Wide mutational spectrum of a gene involved in hormone action and insecticide resistance in Drosophila melanogaster

Juvenile hormones direct primordial germ cell migration to the embryonic gonad

Germ cells are essential to sexual reproduction. Across the animal kingdom, extracellular signaling isoprenoids, such as retinoic acids (RAs) in vertebrates and juvenile hormones (JHs) in invertebrates, facilitate multiple processes in reproduction. Here we investigated the role of these potent signaling molecules in embryonic germ cell development, using JHs in Drosophila melanogaster as a model system. In contrast to their established endocrine roles during larval and adult germline development, we found that JH signaling acts locally during embryonic development. Using an in vivo biosensor, we observed active JH signaling first within and near primordial germ cells (PGCs) as they migrate to the developing gonad. Through in vivo and in vitro assays, we determined that JHs are both necessary and sufficient for PGC migration. Analysis into the mechanisms of this newly uncovered paracrine JH function revealed that PGC migration was compromised when JHs were decreased or increased, suggesting that specific titers or spatiotemporal JH dynamics are required for robust PGC colonization of the gonad. Compromised PGC migration can impair fertility and cause germ cell tumors in many species, including humans. In mammals, retinoids have many roles in development and reproduction. We found that like JHs in Drosophila, RA was sufficient to impact mouse PGC migration in vitro. Together, our study reveals a previously unanticipated role of isoprenoids as local effectors of pre-gonadal PGC development and suggests a broadly shared mechanism in PGC migration.

Juvenile hormone receptor Methoprene tolerant: Functions and applications

During the past 15years, after confirming Methoprene tolerant (Met) as a juvenile hormone (JH) receptor, tremendous progress has been made in understanding the function of Met in supporting JH signal transduction. Met role in JH regulation of development, including metamorphosis, reproduction, diapause, cast differentiation, behavior, im`munity, sleep and epigenetic modifications, have been elucidated. Met's Heterodimeric partners involved in performing some of these functions were discovered. The availability of JH response elements (JHRE) and JH receptor allowed the development of screening assays in cell lines and yeast. These screening assays facilitated the identification of new chemicals that function as JH agonists and antagonists. These new chemicals and others that will likely be discovered in the near future by using JH receptor and JHRE will lead to highly effective species-specific environmentally friendly insecticides for controlling pests and disease vectors.

The juvenile hormone receptor as a target of juvenoid "insect growth regulators"

Synthetic compounds that mimic the action of juvenile hormones (JHs) are founding members of a class of insecticides called insect growth regulators (IGRs). Like JHs, these juvenoids block metamorphosis of insect larvae to reproductive adults. Many biologically active juvenoids deviate in their chemical structure considerably from the sesquiterpenoid JHs, raising questions about the mode of action of such JH mimics. Despite the early deployment of juvenoid IGRs in the mid-1970s, their molecular effect could not be understood until recent discoveries of JH signaling through an intracellular JH receptor, namely the ligand-binding transcription factor Methoprene-tolerant (Met). Here, we briefly overview evidence defining three widely employed and chemically distinct juvenoid IGRs (methoprene, pyriproxyfen, and fenoxycarb), as agonist ligands of the JH receptor. We stress that knowledge of the target molecule is critical for using these compounds both as insecticides and as research tools.

Drosophila melanogaster as a powerful tool for studying insect toxicology

Insecticides are valuable and widely used tools for the control of pest insects. Despite the use of synthetic insecticides for >50 years, we continue to have a limited understanding of the genes that influence the key steps of the poisoning process. Major barriers for improving our understanding of insecticide toxicity have included a narrow range of tools and/or a large number of candidate genes that could be involved in the poisoning process. Herein, we discuss the numerous tools and resources available in Drosophila melanogaster that could be brought to bear to improve our understanding of the processes determining insecticide toxicity. These include unbiased approaches such as forward genetic screens, population genetic methods and candidate gene approaches. Examples are provided to showcase how D. melanogaster has been successfully used for insecticide toxicology studies in the past, and ideas for future studies using this valuable insect are discussed.

