Approaches and Tools to Study the Roles of Juvenile Hormones in Controlling Insect Biology

Simple Summary

The juvenile hormones (JHs) play critical roles during insect development and reproduction. The numerous effects of JHs have generated multiple basic scientific questions, as well as prospects for the development of insecticidal endocrine disruptors. There is an increasing need for methods to identify and quantify endogenous JHs. The low titers and difficulties in working with these lipophilic compounds have often hindered the study of JH biology. In this article, we review the existing information on the detection and quantification of JH from insect samples, the development of approaches to manipulate JH titers, and the use of next-generation tools to modulate JH homeostasis.

Abstract

The juvenile hormones (JHs) are a group of sesquiterpenoids synthesized by the corpora allata. They play critical roles during insect development and reproduction. To study processes that are controlled by JH, researchers need methods to identify and quantify endogenous JHs and tools that can be used to increase or decrease JH titers in vitro and in vivo. The lipophilic nature of JHs, coupled with the low endogenous titers, make handling and quantification challenging. JH titers in insects can easily be increased by the topical application of JH analogs, such as methoprene. On the other hand, experimentally reducing JH titers has been more difficult. New approaches to modulate JH homeostasis have been established based on advances in RNA interference and CRISPR/Cas9-based genome editing. This review will summarize current advances in: (1) the detection and quantification of JHs from insect samples; (2) approaches to manipulating JH titers; and (3) next-generation tools to modulate JH homeostasis.

MicroRNA miR-927 targets the juvenile hormone primary response gene Krüppel homolog1 to control Drosophila developmental growth

Krüppel homolog1 (Kr-h1) is a juvenile hormone (JH) response transcriptional factor that transduces JH signalling to repress insect metamorphosis in both hemimetabolous and holometabolous insects. While few studies about microRNAs (miRNAs) downregulating Kr-h1 expression to mediate insect metamorphosis have been demonstrated in hemimetabolous insects, the miRNAs that target the Kr-h1 of holometabolous insects have not been reported. Here, we identified two miR-927 binding sites within the 3'UTR region of Kr-h1 in Drosophila melanogaster, and miR-927 was found to downregulate the expression of Kr-h1. The expression profiles of miR-927 and Kr-h1 displayed relatively opposite pattern during most of the larval development stages. Overexpression of miR-927 in the fat body significantly decreased the expression of Kr-h1 and resulted in reduced oviposition, increased mortality, delayed pupation, and reduced pupal size. Notably, the co-overexpression of Kr-h1 rescued the developmental and growth defects associated with miR-927 overexpression, indicating that Kr-h1 is a biologically relevant target of miR-927. Moreover, the expression of miR-927 was found to be repressed by JH and its receptor Met/gce, forming a positive regulatory loop of JH signalling. Overall, our studies support a conserved role for the JH/miRNA/Kr-h1 regulatory axis in growth control during insect development.

Endocrine regulation of immunity in insects

Organisms have constant contact with potentially harmful agents that can compromise their fitness. However, most of the times these agents fail to cause serious disease by virtue of the rapid and efficient immune responses elicited in the host that can range from behavioural adaptations to immune system triggering. The immune system of insects does not comprise the adaptive arm, making it less complex than that of vertebrates, but key aspects of the activation and regulation of innate immunity are conserved across different phyla. This is the case for the hormonal regulation of immunity as a part of the broad organismal responses to external conditions under different internal states. In insects, depending on the physiological circumstances, distinct hormones either enhance or suppress the immune response integrating individual (and often collective) responses physiologically and behaviourally. In this review, we provide an overview of our current knowledge on the endocrine regulation of immunity in insects, its mechanisms and implications on metabolic adaptation and behaviour. We highlight the importance of this multilayered regulation of immunity in survival and reproduction (fitness) and its dependence on the hormonal integration with other mechanisms and life-history traits.

Two Undervalued Functions of the Golgi Apparatus: Removal of Excess Ca2+ and Biosynthesis of Farnesol-Like Sesquiterpenoids, Possibly as Ca2+-Pump Agonists and Membrane "Fluidizers-Plasticizers"

The extensive literature dealing with the Golgi system emphasizes its role in protein secretion and modification, usually without specifying from which evolutionary ancient cell physiological necessity such secretion originated. Neither does it specify which functional requirements the secreted proteins must meet. From a reinterpretation of some classical and recent data gained mainly, but not exclusively, from (insect) endocrinology, the view emerged that the likely primordial function of the rough endoplasmic reticulum (RER)–Golgi complex in all eukaryotes was not the secretion of any type of protein but the removal of toxic excess Ca²⁺ from the cytoplasm. Such activity requires the concurrent secretion of large amounts of Ca²⁺-carrying/transporting proteins acting as a micro-conveyor belt system inside the RER–Golgi. Thus, (fitness increasing) protein secretion is subordinate to Ca²⁺ removal. Milk with its high content of protein and Ca²⁺ (60–90 mM vs. 100 nM in unstimulated mammary gland cells) is an extreme example. The sarco(endo)plasmatic reticulum Ca²⁺-ATPases (SERCAs) and SPCA1a Ca²⁺/Mn²⁺ transport ATPases are major players in Ca²⁺ removal through the Golgi. Both are blocked by the sesquiterpenoid thapsigargin. This strengthens the hypothesis (2014) that endogenous farnesol-like sesquiterpenoids (FLSs) may act as the long sought for but still unidentified agonist(s) for Ca²⁺-pumps in both the ER and Golgi. A second putative function also emerges. The fusion of both the incoming and outgoing transport vesicles, respectively, at the cis- and trans- side of Golgi stacks, with the membrane system requiring high flexibility and fast self-closing of the involved membranes. These properties may—possibly partially—be controlled by endogenous hydrophobic membrane “fluidizers” for which FLSs are prime candidates. A recent reexamination of unexplained classical data suggests that they are likely synthesized by the Golgi itself. This game-changing hypothesis is endorsed by several arguments and data, some of which date from 1964, that the insect corpus allatum (CA), which is the major production site of farnesol-esters, has active Golgi systems. Thus, in addition to secreting FLS, in particular juvenile hormone(s), it also secretes a protein(s) or peptide(s) with thus far unknown function. This paper suggests answers to various open questions in cell physiology and general endocrinology.

RNAi-Mediated Knockdown of Transcription Factor E93 in Nymphs of the Desert Locust (Schistocerca gregaria) Inhibits Adult Morphogenesis and Results in Supernumerary Juvenile Stages

Postembryonic development of insects is coordinated by juvenile hormone (JH) together with ecdysteroids. Whereas the JH early response gene krüppel-homolog 1 (Kr-h1) plays a crucial role in the maintenance of juvenile characteristics during consecutive larval stages, the ecdysteroid-inducible early gene E93 appears to be a key factor promoting metamorphosis and adult morphogenesis. Here, we report on the developmental and molecular consequences of an RNAi-mediated knockdown of SgE93 in the desert locust, Schistocerca gregaria, a hemimetabolan species. Our experimental data show that injection of gregarious locust nymphs with a double-stranded RNA construct targeting the SgE93 transcript inhibited the process of metamorphosis and instead led to supernumerary nymphal stages. These supernumerary nymphal instars still displayed juvenile morphological features, such as a nymphal color scheme and body shape, while they reached the physical body size of the adult locusts, or even surpassed it after the next supernumerary molt. Interestingly, when compared to control locusts, the total duration of the fifth and normally final nymphal (N5) stage was shorter than normal. This appeared to correspond with temporal and quantitative changes in hemolymph ecdysteroid levels, as well as with altered expression of the rate-limiting Halloween gene, Spook (SgSpo). In addition, the levels of the ecdysone receptor (SgEcR) and retinoïd X receptor (SgRXR) transcripts were altered, indicating that silencing SgE93 affects both ecdysteroid synthesis and signaling. Upon knockdown of SgE93, a very potent upregulation of the SgKr-h1 transcript levels was observed in both head and fat body, while no significant changes were detected in the transcript levels of SgJHAMT and SgCYP15A1, the enzymes that catalyze the two final steps in JH biosynthesis. Moreover, the process of molting was disturbed in these supernumerary nymphs. While attempting ecdysis to the next stage, 50% of the N6 and all N7 nymphal instars eventually died. S. gregaria is a very harmful, swarm-forming pest species that destroys crops and threatens food security in many of the world's poorest countries. We believe that a better knowledge of the mechanisms of postembryonic development may contribute to the discovery of novel, more selective and sustainable strategies for controlling gregarious locust populations. In this context, identification of molecular target candidates that are capable of significantly reducing the fitness of this devastating swarming pest will be of crucial importance.

