in Zootermopsis nevadensis (Isoptera, Archotermopsidae)

Eusocial insects have highly sophisticated societies, showing a conspicuous division of labor associated with different phenotypes. These castes show specific morphologies adapted to discrete tasks. Termite castes are divided into reproductives, workers, and soldiers. Individuals with soldier-like heads as well as developed gonads have been recorded in several primitive families, and are called reproductive soldiers. In some termite species, however, a trade-off-like developmental relationship has been shown between soldier and imaginal characteristics. Thus, while the mechanism that regulates the development of both characteristics in the same individual is interesting, the details are still unclear. We focused on juvenile hormone (JH), which is involved not only in termite caste differentiation, but also in the gonad development of many insects, and we aimed to clarify the effects of JH on the differentiation of reproductive soldiers in Zootermopsis nevadensis. We succeeded in the induction of individuals with reproductive soldier-like gross morphologies by JH analog (JHA) application to several developmental stages. We also observed that gonad development was significantly promoted by JHA application after molts in the induced reproductive soldier-like individuals, but not in natural soldiers. Finally, we confirmed that the gene expression level of vitellogenin was extremely high in the induced reproductive soldier-like individuals following JHA treatment after the molt. These results suggested that soldiers do not have regulatory mechanisms for gonad development involving JH, and the regulation of reproductive soldiers development is different from that of soldiers. Reproductive soldiers may have evolved independently from the soldier caste rather than from an intermediate stage of soldier evolution.

Methoprene-tolerant 1 regulates gene transcription to maintain insect larval status

Insect molting and metamorphosis are regulated by two hormones: 20-hydroxyecdysone (20E) and juvenile hormone (JH). The hormone 20E regulates gene transcription via the nuclear receptor EcR to promote metamorphosis, whereas JH regulates gene transcription via its intracellular receptor methoprene-tolerant (Met) to prevent larval-pupal transition. However, the function and mechanism of Met in various insect developments are not well understood. We propose that Met1 plays a key role in maintaining larval status not only by promoting JH-responsive gene transcription but also by repressing 20E-responsive gene transcription in the Lepidopteran insect Helicoverpa armigera. Met1 protein is increased during feeding stage and decreased during molting and metamorphic stages. Met1 is upregulated by JH III and a low concentration of 20E independently, but is downregulated by a high concentration of 20E. Knockdown of Met1 in larvae causes precocious pupation, decrease in JH pathway gene expression, and increase in 20E pathway gene expression. Met1 interacts with heat shock protein 90 and binds to JH response element to regulate Krüppel homolog 1 transcription in JH III induction. Met1 interacts with ultraspiracle protein 1 (USP1) to repress 20E transcription complex EcRB1/USP1 formation and binding to ecdysone response element. These data indicate that JH via Met1 regulates JH pathway gene expression and represses 20E pathway gene expression to maintain the larval status.

Genome-wide comparison of genes involved in the biosynthesis, metabolism, and signaling of juvenile hormone between silkworm and other insects

Juvenile hormone (JH) contributes to the regulation of larval molting and metamorphosis in insects. Herein, we comprehensively identified 55 genes involved in JH biosynthesis, metabolism and signaling in the silkworm (Bombyx mori) as well as 35 in Drosophila melanogaster, 35 in Anopheles gambiae, 36 in Apis mellifera, 47 in Tribolium castaneum, and 44 in Danaus plexippus. Comparative analysis showed that each gene involved in the early steps of the mevalonate (MVA) pathway, in the neuropeptide regulation of JH biosynthesis, or in JH signaling is a single copy in B. mori and other surveyed insects, indicating that these JH-related pathways or steps are likely conserved in all surveyed insects. However, each gene participating in the isoprenoid branch of JH biosynthesis and JH metabolism, together with the FPPS genes for catalyzing the final step of the MVA pathway of JH biosynthesis, exhibited an obvious duplication in Lepidoptera, including B. mori and D. plexippus. Microarray and real-time RT-PCR analysis revealed that different copies of several JH-related genes presented expression changes that correlated with the dynamics of JH titer during larval growth and metamorphosis. Taken together, the findings suggest that duplication-derived copy variation of JH-related genes might be evolutionarily associated with the variation of JH types between Lepidoptera and other insect orders. In conclusion, our results provide useful clues for further functional analysis of JH-related genes in B. mori and other insects.

Social interactions affecting caste development through physiological actions in termites

A colony of social insects is not only an aggregation of individuals but also a functional unit. To achieve adaptive social behavior in fluctuating environmental conditions, in addition to coordination of physiological status in each individual, the whole colony is coordinated by interactions among colony members. The study on the regulation of social-insect colonies is termed "social physiology." Termites, a major group of social insects, exhibit many interesting phenomena related to social physiology, such as mechanisms of caste regulation in a colony. In their colonies, there are different types of individuals, i.e., castes, which show distinctive phenotypes specialized in specific colony tasks. Termite castes comprise reproductives, soldiers and workers, and the caste composition can be altered depending on circumstances. For the regulation of caste compositions, interactions among individuals, i.e., social interactions, are thought to be important. In this article, we review previous studies on the adaptive meanings and those on the proximate mechanisms of the caste regulation in termites, and try to understand those comprehensively in terms of social physiology. Firstly, we summarize classical studies on the social interactions. Secondly, previous studies on the pheromone substances that mediate the caste regulatory mechanisms are overviewed. Then, we discuss the roles of a physiological factor, juvenile hormone (JH) in the regulation of caste differentiation. Finally, we introduce the achievements of molecular studies on the animal sociality (i.e., sociogenomics) in terms of social physiology. By comparing the proximate mechanisms of social physiology in termites with those in hymenopterans, we try to get insights into the general principles of social physiology in social animals.