Synergism of the IGRs Methoprene and Pyriproxyfen Against Larval Cat Fleas (Siphonaptera: Pulicidae)

Insect growth regulators (IGRs) methoprene and pyriproxyfen are widely used as topical treatments to pets or applied to the indoor environment to control cat fleas, Ctenocephalides felis (Bouché). The toxicity of methoprene, pyriproxyfen, and combinations of both IGRs to cat flea larvae was determined. The LC₅₀ of methoprene and pyriproxyfen applied to larval rearing medium was 0.39 and 0.19 ppm, respectively. Combinations of methoprene:pyriproxyfen in ratios of 1:1, 5:1, 10:1, and 20:1 produced LC₅₀s of 0.06, 0.09, 0.19, and 0.13 ppm, respectively. The pyriproxyfen synergized the activity of methoprene as indicated by the combination indices (CI). The ratio of methoprene:pyriproxyfen (40:1) provided an LC₅₀ of 0.42 ppm and the pyriproxyfen was not synergistic. Combinations of pyriproxyfen:methoprene in ratios of 5:1, 10:1, and 20:1 provided LC₅₀s of 0.14, 0.20, 0.20 ppm, respectively, and the methoprene did not synergize the activity of pyriproxyfen. The dose-reduction indices (DRIs) indicated that the concentrations of IGRs in the combinations of methoprene:pyriproxyfen (ratios of 20:1 or less) could be reduced by at least one-third of the amount required by methoprene alone to provide similar larval mortality. Combinations of methoprene and pyriproxyfen may be effective in increasing the residual activity on pets and assist in reducing the likelihood of insecticide resistance developing to IGRs.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

Juvenile hormone regulates body size and perturbs insulin signaling in Drosophila

The role of juvenile hormone (JH) in regulating the timing and nature of insect molts is well-established. Increasing evidence suggests that JH is also involved in regulating final insect size. Here we elucidate the developmental mechanism through which JH regulates body size in developing Drosophila larvae by genetically ablating the JH-producing organ, the corpora allata (CA). We found that larvae that lack CA pupariated at smaller sizes than control larvae due to a reduced larval growth rate. Neither the timing of the metamorphic molt nor the duration of larval growth was affected by the loss of JH. Further, we show that the effects of JH on growth rate are dependent on the forkhead box O transcription factor (FOXO), which is negatively regulated by the insulin-signaling pathway. Larvae that lacked the CA had elevated levels of FOXO activity, whereas a loss-of-function mutation of FOXO rescued the effects of CA ablation on final body size. Finally, the effect of JH on growth appears to be mediated, at least in part, via ecdysone synthesis in the prothoracic gland. These results indicate a role of JH in regulating growth rate via the ecdysone- and insulin-signaling pathways.

Regulation of onset of female mating and sex pheromone production by juvenile hormone in Drosophila melanogaster

Juvenile hormone (JH) coordinates timing of female reproductive maturation in most insects. In Drosophila melanogaster, JH plays roles in both mating and egg maturation. However, very little is known about the molecular pathways associated with mating. Our behavioral analysis of females genetically lacking the corpora allata, the glands that produce JH, showed that they were courted less by males and mated later than control females. Application of the JH mimic, methoprene, to the allatectomized females just after eclosion rescued both the male courtship and the mating delay. Our studies of the null mutants of the JH receptors, Methoprene tolerant (Met) and germ cell-expressed (gce), showed that lack of Met in Met(27) females delayed the onset of mating, whereas lack of Gce had little effect. The Met(27) females were shown to be more attractive but less behaviorally receptive to copulation attempts. The behavioral but not the attractiveness phenotype was rescued by the Met genomic transgene. Analysis of the female cuticular hydrocarbon profiles showed that corpora allata ablation caused a delay in production of the major female-specific sex pheromones (the 7,11-C27 and -C29 dienes) and a change in the cuticular hydrocarbon blend. In the Met(27) null mutant, by 48 h, the major C27 diene was greatly increased relative to wild type. In contrast, the gce(2.5k) null mutant females were courted similarly to control females despite changes in certain cuticular hydrocarbons. Our findings indicate that JH acts primarily via Met to modulate the timing of onset of female sex pheromone production and mating.