Diversity of Insect Sesquiterpenoid Regulation

Insects are arguably the most successful group of animals in the world in terms of both species numbers and diverse habitats. The sesquiterpenoids juvenile hormone, methyl farnesoate, and farnesoic acid are well known to regulate metamorphosis, reproduction, sexual dimorphism, eusociality, and defense in insects. Nevertheless, different insects have evolved with different sesquiterpenoid biosynthetic pathway as well as products. On the other hand, non-coding RNAs such as microRNAs have been implicated in regulation of many important biological processes, and have recently been explored in the regulation of sesquiterpenoid production. In this review, we summarize the latest findings on the diversity of sesquiterpenoids reported in different groups of insects, as well as the recent advancements in the understanding of regulation of sesquiterpenoid production by microRNAs.

Precocious Downregulation of Krüppel-Homolog 1 in the Migratory Locust, Locusta migratoria, Gives Rise to An Adultoid Phenotype with Accelerated Ovarian Development but Disturbed Mating and Oviposition

Krüppel-homolog 1 (Kr-h1) is a zinc finger transcription factor maintaining the status quo in immature insect stages and promoting reproduction in adult insects through the transduction of the Juvenile Hormone (JH) signal. Knockdown studies have shown that precocious silencing of Kr-h1 in the immature stages results in the premature development of adult features. However, the molecular characteristics and reproductive potential of these premature adult insect stages are still poorly understood. Here we report on an adult-like or ‘adultoid’ phenotype of the migratory locust, Locusta migratoria, obtained after a premature metamorphosis induced by the silencing of LmKr-h1 in the penultimate instar. The freshly molted adultoid shows precocious development of adult features, corresponding with increased transcript levels of the adult specifier gene LmE93. Furthermore, accelerated ovarian maturation and vitellogenesis were observed in female adultoids, coinciding with elevated expression of LmCYP15A1 in corpora allata (CA) and LmKr-h1 and vitellogenin genes (LmVg) in fat body, whereas LmE93 and Methoprene-tolerant (LmMet) transcript levels decreased in fat body. In adultoid ovaries, expression of the Halloween genes, Spook (LmSpo) and Phantom (LmPhm), was elevated as well. In addition, the processes of mating and oviposition were severely disturbed in these females. L. migratoria is a well-known, swarm-forming pest insect that can destroy crops and harvests in some of the world’s poorest countries. As such, a better understanding of factors that are capable of significantly reducing the reproductive potential of this pest may be of crucial importance for the development of novel locust control strategies.

Pheromonal Regulation of the Reproductive Division of Labor in Social Insects

The reproductive altruism in social insects is an evolutionary enigma that has been puzzling scientists starting from Darwin. Unraveling how reproductive skew emerges and maintains is crucial to understand the reproductive altruism involved in the consequent division of labor. The regulation of adult worker reproduction involves conspecific inhibitory signals, which are thought to be chemical signals by numerous studies. Despite the primary identification of few chemical ligands, the action modes of primer pheromones that regulate reproduction and their molecular causes and effects remain challenging. Here, these questions were elucidated by comprehensively reviewing recent advances. The coordination with other modalities of queen pheromones (QPs) and its context-dependent manner to suppress worker reproduction were discussed under the vast variation and plasticity of reproduction during colony development and across taxa. In addition to the effect of QPs, special attention was paid to recent studies revealing the regulatory effect of brood pheromones. Considering the correlation between pheromone and hormone, this study focused on the production and perception of pheromones under the endocrine control and highlighted the pivotal roles of nutrition-related pathways. The novel chemicals and gene pathways discovered by recent works provide new insights into the understanding of social regulation of reproductive division of labor in insects.

Rhodnius, Golden Oil, and Met: A History of Juvenile Hormone Research

Juvenile hormone (JH) is a unique sesquiterpenoid hormone which regulates both insect metamorphosis and insect reproduction. It also may be utilized by some insects to mediate polyphenisms and other life history events that are environmentally regulated. This article details the history of the research on this versatile hormone that began with studies by V. B. Wigglesworth on the "kissing bug" Rhodnius prolixus in 1934, through the discovery of a natural source of JH in the abdomen of male Hyalophora cecropia moths by C. M. Williams that allowed its isolation ("golden oil") and identification, to the recent research on its receptor, termed Methoprene-tolerant (Met). Our present knowledge of cellular actions of JH in metamorphosis springs primarily from studies on Rhodnius and the tobacco hornworm Manduca sexta, with recent studies on the flour beetle Tribolium castaneum, the silkworm Bombyx mori, and the fruit fly Drosophila melanogaster contributing to the molecular understanding of these actions. Many questions still need to be resolved including the molecular basis of competence to metamorphose, differential tissue responses to JH, and the interaction of nutrition and other environmental signals regulating JH synthesis and degradation.

Dual roles of juvenile hormone signaling during early oogenesis in Drosophila

Juvenile hormone (JH) signaling plays crucial roles in insect metamorphosis and reproduction. Function of JH signaling in germline stem cells (GSCs) remains largely unknown. Here, we found that the number of GSCs significantly declined in the ovaries of Met, Gce and JHAMT mutants. Then we inhibited JH signaling in selected cell types of ovaries by expressing Met and Gce or Kr-h1 double-stranded RNAs (dsRNAs) using different Gal4 drivers. Blocking of JH signaling in muscle cells has no effect on GSC numbers. Blocking of JH signaling in cap cells reduced GSCs cells. Inductive expression of Met and Gce dsRNA but not Kr-h1 by Nos-Gal4 increased GSC cells. These results indicate that JH signaling plays an important role in GSC maintenance.

Impacts of Methyl Farnesoate and 20-Hydroxyecdysone on Larval Mortality and Metamorphosis in the Kuruma Prawn Marsupenaeus japonicus

Physiological functions of juvenile hormone (JH) and molting hormone have been demonstrated in insects. JH, molting hormone and their mimics (insect growth regulators, IGRs) show endocrine-disrupting effects not only on target pest insects but also on other arthropod species such as crustaceans. However, little is known about the endocrine-disrupting effects of IGRs on benthic crustaceans. In this study, laboratory experiments were conducted to investigate effects of representative innate JH in crustaceans (methyl farnesoate, MF) and molting hormone (20-hydroxyecdysone, 20E, active form of ecdysteroid) on larval stages of the kuruma prawn Marsupenaeus japonicus, which is a decapod crustacean living in warm seawater. Larval development of kuruma prawn progresses in the order of nauplius, zoea, mysis, and then post-larvae with molting and metamorphosis, but it is unknown whether both MF and 20E have crucial roles in metamorphosis and molting of this species. Treatments of either MF or 20E on shrimp larvae were attempted at each developmental stage and those effects were validated. In terms of EC50 values between mortality and metamorphosis, there were apparent differences in the transition from nauplius to zoea (MF: 7.67 and 0.12 μM; 20E: 3.84 and 0.06 μM in survival and metamorphic rates, respectively). In contrast, EC50 values in MF and 20E treatments showed high consistency in the transitions between zoea to mysis (EC50 values for survival; MF: 1.25 and 20E: 0.22 μM), and mysis to post-larvae (EC50 values for survival; MF: 0.65 and 20E: 0.46 μM). These data suggest that nauplius has strong resistance against exposure to MF and 20E. Moreover, both chemicals induced high mortality triggered by the disruption of molting associated with metamorphosis. To our knowledge, this is the first experimental evidence that investigates in vivo physiological functions of MF and 20E in the larval stages of kuruma prawn, shedding light on not only ecotoxicological impacts of IGRs released into nature, but also endocrine mechanisms underlying larval development with metamorphosis in benthic decapod crustaceans.