Four-way regulation of mosquito yolk protein precursor genes by juvenile hormone-, ecdysone-, nutrient-, and insulin-like peptide signaling pathways

Anautogenous mosquito females require a meal of vertebrate blood in order to initiate the production of yolk protein precursors by the fat body. Yolk protein precursor gene expression is tightly repressed in a state-of-arrest before blood meal-related signals activate it and expression levels rise rapidly. The best understood example of yolk protein precursor gene regulation is the vitellogenin-A gene (vg) of the yellow fever mosquito Aedes aegypti. Vg-A is regulated by (1) juvenile hormone signaling, (2) the ecdysone-signaling cascade, (3) the nutrient sensitive target-of-rapamycin signaling pathway, and (4) the insulin-like peptide (ILP) signaling pathway. A plethora of new studies have refined our understanding of the regulation of yolk protein precursor genes since the last review on this topic in 2005 (Attardo et al., 2005). This review summarizes the role of these four signaling pathways in the regulation of vg-A and focuses upon new findings regarding the interplay between them on an organismal level.

Post-feeding physiology in Rhodnius prolixus: the possible role of FGLamide-related allatostatins

Allatostatins (ASTs) are neuropeptides that were first identified as inhibitors of juvenile hormone biosynthesis by the corpora allata of some insect species. The FGLamide-related ASTs (FGLa/ASTs) belong to one of three families of insect ASTs. Previously, we showed that Rhodnius prolixus FGLa/ASTs (Rhopr-FGLa/ASTs) are present throughout the R. prolixus central nervous system and are associated with 5 dorsal unpaired median (DUM) neurons in the mesothoracic ganglionic mass. A similar set of neurons contain serotonin which is a diuretic hormone in R. prolixus. Rhopr-FGLa/ASTs inhibit both spontaneous contractions of the anterior midgut and leucokinin-1-induced hindgut contractions. Since these tissues are involved with post-feeding diuresis, these data suggest a possible role for FGLa/ASTs in events associated with feeding, and a possible interaction with serotonin. To investigate this possibility, we have examined the DUM neurons in more detail with regard to their peptide content, examined the potential release of Rhopr-FGLa/ASTs into the haemolymph following feeding, and further investigated the effects of Rhopr-FGLa/ASTs on feeding-related tissues. There are 10 DUM neurons in the abdominal neuromeres, 5 of which express serotonin-like immunoreactivity and the other 5 express FGLa/AST-like immunoreactivity. FGLa/AST-like immunoreactivity is reduced in the 5 DUM neuron cell bodies and their neurohaemal sites on abdominal nerves at 3-5 h post feeding. Rhopr-FGLa/ASTs do not inhibit serotonin-stimulated anterior midgut absorption or Malpighian tubule secretion but do inhibit hindgut contractions induced by an endogenous kinin, suggesting that they may only indirectly affect post-feeding diuresis in R. prolixus.

Sesquiterpene action, and morphogenetic signaling through the ortholog of retinoid X receptor, in higher Diptera

Morphogenetic signaling by small terpenoid hormones is a common feature of both vertebrate and invertebrate development. Most attention on insect developmental signaling by small terpenoids has focused on signaling by juvenile hormone through bHLH-PAS proteins (e.g., the MET protein), especially as that signaling axis intersects with ecdysteroid action through the receptor EcR. However, a series of endocrine and pharmacological studies on pupariation in cyclorrhaphous Diptera have remained persistently refractory to explanation with the above two-axis model. Recently, the terpenoid compound methyl farnesoate has been physicochemically demonstrated to exist in circulation at physiological concentrations, in several mecopterid orders, including Diptera. In addition, it has also been recently demonstrated that the receptor to which methyl farnesoate binds with nanomolar affinity (ultraspiracle, an ortholog of retinoid X receptor) requires a functioning ligand binding pocket to sustain the morphogenetic transition to puparium formation. This review evaluates endocrine and pharmacological evidence for developmental pathways reached by methyl farnesoate action, and assesses the participation of the retinoid X receptor ligand pocket in signal transduction to those developmental endpoints.

Mechanisms regulating nutrition-dependent developmental plasticity through organ-specific effects in insects

Nutrition, via the insulin/insulin-like growth factor (IIS)/Target of Rapamycin (TOR) signaling pathway, can provide a strong molding force for determining animal size and shape. For instance, nutrition induces a disproportionate increase in the size of male horns in dung and rhinoceros beetles, or mandibles in staghorn or horned flour beetles, relative to body size. In these species, well-fed male larvae produce adults with greatly enlarged horns or mandibles, whereas males that are starved or poorly fed as larvae bear much more modest appendages. Changes in IIS/TOR signaling plays a key role in appendage development by regulating growth in the horn and mandible primordia. In contrast, changes in the IIS/TOR pathway produce minimal effects on the size of other adult structures, such as the male genitalia in fruit flies and dung beetles. The horn, mandible and genitalia illustrate that although all tissues are exposed to the same hormonal environment within the larval body, the extent to which insulin can induce growth is organ specific. In addition, the IIS/TOR pathway affects body size and shape by controlling production of metamorphic hormones important for regulating developmental timing, like the steroid molting hormone ecdysone and sesquiterpenoid hormone juvenile hormone. In this review, we discuss recent results from Drosophila and other insects that highlight mechanisms allowing tissues to differ in their sensitivity to IIS/TOR and the potential consequences of these differences on body size and shape.

Partial venom gland transcriptome of a Drosophila parasitoid wasp, Leptopilina heterotoma, reveals novel and shared bioactive profiles with stinging Hymenoptera

Analysis of natural host-parasite relationships reveals the evolutionary forces that shape the delicate and unique specificity characteristic of such interactions. The accessory long gland-reservoir complex of the wasp Leptopilina heterotoma (Figitidae) produces venom with virus-like particles. Upon delivery, venom components delay host larval development and completely block host immune responses. The host range of this Drosophila endoparasitoid notably includes the highly-studied model organism, Drosophila melanogaster. Categorization of 827 unigenes, using similarity as an indicator of putative homology, reveals that approximately 25% are novel or classified as hypothetical proteins. Most of the remaining unigenes are related to processes involved in signaling, cell cycle, and cell physiology including detoxification, protein biogenesis, and hormone production. Analysis of L. heterotoma's predicted venom gland proteins demonstrates conservation among endo- and ectoparasitoids within the Apocrita (e.g., this wasp and the jewel wasp Nasonia vitripennis) and stinging aculeates (e.g., the honey bee and ants). Enzyme and KEGG pathway profiling predicts that kinases, esterases, and hydrolases may contribute to venom activity in this unique wasp. To our knowledge, this investigation is among the first functional genomic studies for a natural parasitic wasp of Drosophila. Our findings will help explain how L. heterotoma shuts down its hosts' immunity and shed light on the molecular basis of a natural arms race between these insects.