The juvenile hormone signaling pathway in insect development

The molecular action of juvenile hormone (JH), a regulator of vital importance to insects, was until recently regarded as a mystery. The past few years have seen an explosion of studies of JH signaling, sparked by a finding that a JH-resistance gene, Methoprene-tolerant (Met), plays a critical role in insect metamorphosis. Here, we summarize the recently acquired knowledge on the capacity of Met to bind JH, which has been mapped to a particular ligand-binding domain, thus establishing this bHLH-PAS protein as a novel type of an intracellular hormone receptor. Next, we consider the significance of JH-dependent interactions of Met with other transcription factors and signaling pathways. We examine the regulation and biological roles of genes acting downstream of JH and Met in insect metamorphosis. Finally, we discuss the current gaps in our understanding of JH action and outline directions for future research.

Sequences that direct subcellular traffic of the Drosophila methoprene-tolerant protein (MET) are located predominantly in the PAS domains

Methoprene-tolerant protein (MET) is a key mediator of antimetamorphic signaling in insects. MET belongs to the family of bHLH-PAS transcription factors which regulate gene expression and determine essential physiological and developmental processes. The ability of many bHLH-PAS proteins to carry out their functions is related to the patterns of intracellular trafficking, which are determined by specific sequences and indicate that a nuclear localization signal (NLS) or a nuclear export signal (NES) is present and active. Therefore, the identification of NLS and NES signals is fundamental in order to understand the intracellular signaling role of MET. Nevertheless, data on the intracellular trafficking of MET are inconsistent, and until now there hasn't been any data on potential NLS and NES sequences. To analyze the trafficking of MET we designed a number of expression vectors encoding full-length MET, as well as various derivatives, that were fused to yellow fluorescent protein (YFP). Confocal microscopy analysis of the subcellular distribution of YFP-MET indicated that while this protein was localized mainly in the nucleus, it was also observed in the cytoplasm. This suggested the presence of both an NLS and NES in MET. Our work has shown that each of the two PAS domains of MET (PAS-A and PAS-B, respectively) contain one NLS and one NES sequence. Additional NES activity was present in the C-terminal fragment. The NLS activity located in PAS-B was dependent on the presence of juvenile hormone (JH), the potential ligand for MET. In contrast to this, JH didn't seem to be required for the NLS in PAS-A to be active. However, on the basis of current knowledge about the function of PAS-A in other bHLH-PAS proteins, we suggest there might be other proteins that control the activity of the NLS and possibly the NES located in the PAS-A of MET. Thus, the intracellular trafficking of MET seems to be regulated by a rather complicated network of different factors.

Evolutionary divergence of the paralogs Methoprene tolerant (Met) and germ cell expressed (gce) within the genus Drosophila

Juvenile hormone (JH) signaling underpins both regulatory and developmental pathways in insects. However, the JH receptor is poorly understood. Methoprene tolerant (Met) and germ cell expressed (gce) have been implicated in JH signaling in Drosophila. We investigated the evolution of Met and gce across 12 Drosophila species and found that these paralogs are conserved across at least 63 million years of dipteran evolution. Distinct patterns of selection found using estimates of dN/dS ratios across Drosophila Met and gce coding sequences, along with their incongruent temporal expression profiles in embryonic Drosophila melanogaster, illustrate avenues through which these genes have diverged within the Diptera. Additionally, we demonstrate that the annotated gene CG15032 is the 5' terminus of gce. In mosquitoes and beetles, a single Met-like homolog displays structural similarity to both Met and gce, and the intron locations are conserved with those of gce. We found that Tribolium and mosquito Met orthologs are assembled from Met- and gce-specific domains in a modular fashion. Our results suggest that Drosophila Met and gce experienced divergent evolutionary pressures following the duplication of an ancestral gce-like gene found in less derived holometabolous insects.