Histone Deacetylase 11 Knockdown Blocks Larval Development and Metamorphosis in the Red Flour Beetle, Tribolium castaneum

Post-translational modifications (PTM) such as methylation, acetylation, phosphorylation, and ubiquitination of histones and other proteins regulate expression of genes. The acetylation levels of these proteins are determined by the balance of expression of histone acetyltransferase (HATs) and histone deacetylases (HDACs). We recently reported that class I HDACs (HDAC1 and HDAC3) play important roles in juvenile hormone (JH) suppression of metamorphosis in the red flour beetle, Tribolium castaneum. Here, we report on the function of a single class IV HDAC member, HDAC11. Injection of dsRNA targeting T. castaneum HDAC11 gene into newly molted last instar larvae induced knockdown of the target gene and arrested larval development and prevented metamorphosis into the pupal stage. Dark melanized areas were detected in larvae that showed developmental arrest and mortality. Developmental expression studies showed an increase in HDAC11 mRNA levels beginning at the end of the penultimate larval stage. These higher levels were maintained during the final instar larval and pupal stages. A JH analog, hydroprene, suppressed HDAC11 expression in the larvae. Sequencing of RNA isolated from control and dsHDAC11 injected larvae identified several differentially expressed genes, including those involved in JH action, ecdysone response, and melanization. The acetylation levels of core histones showed an increase in TcA cells exposed to dsHDAC11. Also, an increase in histone H3 acetylation, specifically H3K9, H3K18 and H3K27, were detected in HDAC11 knockdown larvae. These studies report the function of HDAC11 in insects other than Drosophila for the first time and show that HDAC11 influences the acetylation levels of histones and expression of multiple genes involved in T. castaneum larval development.

Upregulation of E93 Gene Expression Acts as the Trigger for Metamorphosis Independently of the Threshold Size in the Beetle Tribolium castaneum

Body size in holometabolous insects is determined by the size at which the juvenile larva undergoes metamorphosis to the pupal stage. To undergo larva-pupa transition, larva must reach a critical developmental checkpoint, the threshold size (TS); however, the molecular mechanisms through which the TS cues this transition remain to be fully characterized. Here, we use the flour beetle Tribolium castaneum to characterize the molecular mechanisms underlying entry into metamorphosis. We found that T. castaneum reaches a TS at the beginning of the last larval instar, which is associated with the downregulation of TcKr-h1 and the upregulation of TcE93 and TcBr-C. Unexpectedly, we found that while there is an association between TS and TcE93 upregulation, it is the latter that constitutes the molecular trigger for metamorphosis initiation. In light of our results, we evaluate the interactions that control the larva-pupa transition and suggest alternative models.

Involvement of Two Paralogous Methoprene-Tolerant Genes in the Regulation of Vitellogenin and Vitellogenin Receptor Expression in the Rice Stem Borer, Chilo suppressalis

Besides the function of preventing metamorphosis in insects, the juvenile hormone (JH) plays a role in female reproduction; however, the underlying mechanism is largely unknown. The methoprene-tolerant (Met) protein belongs to a family of basic helix-loop-helix–Per-Arnt-Sim (bHLH-PAS) transcription factors and functions as the JH intracellular receptor. In this study, two full length cDNAs encoding Met (CsMet1 and CsMet2) were isolated from the rice stem borer, Chilo suppressalis. Structural analysis revealed that both CsMet1 and CsMet2 exhibited typical bHLH, PAS-A, PAS-B, and PAC (PAS C terminal motif) domains. Comparative analysis of transcript level using reverse transcription-quantitative PCR (RT-qPCR) revealed that CsMet1 was predominant in almost all examined developmental stages and tissues. Treatment with methoprene in vivo induces the transcription of both CsMet1 and CsMet2. Notably, injection of dsCsMet1 and dsCsMet2 suppressed the expression levels of vitellogenin (CsVg) and Vg receptor (CsVgR). These findings revealed the potential JH signaling mechanism regulating C. suppressalis reproduction, and provided evidence that RNAi-mediated knockdown of Met holds great potential as a control strategy of C. suppressalis.

Regulatory Mechanisms of Cell Polyploidy in Insects

Polyploidy cells undergo the endocycle to generate DNA amplification without cell division and have important biological functions in growth, development, reproduction, immune response, nutrient support, and conferring resistance to DNA damage in animals. In this paper, we have specially summarized current research progresses in the regulatory mechanisms of cell polyploidy in insects. First, insect hormones including juvenile hormone and 20-hydroxyecdysone regulate the endocycle of variant cells in diverse insect species. Second, cells skip mitotic division in response to developmental programming and conditional stimuli such as wound healing, regeneration, and aging. Third, the reported regulatory pathways of mitotic to endocycle switch (MES), including Notch, Hippo, and JNK signaling pathways, are summarized and constructed into genetic network. Thus, we think that the studies in crosstalk of hormones and their effects on canonical pathways will shed light on the mechanism of cell polyploidy and elucidate the evolutionary adaptions of MES through diverse insect species.

The making of the defensive caste: Physiology, development, and evolution of the soldier differentiation in termites

Termites (Blattodea, Termitoidea, or Isoptera) constitute one of the major lineages of eusocial insects. In termite societies, multiple types of functional individuals, that is, castes, perform divisions of labors to coordinate social behaviors. Among other castes, the soldier caste is distinctive since it is sterile and exclusively specialized into defensive behavior with largely modified morphological features. Therefore, many of the previous studies have been focused on soldiers, in terms of ecology, behavior, and evolution as well as developmental and physiological mechanisms. This article overviews the accumulation of studies especially focusing on the developmental and physiological mechanisms underlying the soldier differentiation in termites. Furthermore, the evolutionary trajectories that have led the acquisition of soldier caste and have diversified the soldier characteristics in association with the social evolution are discussed.

RNA interference-mediated knockdown of the transcription factor Krüppel homologue 1 suppresses vitellogenesis in Chilo suppressalis

Vitellogenesis in holometabolous insects involves the production and secretion of vitellogenin (Vg) and other yolk protein precursors in developing oocyte by the fat body, all of which is predominantly orchestrated by juvenile hormone (JH). Krüppel homologue 1 (Kr-h1) is a zinc finger transcription factor that has been demonstrated to be a JH-early inducible gene and to contribute to reproduction. However, the exact molecular function of Kr-h1 in insect reproduction is poorly understood. In the current study, we used the notorious pest Chilo suppressalis as a model system to investigate the role of Kr-h1 in female reproduction. Cloning and sequencing C. suppressalis Kr-h1 revealed that it shares high identity with its homologues from other lepidopteran insects. Moreover, RNA interference-mediated knockdown of CsKr-h1 substantially reduced the transcription of Vg in the fat body, dramatically decreased yolk protein deposition and also impaired oocyte maturation and ovarian development, indicating that Kr-h1 is indispensable for normal vitellogenesis in C. suppressalis. Based on these results, we conclude that Kr-h1 is crucial to reproduction in insects and that targeting this gene could potentially be a new way to suppress rice pests.