De novo characterization of transcriptome and gene expression dynamics in epidermis during the larval-pupal metamorphosis of common cutworm

Larval cuticle is degraded and replaced by the pupal counterpart during larval-pupal metamorphosis in the holometabolous insects. In addition to the extrinsic transformation, the epidermis goes through significant changes at molecular levels. To elucidate the intrinsic mechanism of epidermal metamorphosis, the dynamics of chitin content in the cuticle was examined in an important agricultural lepidopteran, the common cutworm, and the transcriptome was analyzed using Illumina sequencing technology. Gene expression profiles during the metamorphosis were further studied by both the digital gene expression (DGE) system and real-time quantitative PCR. The results showed that the chitin content decreased in prepupae and then increased in pupae. A total of 58 million sequencing reads were obtained and assembled into 70,346 unigenes. Over 9000 unigenes were identified to express differentially during the transformation process. As compared with the 6th instar feeding larvae, the most significant changes took place in the proteasome and metabolic pathways in prepupae and pupae, respectively. The cytochrome P450s, VHDLs, chitinase, serine protease and genes involved in sex pheromone biosynthesis changed their mRNA levels remarkably. Three chitinolytic enzymes (chitinase, β-N-acetylglucosaminidase and chitin deacetylase) showed distinct mRNA expression patterns, the former two enzymes revealed the highest expression in prepupae, however the latter one showed its climax mRNA level in pupae. The gene expression patterns suggest that chitinase and β-N-acetylglucosaminidase may be responsible for the degradation of larval cuticles, whereas chitin deacetylase may help to degrade the pupal counterparts. Gene expression dynamics also implied that the chitin of pupal cuticle might be formed by recycling of the degraded chitin of larval cuticle rather than through de novo synthesis. The 20E-induced nuclear receptors seem to be important factors regulating chitin metabolic enzymes during the cuticle remodeling. Our data provide a comprehensive resource for exploring the molecular mechanism of epidermal metamorphosis in insects.

Characterization of Hovi-mEH1, a microsomal epoxide hydrolase from the glassy-winged sharpshooter Homalodisca vitripennis

Epoxide hydrolase (EH) is an enzyme in the α/β-hydrolase fold superfamily that uses a water molecule to transform an epoxide to its corresponding diol. In insects, EHs metabolize among other things critical developmental hormones called juvenile hormones (JHs). EHs also play roles in the detoxification of toxic compounds that are found in the insect's diet or environment. In this study, a full-length cDNA encoding an epoxide hydrolase, Hovi-mEH1, was obtained from the xylem-feeding insect Homalodisca vitripennis. H. vitripennis, commonly known as the glassy-winged sharpshooter, is an economically important vector of plant pathogenic bacteria such as Xylella fastidiosa. Hovi-mEH1 hydrolyzed the general EH substrates cis-stilbene oxide and trans-diphenylpropene oxide with specific activities of 47.5 ± 6.2 and 1.3 ± 0.5 nmol of diol formed min⁻¹ mg⁻¹, respectively. Hovi-mEH1 metabolized JH III with a Vmax of 29.3 ± 1.6 nmol min⁻¹ mg⁻¹, kcat of 0.03 s⁻¹, and KM of 13.8 ± 2.0 μM. These Vmax and kcat values are similar to those of known JH metabolizing EHs from lepidopteran and coleopteran insects. Hovi-mEH1 showed 99.1% identity to one of three predicted EH-encoding sequences that were identified in the transcriptome of H. vitripennis. Of these three sequences only Hovi-mEH1 clustered with known JH metabolizing EHs. On the basis of biochemical, phylogenetic, and transcriptome analyses, we hypothesize that Hovi-mEH1 is a biologically relevant JH-metabolizing enzyme in H. vitripennis.

Juvenile hormone and insulin suppress lipolysis between periods of lactation during tsetse fly pregnancy

Tsetse flies are viviparous insects that nurture a single intrauterine progeny per gonotrophic cycle. The developing larva is nourished by the lipid-rich, milk-like secretions from a modified female accessory gland (milk gland). An essential feature of the lactation process involves lipid mobilization for incorporation into the milk. In this study, we examined roles for juvenile hormone (JH) and insulin/IGF-like (IIS) signaling pathways during tsetse pregnancy. In particular, we examined the roles for these pathways in regulating lipid homeostasis during transitions between non-lactating (dry) and lactating periods. The dry period occurs over the course of oogenesis and embryogenesis, while the lactation period spans intrauterine larvigenesis. Genes involved in the JH and IIS pathways were upregulated during dry periods, correlating with lipid accumulation between bouts of lactation. RNAi suppression of Forkhead Box Sub Group O (FOXO) expression impaired lipolysis during tsetse lactation and reduced fecundity. Similar reduction of the JH receptor Methoprene tolerant (Met), but not its paralog germ cell expressed (gce), reduced lipid accumulation during dry periods, indicating functional divergence between Met and gce during tsetse reproduction. Reduced lipid levels following Met knockdown led to impaired fecundity due to inadequate fat reserves at the initiation of milk production. Both the application of the JH analog (JHA) methoprene and injection of insulin into lactating females increased stored lipids by suppressing lipolysis and reduced transcripts of lactation-specific genes, leading to elevated rates of larval abortion. To our knowledge, this study is the first to address the molecular physiology of JH and IIS in a viviparous insect, and specifically to provide a role for JH signaling through Met in the regulation of lipid metabolism during insect lactation.