A role for juvenile hormone in the prepupal development of Drosophila melanogaster

To elucidate the role of juvenile hormone (JH) in metamorphosis of Drosophila melanogaster, the corpora allata cells, which produce JH, were killed using the cell death gene grim. These allatectomized (CAX) larvae were smaller at pupariation and died at head eversion. They showed premature ecdysone receptor B1 (EcR-B1) in the photoreceptors and in the optic lobe, downregulation of proliferation in the optic lobe, and separation of R7 from R8 in the medulla during the prepupal period. All of these effects of allatectomy were reversed by feeding third instar larvae on a diet containing the JH mimic (JHM) pyriproxifen or by application of JH III or JHM at the onset of wandering. Eye and optic lobe development in the Methoprene-tolerant (Met)-null mutant mimicked that of CAX prepupae, but the mutant formed viable adults, which had marked abnormalities in the organization of their optic lobe neuropils. Feeding Met(27) larvae on the JHM diet did not rescue the premature EcR-B1 expression or the downregulation of proliferation but did partially rescue the premature separation of R7, suggesting that other pathways besides Met might be involved in mediating the response to JH. Selective expression of Met RNAi in the photoreceptors caused their premature expression of EcR-B1 and the separation of R7 and R8, but driving Met RNAi in lamina neurons led only to the precocious appearance of EcR-B1 in the lamina. Thus, the lack of JH and its receptor Met causes a heterochronic shift in the development of the visual system that is likely to result from some cells 'misinterpreting' the ecdysteroid peaks that drive metamorphosis.

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.

bHLH-PAS family transcription factor methoprene-tolerant plays a key role in JH action in preventing the premature development of adult structures during larval-pupal metamorphosis

The biological actions of juvenile hormones are well studied; they regulate almost all aspects of an insect's life. However, the molecular actions of these hormones are not well understood. Recent studies in the red flour beetle, Tribolium castaneum, demonstrated the utility of this insect as a model system to study JH action. These studies confirmed that the bHLH-PAS family transcription factor, methoprene-tolerant (TcMet,) plays a key role in JH action during larval stages. In this study, we investigated the role of TcMet in JH action during larval-pupal metamorphosis. The phenotypes of TcMet RNAi insects shared similarity with the phenotypes of some allatectomized lepidopteran larvae that were attempting to undergo precocious larval-pupal metamorphosis. Knocking-down TcMet during the final instar also disrupted larval-pupal ecdysis, resulting in the development of adultoid underneath the larval skin. However, the loss of TcMet did not completely block remodeling of internal tissues such as midgut. T. castaneum larvae injected with TcMet dsRNA demonstrated a resistance to a JH analog (JHA), hydroprene, irrespective of time and route of application. Knocking-down TcMet also caused down regulation of JH-response genes, JHE and Kr-h1 suggesting that TcMet might be involved in the expression of these genes. Based on the phenotype, gene expression, and JHA action studies in TcMet RNAi insects, this study concludes that Met plays a key role in JH action for preventing the premature development of adult structures during larval-pupal metamorphosis.

Identification and characterization of a juvenile hormone response element and its binding proteins

Juvenile hormones (JH) regulate a wide variety of developmental and physiological processes in insects. Comparison of microarray data on JH-induced genes in the fruit fly, Drosophila melanogaster, L57 cells and in the honey bee, Apis mellifera, identified 16 genes that are induced in both species. Analysis of promoter regions of these 16 D. melanogaster genes identified DmJHRE1 (D. melanogaster JH response element 1). In L57 cells, the reporter gene regulated by DmJHRE1 was induced by JH III. Two proteins (FKBP39 and Chd64) that bind to DmJHRE1 were identified. FKBP39 and Chd64 double-stranded RNA inhibited JH III induction of a reporter gene regulated by DmJHRE1. FKBP39 and Chd64 proteins expressed in yeast bound to DmJHRE1. Two-hybrid and pull-down assays showed that these two proteins interact with each other as well as with ecdysone receptor, ultraspiracle, and methoprene-tolerant protein. Developmental expression profiles and JH induction of mRNA for FKBP39 and Chd64 proteins and their interaction with proteins known to be involved in both JH (methoprene-tolerant protein) and ecdysteroid action (ecdysone receptor and ultraspiracle) suggest that these proteins probably play important roles in cross-talk between JH and ecdysteroids.