Cloning and characterization of Methoprene-tolerant (Met) and Krüppel homolog 1 (Kr-h1) genes in the wheat blossom midge, Sitodiplosis mosellana

Juvenile hormone (JH), a growth regulator, inhibits ecdysteroid-induced metamorphosis and controls insect development and diapause. Methoprene-tolerant (Met) and Krüppel homolog 1 (Kr-h1) are two proteins involved in JH action. To gain some insight into their function in development of Sitodiplosis mosellana, an insect pest undergoing obligatory larval diapause at the mature 3rd instar stage, we cloned full-length complementary DNAs of Met and Kr-h1 from this species. SmMet encoded a putative protein, which contained three domains typical of the bHLH-PAS family and eight conserved amino acid residues important for JH binding. SmKr-h1 encoded a protein showing high sequence homology to its counterparts in other species, and contained all eight highly conserved Zn-finger motifs for DNA-binding. Expression patterns of SmMet and SmKr-h1 were developmentally regulated and JH III responsive as well. Their mRNA abundance increased as larvae entered early 3rd instar, pre-diapause and maintenance stages, and peaked during post-diapause quiescence, a pattern correlated with JH titers in this species. Different from reduced expression of SmMet, SmKr-h1 mRNA increased at mid-to-late period of post-diapause development. Topical application of JH III on diapausing larvae also induced the two genes in a dose-dependent manner. Expression of SmMet and SmKr-h1 clearly declined in the pre-pupal phase, and was significantly higher in female adults than male adults. These results suggest that JH-responsive SmMet and SmKr-h1 might play key roles in diapause induction and maintenance as well as in post-diapause quiescence and adult reproduction, whereas metamorphosis from larvae to pupae might be correlated with their reduced expression.

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.

Krüppel homolog 1 and E93: The doorkeeper and the key to insect metamorphosis

Insect metamorphosis is regulated by two main hormones: ecdysone (20E), which promotes molting, and juvenile hormone (JH), which inhibits adult morphogenesis. The transduction mechanisms for the respective hormonal signals include the transcription factors Krüppel homolog 1 (Kr-h1) and E93, which are JH- and 20E-dependent, respectively. Kr-h1 is the main effector of the antimetamorphic action of JH, while E93 is a key promoter of metamorphosis. The ancestral regulatory axis of metamorphosis, which operates in insects with hemimetabolan (gradual) metamorphosis and is known as the MEKRE93 pathway, is based on Kr-h1 repression of E93. In the last juvenile stage, when the production of JH dramatically decreases, Kr-h1 expression is almost completely interrupted, E93 becomes upregulated and metamorphosis proceeds. The holometabolan (complete) metamorphosis mode of development includes the peculiar pupal stage, a sort of intermediate between the final larval instar and the adult stage. In holometabolan species, Broad-Complex (BR-C) transcription factors determine the pupal stage and E93 stimulates the expression of BR-C in the prepupa. The MEKRE93 pathway is conserved in holometabolan insects, which have added the E93/BR-C interaction loop to the ancestral (hemimetabolan) pathway during the evolution from hemimetaboly to holometaboly.

Crosstalk among Indoleamines, Neuropeptides and JH/20E in Regulation of Reproduction in the American Cockroach, Periplaneta americana

Although the regulation of vitellogenesis in insects has been mainly discussed in terms of ‘classical’ lipid hormones, juvenile hormone (JH), and 20-hydroxyecdysone (20E), recent data support the notion that this process must be adjusted in harmony with a nutritional input/reservoir and involvement of certain indoleamines and neuropeptides in regulation of such process. This study focuses on crosstalks among these axes, lipid hormones, monoamines, and neuropeptides in regulation of vitellogenesis in the American cockroach Periplaneta americana with novel aspects in the roles of arylalkylamine N-acetyltransferase (aaNAT), a key enzyme in indoleamine metabolism, and the enteroendocrine peptides; crustacean cardioactive peptide (CCAP) and short neuropeptide F (sNPF). Double-stranded RNA against aaNAT (dsRNA^aaNAT) was injected into designated-aged females and the effects were monitored including the expressions of aaNAT itself, vitellogenin 1 and 2 (Vg1 and Vg2) and the vitellogenin receptor (VgR) mRNAs, oocyte maturation and changes in the hemolymph peptide concentrations. Effects of peptides application and 20E were also investigated. Injection of dsRNA^aaNAT strongly suppressed oocyte maturation, transcription of Vg1, Vg2, VgR, and genes encoding JH acid- and farnesoate O-methyltransferases (JHAMT and FAMeT, respectively) acting in the JH biosynthetic pathway. However, it did not affect hemolymph concentrations of CCAP and sNPF. Injection of CCAP stimulated, while sNPF suppressed oocyte maturation and Vgs/VgR transcription, i.e., acting as allatomedins. Injection of CCAP promoted, while sNPF repressed ecdysteroid (20E) synthesis, particularly at the second step of Vg uptake. 20E also affected the JH biosynthetic pathway and Vg/VgR synthesis. The results revealed that on the course of vitellogenesis, JH- and 20E-mediated regulation occurs downstream to indoleamines- and peptides-mediated regulations. Intricate mutual interactions of these regulatory routes must orchestrate reproduction in this species at the highest potency.

Immunohistochemical and Direct Mass Spectral Analyses of Plautia stali Myoinhibitory Peptides in the Cephalic Ganglia of the Brown-Winged Green Bug Plautia stali

For seasonal adaptation, the brown-winged green bug Plautia stali (Hemiptera: Pentatomidae) enters reproductive diapause by suppressing juvenile hormone biosynthesis. Plautia stali myoinhibitory peptides (Plast-MIPs) are known to have allatostatic effects and to suppress juvenile hormone biosynthesis. We examined Plast-MIP-producing neurons in the brain with immunohistochemistry and Fourier transform ion cyclotron resonance mass spectrometry. Rabbit polyclonal antiserum against Plast-MIP revealed immunoreactive cells in seven regions of the brain, including the posterior antennal lobe, basal optic lobe, dorsal anterior protocerebrum, ventrolateral protocerebrum, pars intercerebralis, posterior protocerebrum, and dorsal posterior region to the calyx of the mushroom body, aside from the gnathal ganglion. Anatomical locations of the immunoreactive cells in the pars intercerebralis and dorsal posterior region to the mushroom body calyx partly overlapped with the cell body location stained by retrograde dye fills from the corpus allatum and corpus cardiacum complex. Direct mass spectrometry revealed the molecular ion peaks corresponding to the predictive mass of Plast-MIPs in the pars intercerebralis and the corpus allatum-corpus cardiacum complex. Plast-MIP immunoreactivity in different cell types suggests that Plast-MIPs have different functions in the cephalic ganglia. Considering the anatomical location of neurons projecting to the corpus allatum-corpus cardiacum and results of mass spectrometry, Plast-MIP immunoreactive cells in the pars intercerebralis may play a role in suppressing juvenile hormone biosynthesis.

Methoprene-Induced Genes in Workers of Formosan Subterranean Termites (Coptotermes formosanus Shiraki)

Termites have a distinct polyphenism controlled by concise hormonal and molecular mechanisms. Workers undergo double molts to transform into soldiers (worker–presoldier–soldier). Juvenile hormone analogs, such as methoprene, can induce workers to transform into presoldiers. However, the molecular mechanism underlying the worker-to-presoldier transformation in Coptotermes formosanus Shiraki is still not clear. We sequenced the transcriptome of workers four days after they had fed on methoprene-treated filter paper and control group workers, which fed on acetone-treated filter paper. The transcriptome of C. formosanus was assembled using the de novo assembly method. Expression levels of unigenes in the methoprene-treated group and the control group were compared. The differentially expressed genes were further analyzed by Gene Ontology (GO) term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. Tetrapyrrole binding, oxidoreductase activity, and metal ion binding were the only three enriched GO terms. Juvenile hormone synthesis was the first ranked enriched pathway. Carbohydrate, amino acid, and lipid metabolism pathways were also enriched. These three pathways may be related to fat body development, which is critical for presoldier formation. Our results have demonstrated the significance of JH synthesis pathways, and pathways related to fat body development in the artificial induction of presoldiers.

A Life's Journey Through Insect Metamorphosis

This autobiographical article describes the research career of Lynn M. Riddiford from its early beginnings in a summer program for high school students at Jackson Laboratory to the present "retirement" at the Friday Harbor Laboratories. The emphasis is on her forays into many areas of insect endocrinology, supported by her graduate students and postdoctoral associates. The main theme is the hormonal regulation of metamorphosis, especially the roles of juvenile hormone (JH). The article describes the work of her laboratory first in the elucidation of the endocrinology of the tobacco hornworm, Manduca sexta, and later in the molecular aspects of the regulation of cuticular and pigment proteins and of the ecdysone-induced transcription factor cascade during molting and metamorphosis. Later studies utilized Drosophila melanogaster to answer further questions about the actions of JH.