The insulin/TOR signal transduction pathway is involved in the nutritional regulation of juvenile hormone synthesis in Aedes aegypti

Juvenile hormone (JH) levels must be modulated to permit the normal progress of development and reproductive maturation in mosquitoes. JH is part of a transduction system that assesses nutritional information and controls reproduction in mosquitoes. Adult female Aedes aegypti show nutritionally-dependent dynamic changes in corpora allata (CA) JH biosynthetic activities. A coordinated expression of most JH biosynthetic enzymes has been described in female pupae and adult mosquitoes; increases or decreases in transcript levels for all the enzymes were concurrent with increases or decreases in JH synthesis; suggesting that transcriptional changes are at least partially responsible for the dynamic changes of JH biosynthesis. The goal of the present study is to identify signaling network components responsible for the nutritional-dependent changes of JH synthesis in the CA of mosquitoes. The insulin/TOR signaling network plays a central role in the transduction of nutritional signals that regulate cell growth and metabolism in insects. These pathways have also been suggested as a link between nutritional signals and JH synthesis regulation in the CA of cockroaches and flies. We used a combination of in vitro studies and in vivo genetic knockdown experiments to explore nutritional signaling pathways in the CA. Our results suggest that the insulin/TOR pathway plays a role in the transduction of the nutritional information that regulates JH synthesis in mosquitoes. Transcriptional regulation of the genes encoding JH biosynthetic enzymes is at least partially responsible for these nutritionally modulated changes of JH biosynthesis.

Functional analysis of a mosquito short-chain dehydrogenase cluster

The short-chain dehydrogenases (SDR) constitute one of the oldest and largest families of enzymes with over 46,000 members in sequence databases. About 25% of all known dehydrogenases belong to the SDR family. SDR enzymes have critical roles in lipid, amino acid, carbohydrate, hormone, and xenobiotic metabolism as well as in redox sensor mechanisms. This family is present in archaea, bacteria, and eukaryota, emphasizing their versatility and fundamental importance for metabolic processes. We identified a cluster of eight SDRs in the mosquito Aedes aegypti (AaSDRs). Members of the cluster differ in tissue specificity and developmental expression. Heterologous expression produced recombinant proteins that had diverse substrate specificities, but distinct from the conventional insect alcohol (ethanol) dehydrogenases. They are all NADP⁺-dependent and they have S-enantioselectivity and preference for secondary alcohols with 8-15 carbons. Homology modeling was used to build the structure of AaSDR1 and two additional cluster members. The computational study helped explain the selectivity toward the (10S)-isomers as well as the reduced activity of AaSDR4 and AaSDR9 for longer isoprenoid substrates. Similar clusters of SDRs are present in other species of insects, suggesting similar selection mechanisms causing duplication and diversification of this family of enzymes.

Characterization of an isopentenyl diphosphate isomerase involved in the juvenile hormone pathway in Aedes aegypti

Isopentenyl diphosphate isomerase (IPPI) is an enzyme involved in the synthesis of juvenile hormone (JH) in the corpora allata (CA) of insects. IPPI catalyzes the conversion of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP); afterward IPP and DMAPP condense in a head-to-tail manner to produce geranyl diphosphate (GPP), this head-to-tail condensation can be repeated, by the further reaction of GPP with IPP, yielding the JH precursor farnesyl diphosphate. An IPPI expressed sequence tag (EST) was obtained from an Aedes aegypti corpora-allata + corpora cardiaca library. Its full-length cDNA encodes a 244-aa protein that shows a high degree of similarity with type I IPPIs from other organisms, particularly for those residues that have important roles in catalysis, metal coordination and interaction with the diphosphate moiety of the IPP. Heterologous expression produced a recombinant protein that metabolized IPP into DMAPP; treatment of DMAPP with phosphoric acid produced isoprene, a volatile compound that was measured with an assay based on a solid-phase micro extraction protocol and direct analysis by gas chromatography. A. aegypti IPPI (AaIPPI) required Mg(2+) or Mn(2+) but not Zn(2+) for full activity and it was entirely inhibited by iodoacetamide. Real time PCR experiments showed that AaIPPI is highly expressed in the CA. Changes in AaIPPI mRNA levels in the CA in the pupal and adult female mosquito corresponded well with changes in JH synthesis (Li et al., 2003). This is the first molecular and functional characterization of an isopentenyl diphosphate isomerase involved in the production of juvenile hormone in the CA of an insect.

The fate of follicles after a blood meal is dependent on previtellogenic nutrition and juvenile hormone in Aedes aegypti

Juvenile hormone (JH) mediates the relationship between fecundity and nutrition during the gonotrophic cycle of the mosquito in three ways: (1) by regulating initial previtellogenic development, (2) by mediating previtellogenic resorption of follicles and (3) by altering intrinsic previtellogenic follicle "quality", physiology, and competitiveness thereby predetermining the fate of follicles after a blood meal. To support a role for JH in mediating the response of ovarian follicles after a blood meal, we explored three main questions: (1) Do changes in nutrition during the previtellogenic resting stage lead to relevant biochemical and molecular changes in the previtellogenic ovary? (2) Do hormonal manipulations during the previtellogenic resting stage lead to the same biochemical and molecular changes? (3) Does nutrition and hormones during the previtellogenic resting stage affect vitellogenic resorption and reproductive output? We examined the accumulation of neutral lipids in the previtellogenic ovary as well as the previtellogenic expression of genes integral to endocytosis and oocyte development such as the: vitellogenin receptor (AaVgR), lipophorin receptor (AaLpRov), heavy-chain clathrin (AaCHC), and ribosomal protein L32 (rpL32) under various previtellogenic nutritional and hormonal conditions. mRNA abundance and neutral lipid content increased within the previtellogenic ovary as previtellogenic mosquitoes were offered increasing sucrose concentrations. Methoprene application mimicked the effect of offering the highest sucrose concentrations on mRNA abundance and lipid accumulation in the previtellogenic ovary. These same nutritional and hormonal manipulations altered the extent of vitellogenic resorption. Mosquitoes offered 20% sucrose during the previtellogenic resting stage had nearly 3 times less vitellogenic resorption than mosquitoes offered 3% sucrose despite taking smaller blood meals and developed ∼10% more eggs during the first gonotrophic cycle. Mosquitoes treated with JH III during the previtellogenic resting stage and then offered a blood meal had a ∼40% reduction in the amount of vitellogenic resorption and developed ∼12% more eggs. Taken together, these results suggest that previtellogenic nutrition alters the extent and pattern of resorption after a blood meal through the effect of JH on mRNA abundance and lipid accumulation in previtellogenic follicles.