Farnesoid secretions of dipteran ring glands: what we do know and what we can know

Harnessing of the Drosophila genetic system toward ascertaining the molecular endocrinology of higher dipteran (cyclorrhaphan) larval development has been a goal for over 70 years, beginning with the data left to us by pioneer researchers from the classical endocrine era. The results of their experiments evidence numerous ring gland activities that are parsimoniously explained as arising from secretions of the larval corpora allatal cells. Utilization of those data toward an understanding of molecular endocrinology of cyclorrhaphan metamorphosis has not yet achieved its hoped for fruition, in part due to a perceived difficulty in identifying larval targets of the molecule "methyl epoxyfarnesoate" (=juvenile hormone III). However, as is reviewed here, it is important to maintain a conceptual distinction between "the target of JH III"Versus "the target(s) of products secreted by the larval corpora allatal cells of ring glands." Recent advances have been made on the identity, regulation and reception of ring gland farnesoid products. When these advances are evaluated together with the above data from the classical endocrine era, there is a new opportunity to frame experimental hypotheses so as to discern underlying mechanisms on cyclorrhaphan larval-pupal metamorphosis that have been heretofore intractable. This paper reconsiders a number of evidenced physiological targets of secretions of corpora allatal cells of the larval ring gland, and places them in the context of more recent biochemical and molecular advances in the field.

Juvenile hormone resistance gene Methoprene-tolerant controls entry into metamorphosis in the beetle Tribolium castaneum

Besides being a spectacular developmental process, metamorphosis is key to insect success. Entry into metamorphosis is controlled by juvenile hormone (JH). In larvae, JH prevents pupal and adult morphogenesis, thus keeping the insect in its immature state. How JH signals to preclude metamorphosis is poorly understood, and a JH receptor remains unknown. One candidate for the JH receptor role is the Methoprene-tolerant (Met) Per-Arnt-Sim (PAS) domain protein [also called Resistance to JH, Rst (1)JH], whose loss confers tolerance to JH and its mimic methoprene in the fruit fly Drosophila melanogaster. However, Met deficiency does not affect the larval-pupal transition, possibly because this process does not require JH absence in Drosophila. By contrast, the red flour beetle Tribolium castaneum is sensitive to developmental regulation by JH, thus making an ideal system to examine the role of Met in the antimetamorphic JH action. Here we show that impaired function of the Met ortholog TcMet renders Tribolium resistant to the effects of ectopic JH and, in a striking contrast to Drosophila, causes early-stage beetle larvae to undergo precocious metamorphosis. This is evident as TcMet-deficient larvae pupate prematurely or develop specific heterochronic phenotypes such as pupal-like cuticular structures, appendages, and compound eyes. Our results demonstrate that TcMet functions in JH response and provide the critical evidence that the putative JH receptor Met mediates the antimetamorphic effect of JH.

Drosophila melanogaster Methoprene-tolerant (Met) gene homologs from three mosquito species: Members of PAS transcriptional factor family

The Methoprene-tolerant (Met) gene in Drosophila melanogaster has been shown to function in juvenile hormone (JH) action. Met homologs were isolated from three mosquito species, Culex pipiens, Aedes aegypti and Anopheles gambiae. Sequence similarity was found to be high in bHLH and PAS conserved domains, and the majority of the 7-9 introns in AaMet and AgMet are located in either identical or similar positions, indicating evolutionary relatedness. Sequence comparison with Met and the similar germ-cell expressed (gce) gene in D. melanogaster showed that the mosquito genes are more similar to gce than to Met. Moreover, the multiple introns in AgMet and AaMet are more similar in number with the 7 introns in Dmgce than to the single intron in DmMet; in fact, six intron positions in AaMet and AgMet are similar to those in Dmgce. Efforts to identify a second homologous gene in mosquitoes were unsuccessful, suggesting a single gene in lower Diptera, consistent with the single gene uncovered in genomic sequencing of Ae. aegypti and An. gambiae. These results suggest that a gene duplication occurred during the evolution of higher Diptera, resulting in Met and gce.