Epigenetic Regulator CoREST Controls Social Behavior in Ants

Ants acquire distinct morphological and behavioral phenotypes arising from a common genome, underscoring the importance of epigenetic regulation. In Camponotus floridanus, "Major" workers defend the colony, but can be epigenetically reprogrammed to forage for food analogously to "Minor" workers. Here, we utilize reprogramming to investigate natural behavioral specification. Reprogramming of Majors upregulates Minor-biased genes and downregulates Major-biased genes, engaging molecular pathways fundamental to foraging behavior. We discover the neuronal corepressor for element-1-silencing transcription factor (CoREST) is upregulated upon reprogramming and required for the epigenetic switch to foraging. Genome-wide profiling during reprogramming reveals CoREST represses expression of enzymes that degrade juvenile hormone (JH), a hormone elevated upon reprogramming. High CoREST, low JH-degrader expression, and high JH levels are mirrored in natural Minors, revealing parallel mechanisms of natural and reprogrammed foraging. These results unveil chromatin regulation via CoREST as central to programming of ant social behavior, with potential far-reaching implications for behavioral epigenetics.

Knockdown of Methoprene-Tolerant Arrests Ovarian Development in the Sogatella furcifera (Hemiptera: Delphacidae)

Juvenile hormone (JH) is responsible for repressing larval metamorphosis and inducing vitellogenesis and egg production in insects. Methoprene-tolerant (Met) is known to be an intracellular receptor and transducer of JH. We examined the role of Met in ovarian development in the rice pest Sogatella furcifera (Horváth). We first cloned and sequenced S. furcifera Met (SfMet). The SfMet protein belongs to the basic helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) family with a bHLH domain and two PAS domains (PAS-A and PAS-B). SfMet was expressed in all developmental stages and tissues but was most highly expressed in the ovaries of adult females. Furthermore, RNA interference (RNAi) mediated silencing of SfMet substantially reduced the expression of SfVg, decreased yolk protein deposition and blocked oocyte maturation and ovarian development. These results demonstrate that SfMet plays a key role in female reproduction in S. furcifera and suggest that targeting this gene could be an effective way of controlling this pest.

Social Context Enhances Hormonal Modulation of Pheromone Detection in Drosophila

Critical to evolutionary fitness, animals regulate social behaviors by integrating signals from both their external environments and internal states. Here, we find that population density modulates the courtship behavior of male Drosophila melanogaster in an age-dependent manner. In a competitive mating assay, males reared in a social environment have a marked advantage in courting females when pitted against males reared in isolation. Group housing promotes courtship in mature (7-day) but not immature (2-day) males; this behavioral plasticity requires the Or47b pheromone receptor. Using single-sensillum recordings, we find that group housing increases the response of Or47b olfactory receptor neurons (ORNs) only in mature males. The effect of group housing on olfactory response and behavior can be mimicked by chronically exposing single-housed males to an Or47b ligand. At the molecular level, group housing elevates Ca²⁺ levels in Or47b ORNs, likely leading to CaMKI-mediated activation of the histone-acetyl transferase CBP. This signaling event in turn enhances the efficacy of juvenile hormone, an age-related regulator of reproductive maturation in flies. Furthermore, the male-specific Fruitless isoform (Fru^M) is required for the sensory plasticity, suggesting that Fru^M functions as a downstream genomic coincidence detector in Or47b ORNs-integrating reproductive maturity, signaled by juvenile hormone, and population density, signaled by CBP. In all, we identify a neural substrate and activity-dependent mechanism by which social context can directly influence pheromone sensitivity, thereby modulating social behavior according to animals' life-history stage.

Juvenile Hormone III but Not 20-Hydroxyecdysone Regulates the Embryonic Diapause of Aedes albopictus

Diapause is an alternative developmental trajectory allowing insects to enter dormancy and persist through predictable periods of seasonally unfavorable conditions. This crucial ecological adaptation defines the geographic and seasonal abundance of many insect pollinators, pests, and vectors. Understanding the hormonal changes by which insects coordinate the perception of external, diapause-inducing cues with the physiological mechanisms that lead to developmental arrest is a long-standing goal in biology. The hormonal regulation of diapause tends to vary by the life stage at which development arrest occurs; for example, diapause is typically regulated by ecdysteroids in larvae and pupae, and by juvenile hormones in adults. However, little is known about the hormonal control of embryonic diapause, particularly in Diptera. To address this fundamental gap, we directly measured 20-hydroxyecdysone (20HE) (via LC-MS/MS) and juvenile hormone III (JH3) (via GC-MS) in diapause and non-diapause eggs of the Asian tiger mosquito, Aedes albopictus. While 20HE abundance did not differ, diapause eggs had lower JH3 abundance than non-diapause eggs. These results are corroborated by transcriptional and manipulative evidence suggesting that reduced JH3 regulates diapause in this medically important mosquito.

Amplification of Drosophila Olfactory Responses by a DEG/ENaC Channel

Insect olfactory receptors operate as ligand-gated ion channels that directly transduce odor stimuli into electrical signals. However, in the absence of any known intermediate transduction steps, it remains unclear whether and how these ionotropic inputs are amplified in olfactory receptor neurons (ORNs). Here, we find that amplification occurs in the Drosophila courtship-promoting ORNs through Pickpocket 25 (PPK25), a member of the degenerin/epithelial sodium channel family (DEG/ENaC). Pharmacological and genetic manipulations indicate that, in Or47b and Ir84a ORNs, PPK25 mediates Ca²⁺-dependent signal amplification via an intracellular calmodulin-binding motif. Additionally, hormonal signaling upregulates PPK25 expression to determine the degree of amplification, with striking effects on male courtship. Together, these findings advance our understanding of sensory neurobiology by identifying an amplification mechanism compatible with ionotropic signaling. Moreover, this study offers new insights into DEG/ENaC activation by highlighting a novel means of regulation that is likely conserved across species.

Mechanism of trace acetamiprid-caused reproductive disorders in silkworm, Bombyx mori

BACKGROUND

Neonicotinoid insecticides are widely used in the prevention and control of pests in agriculture and forestry, but they can also affect the development of nontarget economic insects. In order to determine the effects of trace acetamiprid on the development of reproductive system of silkworm, we studied the gonad development and endogenous hormone metabolism in silkworms exposed to trace acetamiprid.

RESULT

The silkworm showed mild poisoning symptoms after being exposed to trace acetamiprid (0.01 mg L^-1 ). After 96 h of acetamiprid exposure, the body weight of silkworm was decreased by 7.67% and the ovary and fallopian tube were abnormally developed. The egg production in adults was decreased by 197 ± 29, and the egg weight was decreased by 0.52 ± 0.01 mg. Quantitative RT-PCR analysis showed that the relative expression levels of ovarian development-related genes Vg, Ovo, Otu, Sxl-S and Sxl-L were downregulated by 0.71, 0.77, 0.47, 0.67 and 0.88-fold, respectively. The transcriptional expression of ecdysone metabolism-related gene (EcR) in the ovary was downregulated by 0.46-fold while the expression of juvenile hormone-related gene (JHBP2) was upregulated by 1.36-fold. In hemolymph of acetamiprid exposed larvae, the content of ecdysone was reduced whereas the content of juvenile hormone was increased.

CONCLUSION

Trace acetamiprid may cause reproductive dysfunction in silkworms through regulating the metabolism of endogenous hormones. Our study provides a reference for elucidating the mechanism of acetamiprid-induced reproductive disorders in insects. © 2019 Society of Chemical Industry.