Molecular Mechanisms of Transcription Activation by Juvenile Hormone: A Critical Role for bHLH-PAS and Nuclear Receptor Proteins

Juvenile hormone (JH) is responsible for controlling many biological processes. In several insect species JH has been implicated as a key regulator of developmental timing, preventing the premature onset of metamorphosis during larval growth periods. However, the molecular basis of JH action is not well-understood. In this review, we highlight recent advances which demonstrate the importance of transcription factors from the bHLH-PAS and nuclear receptor families in mediating the response to JH.

Functional characterization of an allatotropin receptor expressed in the corpora allata of mosquitoes

Allatotropin is an insect neuropeptide with pleiotropic actions on a variety of different tissues. In the present work we describe the identification, cloning and functional and molecular characterization of an Aedes aegypti allatotropin receptor (AeATr) and provide a detailed quantitative study of the expression of the AeATr gene in the adult mosquito. Analysis of the tissue distribution of AeATr mRNA in adult female revealed high transcript levels in the nervous system (brain, abdominal, thoracic and ventral ganglia), corpora allata-corpora cardiaca complex and ovary. The receptor is also expressed in heart, hindgut and male testis and accessory glands. Separation of the corpora allata (CA) and corpora cardiaca followed by analysis of gene expression in the isolated glands revealed expression of the AeATr primarily in the CA. In the female CA, the AeATr mRNA levels were low in the early pupae, started increasing 6h before adult eclosion and reached a maximum 24h after female emergence. Blood feeding resulted in a decrease in transcript levels. The pattern of changes of AeATr mRNA resembles the changes in JH biosynthesis. Fluorometric Imaging Plate Reader recordings of calcium transients in HEK293 cells expressing the AeATr showed a selective response to A. aegypti allatotropin stimulation in the low nanomolar concentration range. Our studies suggest that the AeATr play a role in the regulation of JH synthesis in mosquitoes.

Integrating body and organ size in Drosophila: recent advances and outstanding problems

OVER THE PAST TWO DECADES, FUNDAMENTAL STRIDES IN PHYSIOLOGY AND GENETICS HAVE ALLOWED US TO FINALLY GRASP THE DEVELOPMENTAL MECHANISMS REGULATING BODY SIZE, PRIMARILY IN ONE MODEL ORGANISM: the fruit fly Drosophila melanogaster. In Drosophila, as in all animals, final body size is regulated by the rate and duration of growth. These studies have identified important roles for the insulin and the target of rapamycin (TOR) signaling pathways in regulating the growth rate of the larva, the stage most important in determining final adult size. Furthermore, they have shown that the insulin/TOR pathway interacts with hormonal systems, like ecdysone and juvenile hormone, to regulate the timing of development and hence the duration of growth. This interaction allows the growing larvae to integrate cues from the environment with environmentally sensitive developmental windows to ensure that optimal size and proportions are reached given the larval rearing conditions. Results from this work have opened up new avenues of studies, including how environmental cues are integrated to regulate developmental time and how organs maintain proportional growth. Other researchers interested in the evolution of body size are beginning to apply these results to studies of body size evolution and the generation of allometry. With these new findings, and with the developments to come, the field of size control finds itself in the fortunate position of finally being able to tackle century old questions of how organisms achieve final adult size and proportions. This review discusses the state of the art of size control from a Drosophila perspective, and outlines an approach to resolving outstanding issues.

A coordinated expression of biosynthetic enzymes controls the flux of juvenile hormone precursors in the corpora allata of mosquitoes

Juvenile hormone (JH) is a key regulator of metamorphosis and ovarian development in mosquitoes. Adult female Aedes aegypti mosquitoes show developmental and dynamically regulated changes of JH synthesis. Newly emerged females have corpora allata (CA) with low biosynthetic activity, but they produce high amounts of JH a day later; blood feeding results in a striking decrease in JH synthesis, but the CA returns to a high level of JH synthesis three days later. To understand the molecular bases of these dynamic changes we combined transcriptional studies of 11 of the 13 enzymes of the JH pathway with a functional analysis of JH synthesis. We detected up to a 1000-fold difference in the levels of mRNA in the CA among the JH biosynthetic enzymes studied. There was a coordinated expression of the 11 JH biosynthetic enzymes in female pupae and adult mosquito. Increases or decreases in transcript levels for all the enzymes resulted in increases or decreases of JH synthesis; suggesting that transcript changes are at least partially responsible for the dynamic changes of JH biosynthesis observed. JH synthesis by the CA was progressively increased in vitro by addition of exogenous precursors such as geranyl-diphosphate, farnesyl-diphosphate, farnesol, farnesal and farnesoic acid. These results suggest that the supply of these precursors and not the activity of the last 6 pathway enzymes is rate limiting in these glands. Nutrient reserves play a key role in the regulation of JH synthesis. Nutritionally deficient females had reduced transcript levels for the genes encoding JH biosynthetic enzymes and reduced JH synthesis. Our studies suggest that JH synthesis is controlled by the rate of flux of isoprenoids, which is the outcome of a complex interplay of changes in precursor pools, enzyme levels and external regulators such as nutrients and brain factors. Enzyme levels might need to surpass a minimum threshold to achieve a net flux of precursors through the biosynthetic pathway. In glands with low synthetic activity, the flux of isoprenoids might be limited by the activity of enzymes with low levels of expression.

Nutrient limitation results in juvenile hormone-mediated resorption of previtellogenic ovarian follicles in mosquitoes

Juvenile hormone (JH) is a central hormonal regulator of previtellogenic development in female Aedes aegypti mosquitoes. JH levels are low at eclosion and increase during the first day after adult emergence. This initial rise in JH is essential for female reproductive maturation. After previtellogenic maturation is complete, the mosquito enters a 'state-of-arrest' during which JH synthesis continues at a slower pace and further ovary development is repressed until a blood meal is taken. By examining the relationships between juvenile hormone, follicular resorption and nutrition in A. aegypti, we were able to define a critical role of JH during the previtellogenic resting stage. The rate of follicular resorption in resting stage mosquitoes is dependent on nutritional quality. Feeding water alone caused the rate of follicular resorption to reach over 20% by day 7 after emergence. Conversely, feeding a 20% sucrose solution caused resorption to remain below 5% during the entire experimental period. Mosquitoes fed 3% sucrose show rates of resorption intermediate between water and 20% sucrose and only reached 10% by day 7 after emergence. Follicular resorption is related to JH levels. Ligated abdomens separated from a source of JH (the corpora allata) showed an increase in resorption comparable to similarly aged starved mosquitoes (16%). Resorption in ligated abdomens was reduced to 6% by application of methoprene. The application of methoprene was also sufficient to prevent resorption in intact mosquitoes starved for 48 h (14% starved vs. 4% starved with methoprene). Additionally, active caspases were localized to resorbing follicles indicating that an apoptotic cell-death mechanism is responsible for follicular resorption during the previtellogenic resting stage. Taken together, these results indicate that JH mediates reproductive trade-offs in resting stage mosquitoes in response to nutrition.

Bioassays of compounds with potential juvenoid activity on Drosophila melanogaster: juvenile hormone III, bisepoxide juvenile hormone III and methyl farnesoates

Metabolites of the 6,7,10,11 bisepoxide juvenile hormone III (JHB(3)), and other potential juvenoids, were tested for juvenile hormone activity using early instar or early stage pupae of Drosophila melanogaster. Importantly, methyl farnesoates were tested as they might have JH-like activity on Dipteran juveniles. Larvae were exposed to compounds in medium, or the compounds were applied to white puparia. In the assays employed in the present study, there was no indication for JH activity associated with the metabolites of JHB(3). The activity of methyl farnesoate (MF) was higher than that of JH III and far greater than bisepoxide JH III. As opposed to the two endogenous juvenile hormones, methyl farnesoate has weak activity in the white puparial bioassay. When fluorinated forms of methyl farnesoate, which is unlikely to be converted to JH, were applied to Drosophila medium to which fly eggs were introduced, there was a high degree of larval mortality, but no evidence of subsequent mortality at the pupal stage. One possible explanation for the results is that methyl farnesoate is active as a hormone in larval stages, but has little activity at the pupal stage where only juvenile hormone has a major effect.

Mode of action of methoprene in affecting female reproduction in the African malaria mosquito, Anopheles gambiae

BACKGROUND

One of the most studied actions of juvenile hormone (JH) is its ability to modulate ecdysteroid signaling during insect development and metamorphosis. Previous studies in mosquitoes showed that 20-hydroxyecdysone (20E) regulates vitellogenin synthesis. However, the action of JH and its mimics, e.g. methoprene, on female reproduction of mosquitoes remains unknown.

RESULTS

Here, a major malaria vector, Anopheles gambiae Giles, was used as a model insect to study the action of methoprene on female reproduction. Ecdysteroid titers and expression profiles of ecdysone-regulated genes were determined before and after a blood meal. An ecdysteroid peak was detected at 12 h post blood meal (PBM). The maximum expression of ecdysone-regulated genes, such as ecdysone receptor (EcR), hormone receptor 3 (HR3) and vitellogenin (Vg) gene, coincided with the ecdysteroid peak. Interestingly, topical application of methoprene at 6 h PBM delayed ovarian development and egg maturation by suppressing the expression of ecdysone-regulated genes in female mosquitoes.

CONCLUSION

The data suggest that ecdysteroid titers are correlated with Vg synthesis, and methoprene affects vitellogenesis by modulating ecdysteroid action in A. gambiae.

Juvenile hormone regulation of vitellogenin synthesis in the red flour beetle, Tribolium castaneum

To elucidate the endocrine regulation of vitellogenin (Vg) synthesis in the red flour beetle, Tribolium castaneum, the titers of juvenile hormone (JH) and ecdysteroids in the whole body of female beetles were measured and compared with Vg mRNA levels. Juvenile hormone levels remained high while the ecdysteroid levels declined steadily during 1-5 days post adult emergence (PAE). The Vg mRNA levels began to increase by the end of 3rd day PAE and peaked by the 4th-5th day PAE. Gene expression profiling by microarray and quantitative real-time PCR analyses of RNA isolated from 1 to 5 days PAE beetles revealed that the genes coding for proteins involved in JH biosynthesis and action, but not those involved in 20-hydroxyecdysone (20E) biosynthesis and action had similar expression patterns as the genes coding for Vg. RNA interference (RNAi)-aided knock-down in the expression of these genes showed that both JH and 20E were required for Vg gene expression. However, Vg mRNA was induced by the application of JH III but not by the injection of 20E into the previtellogenic females. These data suggest that JH is required for Vg synthesis in the fat body and 20E influences Vg synthesis through its action on oocyte maturation.

Allatostatin-C receptors in mosquitoes

In the present work we describe the functional and molecular characterization of two Aedes aegypti allatostatin-C receptor paralogs (AeAS-CrA and AeAS-CrB) and provide a detailed quantitative study of the expression of the AS-C receptor genes in an adult insect. The tissue distribution of the two AS-C receptors differed significantly; the mRNA levels of AeAS-CrB in the Malpighian tubules were the highest detected, while transcripts for AeAS-CrA were relatively low in this tissue. In addition, the transcript levels of both receptors were different in the thoracic and abdominal ganglia, corpora allata (CA) and the testis of the male. In the CA, the AeAS-CrB mRNA levels were constant from 0 to 72 h after female emergence, while the AeAS-CrA levels increased at 72 h. To complement the receptor expression studies, we analyzed the tissue specificity for allatostatin-C mRNA in female mosquitoes. Expression was high in abdominal ganglia and brain. Transcript levels of allatostatin-C in the head of females were elevated at eclosion and there were no major changes during the first week of adult life or after blood feeding. Fluorometric Imaging Plate Reader (FLIPR) recordings of calcium transients in HEK293T cells transiently expressing both putative receptors showed that they both responded selectively to allatostatin-C stimulation in the nanomolar concentration range. However, the peptide showed slightly greater affinity for AeAS-CrB than AeAS-CrA. Our studies suggest that some of the pleiotropic effects of allatostatin-C in mosquitoes could be mediated by the different receptor paralogs. Transcriptional regulation of the AS-C receptors may not have a critical role in the changes of CA responsiveness to the peptide that we previously described.