Effects of larval Age at Grafting and Juvenile Hormone on Morphometry and Reproductive Quality Parameters of in Vitro Reared Honey Bees (Hymenoptera: Apidae)

The honey bee queen plays a central role in the Apis mellifera L. (Hymenoptera: Apidae) colony, and her high reproductive capacity is fundamental for building up the workforce of a colony. Caste development in honey bee females involves elaborate physiological pathways unleashed at the beginning of the first larval instars, with juvenile hormone (JH) playing a crucial role. Here we took advantage of established in vitro rearing techniques to conduct a 2 × 2 experimental design and test initial rearing age (young vs old) and JH treatment (JH III vs solvent control) to enlighten the role of nutrient quality and JH in shaping honey bee female fertility, morphological features related to queenliness, and key physiological parameters (hemolymph vitellogenin/Vg, sugar levels, and Vg transcript levels). Our results show that while the age at initial larval rearing had major impacts on external morphology development, where younger larvae exhibited a higher probability to develop into queen-like adults morphotypes, the JH application during the larval stage improved physiological pathways related to ovary development and metabolism during the ontogenic development. We detected that the supplementation of queen larvae with JH promoted important benefits regarding queen fertility as the increase of ovariole number and vg levels at hemolymph, both crucial factors at eggs production. The data presented here provide guidance in efforts to improve honey bee queen quality, especially in light of frequent episodes of queen failures in the beekeeping industry.

Suppression of Gene Juvenile Hormone Diol Kinase Delays Pupation in Heortia vitessoides Moore

Juvenile hormone diol kinase (JHDK) is a critical enzyme involved in juvenile hormone degradation in insects. In this study, HvJHDK in the Heortia vitessoides Moore (Lepidoptera: Crambidae) transcriptional library was cloned. Stage-specific expression patterns of HvJHDK, HvJHEH, and HvJHE as well as juvenile hormone titers were determined. The three tested enzymes participated in juvenile hormone degradation. Moreover, juvenile hormone titers peaked after larval–larval molts, consistent with a role for juvenile hormone in inhibition of metamorphosis. HvJHDK was subsequently suppressed using RNA interference (RNAi) to reveal its functions. Different concentrations of dsJHDK elicited the optimal interference efficiency at different life stages of H. vitessoides. Suppression of HvJHDK decreased HvJHDK content and increased the juvenile hormone titer, thereby resulting in reduced triglyceride content, sharply declined survival rate, clearly lethal phenotypes, and extended larval growth. Moreover, suppression of HvJHDK upregulated HvJHEH and HvJHE expression levels, suggesting that there is feedback regulation in the juvenile hormone metabolic pathway. Taken together, our findings provide molecular references for the selection of novel insecticidal targets.

Complete metamorphosis of insects

The majority of described hexapod species are holometabolous insects, undergoing an extreme form of metamorphosis with an intercalated pupal stage between the larva and adult, in which organs and tissues are extensively remodelled and in some cases completely rebuilt. Here, we review how and why this developmental strategy has evolved. While there are many theories explaining the evolution of metamorphosis, many of which fit under the hypothesis of decoupling of life stages, there are few clear adaptive hypotheses on why complete metamorphosis evolved. We propose that the main adaptive benefit of complete metamorphosis is decoupling between growth and differentiation. This facilitates the exploitation of ephemeral resources and enhances the probability of the metamorphic transition escaping developmental size thresholds. The evolution of complete metamorphosis comes at the cost of exposure to predators, parasites and pathogens during pupal life and requires specific adaptations of the immune system at this time. Moreover, metamorphosis poses a challenge for the maintenance of symbionts and the gut microbiota, although it may also offer the benefit of allowing an extensive change in microbiota between the larval and adult stages. The regulation of metamorphosis by two main players, ecdysone and juvenile hormone, and the related signalling cascades are now relatively well understood. The mechanics of metamorphosis have recently been studied in detail because of the advent of micro-CT and research into the role of cell death in remodelling tissues and organs. We support the argument that the adult stage must necessarily have preceded the larval form of the insect. We do not resolve the still contentious question of whether the larva of insects in general originated through the modification of existing preadult forms or through heterochrony as a modified embryonic stage (pronymph), nor whether the holometabolous pupa arose as a modified hemimetabolous final stage larva. This article is part of the theme issue 'The evolution of complete metamorphosis'.

The innovation of the final moult and the origin of insect metamorphosis

The three modes of insect postembryonic development are ametaboly, hemimetaboly and holometaboly, the latter being considered the only significant metamorphosis mode. However, the emergence of hemimetaboly, with the genuine innovation of the final moult, represents the origin of insect metamorphosis and a necessary step in the evolution of holometaboly. Hemimetaboly derives from ametaboly and might have appeared as a consequence of wing emergence in Pterygota, in the early Devonian. In extant insects, the final moult is mainly achieved through the degeneration of the prothoracic gland (PG), after the formation of the winged and reproductively competent adult stage. Metamorphosis, including the formation of the mature wings and the degeneration of the PG, is regulated by the MEKRE93 pathway, through which juvenile hormone precludes the adult morphogenesis by repressing the expression of transcription factor E93, which triggers this change. The MEKRE93 pathway appears conserved in extant metamorphosing insects, which suggest that this pathway was operative in the Pterygota last common ancestor. We propose that the final moult, and the consequent hemimetabolan metamorphosis, is a monophyletic innovation and that the role of E93 as a promoter of wing formation and the degeneration of the PG was mechanistically crucial for their emergence. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Where did the pupa come from? The timing of juvenile hormone signalling supports homology between stages of hemimetabolous and holometabolous insects

Insect metamorphosis boasts spectacular cases of postembryonic development when juveniles undergo massive morphogenesis before attaining the adult form and function; in moths or flies the larvae do not even remotely resemble their adult parents. A selective advantage of complete metamorphosis (holometaboly) is that within one species the two forms with different lifestyles can exploit diverse habitats. It was the environmental adaptation and specialization of larvae, primarily the delay and internalization of wing development, that eventually required an intermediate stage that we call a pupa. It is a long-held and parsimonious hypothesis that the holometabolous pupa evolved through modification of a final juvenile stage of an ancestor developing through incomplete metamorphosis (hemimetaboly). Alternative hypotheses see the pupa as an equivalent of all hemimetabolous moulting cycles (instars) collapsed into one, and consider any preceding holometabolous larval instars free-living embryos stalled in development. Discoveries on juvenile hormone signalling that controls metamorphosis grant new support to the former hypothesis deriving the pupa from a final pre-adult stage. The timing of expression of genes that repress and promote adult development downstream of hormonal signals supports homology between postembryonic stages of hemimetabolous and holometabolous insects. This article is part of the theme issue 'The evolution of complete metamorphosis'.

The evolution of insect metamorphosis: a developmental and endocrine view

Developmental, genetic and endocrine data from diverse taxa provide insight into the evolution of insect metamorphosis. We equate the larva–pupa–adult of the Holometabola to the pronymph–nymph–adult of hemimetabolous insects. The hemimetabolous pronymph is a cryptic embryonic stage with unique endocrinology and behavioural modifications that probably served as preadaptations for the larva. It develops in the absence of juvenile hormone (JH) as embryonic primordia undergo patterning and morphogenesis, the processes that were arrested for the evolution of the larva. Embryonic JH then drives tissue differentiation and nymph formation. Experimental treatment of pronymphs with JH terminates patterning and induces differentiation, mimicking the processes that occurred during the evolution of the larva. Unpatterned portions of primordia persist in the larva, becoming imaginal discs that form pupal and adult structures. Key transcription factors are associated with the holometabolous life stages: Krüppel-homolog 1 (Kr-h1) in the larva, broad in the pupa and E93 in the adult. Kr-h1 mediates JH action and is found whenever JH acts, while the other two genes direct the formation of their corresponding stages. In hemimetabolous forms, the pronymph has low Broad expression, followed by Broad expression through the nymphal moults, then a switch to E93 to form the adult.

This article is part of the theme issue ‘The evolution of complete metamorphosis’.