Suppressed production of methyl farnesoid hormones yields developmental defects and lethality in Drosophila larvae

A long-unresolved question in the developmental biology of Drosophila melanogaster has been whether methyl farnesoid hormones secreted by the ring gland are necessary for larval maturation and metamorphosis. In this study, we have used RNAi techniques to inhibit 3-Hydroxy-3-Methylglutaryl CoA Reductase (HMGCR) expression selectively in the corpora allatal cells that produce the circulating farnesoid hormones. The developing larvae manifest a number of developmental, metabolic and morphogenetic derangements. These defects included the exhibition of an "ultraspiracle" death phenotype at the 1st to 2nd instar larval molt, similar to that exhibited by animals that are null for the farnesoid receptor ultraspiracle. The few larvae surviving past a second lethal period at the 2nd to 3rd instar larval molt, again with "ultraspiracle" phenotype, often became developmentally arrested after either attaining a misformed puparium or after formation of the white pupa. Survival past the "ultraspiracle" lethal phenotype could be rescued by dietary provision of an endogenous dedicated precursor to the three naturally secreted methyl farnesoid hormones. In addition to these developmental and morphogenetic defects, most larvae that survived to the late second instar exhibited a posterior-originating melanization of the tracheal system. These results support the hypothesis that larval methyl farnesoid hormones are necessary for larval survival and morphogenetic transformation through the larval and pupal metamorphic processes.

Ovarian development of female-female pairs in the termite, Reticulitermes speratus

In the rhinotermitid termite Reticulitermes speratus (Kolbe) (Isoptera: Rhinotermitidae), facultative parthenogenesis is known to occur occasionally and females cooperate with other females to found the colony. To elucidate the ovarian development in these two females, incipient female-female colonies were established under laboratory conditions, and the process of colony development was observed at 0.5, 1.5, 2.5, 3.5, and 7.5 months (stages I-V, respectively) after colony foundation. Ovarian development, vitellogenin gene expression, and juvenile hormone (JH) titers were examined. A precise reproductive cycle in both females was observed, in which the oviposition rate was relatively higher during stages I and II, decreased during stages III and IV, and then increased again at stage V. JH III titer and vitellogenin gene expression changed in parallel throughout the reproductive cycle of these queens. Ovarian maturation and vitellogenesis were similar in both females in a female-female colony at all stages examined, suggesting that no conflicts existed for two females in terms of oviposition.

A shut-down in expression of an insulin-like peptide, ILP-1, halts ovarian maturation during the overwintering diapause of the mosquito Culex pipiens

Short day-length is used to programme adult diapause in the mosquito, Culex pipiens. The downstream endocrine event that halts ovarian maturation is a shut-down in juvenile hormone (JH) production, and recent evidence suggests that the insulin signalling pathway may be a key upstream player in executing this developmental arrest. Genes encoding insulin-like peptides-1, -2 and -5 were identified in C. pipiens, and we report that transcript levels of insulin-like peptides-1 and -5 were significantly lower in diapausing females than in their nondiapausing counterparts. Genes encoding both insulin-like peptides-1 and -5 were suppressed using RNA interference in mosquitoes programmed for nondiapause, and ovarian maturation was monitored. Knocking down insulin-like peptide-1 with RNAi in nondiapausing mosquitoes resulted in a cessation of ovarian development akin to diapause, and this arrest in development could be reversed with an application of JH. Knocking down insulin-like peptide-5 did not alter ovarian development. These results are consistent with a role for insulin-like peptide-1 in the signalling pathway leading from the perception of short day-lengths to the shut-down in JH production that characterizes adult diapause in C. pipiens.

Molecular and functional characterization of a juvenile hormone acid methyltransferase expressed in the corpora allata of mosquitoes

A juvenile hormone acid methyltransferase (JHAMT) was isolated as an abundant EST in a library of the corpora allata of the adult female mosquito Aedes aegypti. Its full length cDNA encodes a 278-aa protein that has 43% amino acid identity with BmJHAMT, a juvenile hormone acid methyltransferase previously cloned from Bombyx mori. Heterologous expression produced a recombinant protein that metabolizes farnesoic acid (FA) into methyl farnesoate, as well as juvenile hormone acid into juvenile hormone III (JH III) with exquisite stereo specificity. Real time PCR experiments showed that JHAMT mRNA levels are not an unequivocal indicator of JH III synthesis rates; the A. aegypti JHAMT gene, silent in female pupae, was transcriptionally activated just 4-6h before adult eclosion. Radiochemical methyltransferase assays using active and inactive corpora allata glands (CA) dissected from sugar and blood-fed females respectively, clearly indicated that significant levels of JHAMT enzymatic activity are present when the CA shows very low spontaneous rates of JH III synthesis. Having the last enzymes of the JH synthetic pathway readily available all the time might be critical for the adult female mosquito to sustain rapid dynamic changes in JH III synthesis in response to nutritional changes or peripheral influences, such as mating or feeding. These results suggest that this gene has different roles in the regulation of JH synthesis in pupal and adult female mosquitoes, and support the hypothesis that the rate-limiting steps in JH III synthesis in adult female mosquitoes are located before entrance of FA into the synthetic pathway.

Egg formation in lepidoptera

Reproductive biology in the Twentieth Century produced comprehensive descriptions of the mechanisms of egg formation in most of the major orders of insects. While many general principles of ovarian development and physiology emerged, every order turned out to have a set of its own special motifs. Discovery of the lepidopteran motifs is summarized in this essay. The emphasis is on developmental mechanisms, beginning with the early growth and differentiation of female germ cells and ending, after many turns in morphogenesis, physiology and biosynthesis, with eggs that are filled with yolk and encased in chorions. Examples of uniquely lepidopteran traits include the cellular composition of ovarian follicles, the number of tubular ovarioles in which they mature, the functions of cell-to-cell junctional complexes in their maturation, their use of glycosaminoglycans to maintain intercellular patency during vitellogenesis, the role of proton and calcium pumps in their ion physiology, a separate postvitellogenic period of water and inorganic ion uptake, and the fine structure and protein composition of their chorions. Discovery of this combination of idiosyncracies was based on advances in the general concepts and techniques of cell and molecular biology and on insights borrowed from studies on other insects. The lepidopteran ovary in turn has contributed much to the understanding of egg formation in insects generally.

Characterization of a juvenile hormone-regulated chymotrypsin-like serine protease gene in Aedes aegypti mosquito

After female mosquitoes ingest blood from vertebrate hosts, exopeptidases and endopeptidases are required for digesting blood proteins in the midgut into amino acids, which female mosquitoes use to build yolk proteins. These proteases are not always present in the midgut, and their diverse expression patterns suggest that production of these enzymes is highly regulated in order to meet specific physiological demands at various stages. Here we report identification of a serine-type protease, JHA15, in the yellow fever mosquito Aedes aegypti. This protein shares high sequence homology with chymotrypsins, and indeed exhibits specific chymotrypsin enzymatic activity. The JHA15 gene is expressed primarily in the midgut of adult female mosquitoes. Our results indicate that its transcription is activated by juvenile hormone in the newly emerged female adults. Although its mRNA profile is similar to that of the early trypsin gene, we found that JHA15 proteins were readily detected in the midgut epithelium cells of both non-blood-fed and blood-fed mosquitoes. Analysis of polysomal RNA further substantiated that synthesis of JHA15 occurs before and shortly after blood feeding. Knocking down expression of JHA15 resulted in no evident phenotypic changes, implying that functional redundancy exists among those proteolytic enzymes.

Specific transcriptional responses to juvenile hormone and ecdysone in Drosophila

Previous studies have shown that ecdysone (E), and its immediate downstream product 20-hydroxyecdysone (20E), can have different biological functions in insects, suggesting that E acts as a distinct hormone. Here, we use Drosophila larval organ culture in combination with microarray technology to identify genes that are transcriptionally regulated by E, but which show little or no response to 20E. These genes are coordinately expressed for a brief temporal interval at the onset of metamorphosis, suggesting that E acts together with 20E to direct puparium formation. We also show that E74B, pepck, and CG14949 can be induced by juvenile hormone III (JH III) in organ culture, and that CG14949 can be induced by JH independently of protein synthesis. In contrast, E74A and E75A show no response to JH in this system. These studies demonstrate that larval organ culture can be used to identify Drosophila genes that are regulated by hormones other than 20E, and provide a basis for studying crosstalk between multiple hormone signaling pathways.

Role of juvenile hormone and allatotropin on nutrient allocation, ovarian development and survivorship in mosquitoes

Teneral reserves are utilized to initiate previtellogenic ovarian development in mosquitoes. Females having emerged with low teneral reserves have reduced juvenile hormone (JH) synthesis and previtellogenic development. We investigated what role JH, allatotropin (AT) and other head-factors play in the regulation of previtellogenic ovarian development and adult survivorship. Factors from the head are essential for corpora allata (CA) activation and reproductive maturation. We have shown that decapitation of females within 9-12h after adult ecdysis prevented normal development of the previtellogenic follicles; however maximum previtellogenic ovarian development could be induced in decapitated females by topically applying a JH analog. When females were decapitated 12 or more hours after emergence nutritional resources had been committed to ovarian development and survivorship was significantly reduced. To study if allatotropin levels correlated with teneral reserves, we measured AT titers in the heads of two adult phenotypes (large and small females) generated by raising larvae under different nutritional diets. In large mosquitoes AT levels increased to a maximum of 45 fmol in day 4; in contrast, the levels of allatotropin in the heads of small mosquitoes remained below 9 fmol during the 7 days evaluated. These results suggest that only when nutrients are appropriate, factors released from the brain induce the CA to synthesize enough JH to activate reproductive maturation.

Programmed cell death in flight muscle histolysis of the house cricket

We have characterized the process of flight muscle histolysis in the female house cricket, Acheta domesticus, through analysis of alterations of tissue wet weight, total protein content, and percent shortening of the dorsal longitudinal flight muscles (DLMs). Our objectives were to (1) define the normal course of histolysis in the cricket, (2) analyze the effects of juvenile hormone (JH) removal and replacement, (3) determine the effects of cycloheximide treatment, and (4) examine patterns of protein expression during histolysis. Our results suggest that flight muscle histolysis in the house cricket is an example of an active, developmentally regulated cell death program induced by an endocrine signal. Initial declines of total protein in DLMs indicated the JH signal that induced histolysis occurred by Day 2 and that histolysis was essentially complete by Day 3. Significant reductions in tissue weight and percent muscle shortening were observed in DLMs from Day 3 crickets. Cervical ligation of Day 1 crickets prevented histolysis but this inhibition could be reversed by continual topical treatments with methoprene (an active JH analog) although ligation of Day 2 crickets did not prevent histolysis. A requirement for active protein expression was demonstrated by analysis of synthesis block by cycloheximide and short-term incorporation of (35)S-methionine. Treatment with cycloheximide prevented histolysis. Autofluorographic imaging of DLM proteins separated by electrophoresis revealed apparent coordinated regulation of protein expression.

Social exploitation of hexamerin: RNAi reveals a major caste-regulatory factor in termites

Lower termites express a unique form of eusocial polyphenism in that totipotent workers can differentiate into either soldier or reproductive caste phenotypes. In this initial effort using RNA interference in termites, we found that two hexamerin genes, Hex-1 and Hex-2, participate in the regulation of caste polyphenism. Our methodology involved a dual gene-silencing approach that used a single short-interfering RNA fragment to silence the two homologous hexamerin genes. We performed validation studies that evaluated effects on nontarget housekeeping genes, silencing of a nonhousekeeping control gene, and effects at the protein level. We found that the two hexamerin proteins, which are inducible by the morphogenetic juvenile hormone and which constitute a significant proportion of total termite protein, suppress juvenile-hormone-dependent worker differentiation to the soldier caste phenotype. This mechanism allows termite colonies to retain high proportions of altruistic worker caste members, thus apparently enhancing colony-inclusive fitness. These findings demonstrate a unique status quo regulatory mechanism for termite worker caste retention and provide an example of previously undescribed preadult developmental/caste-regulatory genes from any social insect.