Monarch butterflies use an environmentally sensitive, internal timer to control overwintering dynamics

The monarch butterfly (Danaus plexippus) complements its iconic migration with diapause, a hormonally controlled developmental programme that contributes to winter survival at overwintering sites. Although timing is a critical adaptive feature of diapause, how environmental cues are integrated with genetically-determined physiological mechanisms to time diapause development, particularly termination, is not well understood. In a design that subjected western North American monarchs to different environmental chamber conditions over time, we modularized constituent components of an environmentally-controlled, internal diapause termination timer. Using comparative transcriptomics, we identified molecular controllers of these specific diapause termination components. Calcium signalling mediated environmental sensitivity of the diapause timer, and we speculate that it is a key integrator of environmental condition (cold temperature) with downstream hormonal control of diapause. Juvenile hormone (JH) signalling changed spontaneously in diapause-inducing conditions, capacitating response to future environmental condition. Although JH is a major target of the internal timer, it is not itself the timer. Epigenetic mechanisms are implicated to be the proximate timing mechanism. Ecdysteroid, JH, and insulin/insulin-like peptide signalling are major targets of the diapause programme used to control response to permissive environmental conditions. Understanding the environmental and physiological mechanisms of diapause termination sheds light on fundamental properties of biological timing, and also helps inform expectations for how monarch populations may respond to future climate change.

Yellowing and YPT gene expression in the desert locust, Schistocerca gregaria: Effects of developmental stages and fasting

The yellow protein of the takeout family (YPT) controls the development of yellow body color in the desert locust. This study focused on two aspects related to YPT in the locust. We first examined the expression pattern of YPT during nymphal stages because yellowing was not obvious during the early instars. YPT expression levels were extremely low in the second and third instars compared with the last two nymphal instars. Warm rearing temperature and juvenile hormone (JH) injection, which stimulated YPT expression in the late instars, had little effect in the second instar, suggesting that YPT expression during the early instars was suppressed and could not be stimulated by either of these factors. We also investigated delayed yellowing in fasting male adults, under the hypothesis that fasting decreased the JH titers and delayed the onset of YPT expression. Yellowing was delayed in fasting adults compared with well-fed adults and YPT expression was stimulated by JH injections at Day 15. However, we failed to obtain evidence that fasting significantly influenced the expression levels of YPT and the JH early-inducible gene Krüppel homolog 1 at Days 15 and 20 post-adult emergence. Results suggest that a YPT-independent mechanism possibly induces delayed yellowing in fasting males.

Flip-Flopping Retinal in Microbial Rhodopsins as a Template for a Farnesyl/Prenyl Flip-Flop Model in Eukaryote GPCRs

Thirty years after the first description and modeling of G protein coupled receptors (GPCRs), information about their mode of action is still limited. One of the questions that is hard to answer is: how do the allosteric changes in the GPCR induced by, e.g., ligand binding in the end activate a G protein-dependent intracellular pathway (e.g., via the cAMP or the phosphatidylinositol signal pathways). Another question relates to the role of prenylation of G proteins. Today's "consensus model" states that protein prenylation is required for the assembly of GPCR-G protein complexes. Although it is well-known that protein prenylation is the covalent addition of a farnesyl- or geranylgeranyl moiety to the C terminus of specific proteins, e.g., α or γ G protein, the reason for this strong covalent binding remains enigmatic. The arguments for a fundamental role for prenylation of G proteins other than just being a hydrophobic linker, are gradually accumulating. We uncovered a dilemma that at first glance may be considered physiologically irrelevant, however, it may cause a true change in paradigm. The consensus model suggests that the only functional role of prenylation is to link the G protein to the receptor. Does the isoprenoid nature of the prenyl group and its exact site of attachment somehow matter? Or, are there valid arguments favoring the alternative possibility that a key role of the G protein is to guide the covalently attached prenyl group to - and it hold it in - a very specific location in between specific helices of the receptor? Our model says that the farnesyl/prenyl group - aided by its covalent attachment to a G protein -might function in GPCRs as a horseshoe-shaped flexible (and perhaps flip-flopping) hydrophobic valve for restricting (though not fully inhibiting) the untimely passage of Ca²⁺, like retinal does for the passage of H⁺ in microbial rhodopsins that are ancestral to many GPCRs.

Stress-related hormones affect carbohydrate metabolism in Drosophila females

A highly conservative insulin signaling pathway, stable work of which is indicated by carbohydrates metabolism, is also known to play an important role in the control of stress resistance. Here we demonstrate that exposure to heat stress leads to a rise in the levels of trehalose and glucose in females of Drosophila melanogaster, but does not affect the expression level of the trehalase (Treh) gene. We have shown that the rise in juvenile hormone (JH) and dopamine decreases levels of both carbohydrates under the normal conditions but brings them to values close to normal following the stress exposure. The data obtained suggest that (a) dopamine and JH involved in the neuroendocrine stress reaction in D. melanogaster also take part in the regulation of carbohydrates metabolism, tending to normalize it after stress; (b) the regulation of trehalose content under stress does not occur at the level of transcription of the degrading enzyme.

Hierarchical Virtual Screening of Potential Insectides Inhibitors of Acetylcholinesterase and Juvenile Hormone from Temephos

Aedes aegypti (Linnaeus, 1762; Diptera: Culicidae) is the main vector transmitting viral diseases such as dengue fever, dengue haemorrhagic fever, urban yellow fever, zika and chikungunya. Worldwide, especially in the Americas and Brazil, many cases of dengue have been reported in recent years, which have shown significant growth. The main control strategy is the elimination of the vector, carried out through various education programs, to change human habits, but the most usual is biological control, together with environmental management and chemical control. The most commonly insecticide used is temephos (an organophosphorus compound), but Aedes aegypti populations have shown resistance and the product is highly toxic, so we chose it as a template molecule to perform a ligand-based virtual screening in the ChemBrigde (DIVERSet-CL subcollection) database, searching for derivatives with similarity in shape (ROCS) and electrostatic potential (EON). Thus, fourty-five molecules were filtered based on their pharmacokinetic and toxicological properties and 11 molecules were selected by a molecular docking study, including binding affinity and mode of interaction. The L46, L66 and L68 molecules show potential inhibitory activity for both the insect (−9.28, −10.08 and −6.78 Kcal/mol, respectively) and human (−6.05, 6.25 and 7.2 Kcal/mol respectively) enzymes, as well as the juvenile hormone protein (−9.2; −10.96 and −8.16 kcal/mol, respectively), showing a significant difference in comparison to the template molecule temephos. Molecules L46, L66 and L68 interacted with important amino acids at each catalytic site of the enzyme reported in the literature. Thus, the molecules here investigated are potential inhibitors for both the acetylcholinesterase enzymes and juvenile hormone protein–from insect and humans, characterizing them as a potential insecticide against the Aedes aegypti mosquito.

Genome-Wide Analysis and Hormone Regulation of Chitin Deacetylases in Silkworm

Chitin deacetylases (CDAs) are a group of enzymes involved in chitin metabolism in insects; they play a critical role in molting, pupation, and the modification of chitin. In this study, we identified several CDAs in the silkworm, Bombyx mori (BmCDA), and investigated the effect of various hormones on their expression in B. mori larvae and embryo cell lines (BmE). Eight genes encoding BmCDAs were identified in the silkworm genome. They showed different expression patterns in different tissues, and were classified into three types based on where they were expressed: the exoskeleton, digestive organs, and genital organs. Moreover, we found that some BmCDAs showed upregulated expression during the molting period, especially during the fourth molting period in larvae. We also verified that the expression of BmCDA1-6 was upregulated by treatment with 20-hydroxyecdysone not only in larvae, but also in BmE cells. Interestingly, juvenile hormone analog treatment also upregulated the expression of some BmCDAs. The overexpression of several transcription factors revealed that the POU transcription factor POUM2 may play a major role in the regulation of BmCDA expression. Finally, the silencing of BmCDA1 and BmCDA2 did not lead to abnormal phenotypes or death, but may have led to delays in silkworm pupation. These results provide important information about lepidopteran insects in terms of chitin deacetylases and the regulation of their expression.

Response Mechanism of Oviposition and Relevant Protein Expression of Bactrocera cucurbitae (Coquillet) to Short-Term High-Temperature Conditions

Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae) is an important pest of vegetables in Asia, the Middle East, Africa, and Hawaii. High temperature can significantly influence B. cucurbitae reproduction. The effect of short-term high-temperature exposure on proteins that affect oviposition was analyzed by proteomics. Among six key target genes for oviposition, the expression of Vitellogenin-1, Vitellogenin-2, and Vitellogenin receptor was similar in B. cucurbitae exposed to higher temperature compared to controls. However, levels of Vitellogenin-3 were reduced. Juvenile hormone (Jh)-inducible protein was downregulated and then upregulated, while the expression of Jh-epoxide hydrolase-2 showed the opposite Jh-inducible protein trend. Therefore, short-term high-temperature stress can cause differential expression of proteins related to oviposition in B. cucurbitae, which in turn further triggers the hormesis of oviposition. High-temperature conditions have become more frequent because of climate warming and are predicted to continue. The data indicate that climate effects on insect reproduction pose a significant threat to agriculture in a world of increasing population.

The Ecdysis Triggering Hormone System, via ETH/ETHR-B, Is Essential for Successful Reproduction of a Major Pest Insect, Bactrocera dorsalis (Hendel)

Ecdysis triggering hormone (ETH), released by the Inka cells, is a master hormone in regulating the ecdysis process in insect. Here we investigated the presence and role of the ETH signaling in the female adult of the oriental fruit fly, Bactrocera dorsalis (Hendel) that is one of the most important invasive pest insects in agriculture worldwide. In the female adult, ETH was confirmed in the Inka cells at the tracheae by immunostaining and also in vitro exposure to ETH stimulated the isolated corpora allata of adult in activity. Then we prepared cDNA of females at 0, 5, 10, 15, and 20 days after adult eclosion, and RT-qPCR showed that the expression pattern of ETH and its receptor ETHR-B started from a peak at the day of adult eclosion (day 0), then dropped to basal levels and increased again between day 10 and 15 which is also the period corresponding to ovary growth. In contrast, ETHR-A was absent with Ct values of >33. The expression patterns of the ecdysteroid-producing Halloween genes Spook and Shade, and the vitellogenin genes Vg1, Vg2, and Vg3 co-occurred with peak levels at days 10–15, and also juvenile hormone acid methyltransferase (JHAMT) showed increased levels on day 15. Further in RNAi assays to better understand the role of ETH and ETHR, dsRNA was injected to adult and this led to a respective decrease in expression of 62 and 56% for ETH and ETHR-B, while ETHR-A stayed absent with Ct values of 33. In these RNAi-females, there was an apparently decreased expression for JHAMT and Vg2, together with a significant decrease of the JH titer and egg production. Injection of the JH mimetic methoprene could rescue Vg2 expression and egg production. Upstream, in dsETH/dsETHR-injected females, 20-hydroxyecdysone (20E) injection rescued the transcriptions of ETH and ETHR and also egg production. In summary, our results shed more light on the pivotal role that the ETH peptide hormone and its receptor ETHR-B play an essential role in the reproduction of the female adult of B. dorsalis, via the regulation of JH and vitellogenin, which are controlled by a pulse of 20E.

Hormonal control and target genes of ftz-f1 expression in the honeybee Apis mellifera: a positive loop linking juvenile hormone, ftz-f1, and vitellogenin

Ftz-f1 is an orphan member of the nuclear hormone receptor superfamily. A 20-hydroxyecdysone pulse allows ftz-f1 gene expression, which then regulates the activity of downstream genes involved in major developmental progression events. In honeybees, the expression of genes like vitellogenin (vg), prophenoloxidase and juvenile hormone-esterase during late pharate-adult development is known to be hormonally controlled in both queens and workers by increasing juvenile hormone (JH) titres in the presence of declining levels of ecdysteroids. Since Ftz-f1 is known for mediating intracellular JH signalling, we hypothesized that ftz-f1 could mediate JH action during the pharate-adult development of honeybees, thus controlling the expression of these genes. Here, we show that ftz-f1 has caste-specific transcription profiles during this developmental period, with a peak coinciding with the increase in JH titre, and that its expression is upregulated by JH and downregulated by ecdysteroids. RNAi-mediated knock down of ftz-f1 showed that the expression of genes essential for adult development (e.g. vg and cuticular genes) depends on ftz-f1 expression. Finally, a double-repressor hypothesis-inspired vg gene knock-down experiment suggests the existence of a positive molecular loop between JH, ftz-f1 and vg.

Analysis of the Drosophila melanogaster anti-ovarian response to honey bee queen mandibular pheromone

Queen mandibular pheromone (QMP) is a potent reproductive signal to which honey bee workers respond by suppressing their ovaries and adopting alloparental roles within the colony. This anti-ovarian effect of QMP on workers can, surprisingly, be induced in other insects, including fruit flies, in which females exposed to synthetic QMP develop smaller ovaries with fewer eggs. In this study, we use the Drosophila melanogaster model to identify the components of synthetic QMP required for the anti-ovarian effect. We found that virgin females respond strongly to 9-oxo-2-decenoic acid and 10-hydroxy-2-decenoic acid (10HDA), suggesting that the decenoic acid components of QMP are essential for the anti-ovarian response. Further, a nuclear factor of activated T-cells reporter system revealed neurones expressing the olfactory receptors Or-56a, Or-49b and Or-98a are activated by QMP in the antenna. In addition, we used olfactory receptor GAL4 drivers and a neuronal activator (a neuronal activating bacterial sodium channel) to test whether the candidate neurones are potential labelled lines for a decenoic acid response. We identified Or-49b as a potential candidate receiver of the 10HDA signal. Finally, the anti-ovarian response to synthetic QMP is not mediated by decreasing the titre of the reproductive hormones ecdysone and juvenile hormone.

Identification of Potential Inhibitors from Pyriproxyfen with Insecticidal Activity by Virtual Screening

Aedes aegypti is the main vector of dengue fever transmission, yellow fever, Zika, and chikungunya in tropical and subtropical regions and it is considered to cause health risks to millions of people in the world. In this study, we search to obtain new molecules with insecticidal potential against Ae. aegypti via virtual screening. Pyriproxyfen was chosen as a template compound to search molecules in the database Zinc_Natural_Stock (ZNSt) with structural similarity using ROCS (rapid overlay of chemical structures) and EON (electrostatic similarity) software, and in the final search, the top 100 were selected. Subsequently, in silico pharmacokinetic and toxicological properties were determined resulting in a total of 14 molecules, and these were submitted to the PASS online server for the prediction of biological insecticide and acetylcholinesterase activities, and only two selected molecules followed for the molecular docking study to evaluate the binding free energy and interaction mode. After these procedures were performed, toxicity risk assessment such as LD50 values in mg/kg and toxicity class using the PROTOX online server, were undertaken. Molecule ZINC00001624 presented potential for inhibition for the acetylcholinesterase enzyme (insect and human) with a binding affinity value of -10.5 and -10.3 kcal/mol, respectively. The interaction with the juvenile hormone was -11.4 kcal/mol for the molecule ZINC00001021. Molecules ZINC00001021 and ZINC00001624 had excellent predictions in all the steps of the study and may be indicated as the most promising molecules resulting from the virtual screening of new insecticidal agents.

Expressing Double-Stranded RNAs of Insect Hormone-Related Genes Enhances Baculovirus Insecticidal Activity

Baculoviruses have already been used for insect pest control, but the slow killing speed limits their further promotion and application. Here we provide a strategy for improving baculovirus insecticidal activity using Helicoverpa armigera nucleopolyhedrovirus (HearNPV) to express double-stranded RNAs (dsRNAs) targeting cotton bollworm (Helicoverpa armigera) juvenile hormone (JH)-related genes. Droplet-feeding bioassays show that the 50% lethal concentration (LC50) values of recombinant baculoviruses expressing the dsRNA of JH acid methyl transferase gene (HaJHAMT) and the JH acid binding protein gene (HaJHBP) were 1.24 × 10⁴ polyhedral inclusion bodies (PIB)/mL and 2.26 × 10⁴ PIB/mL, respectively. Both were much lower than the control value (8.12 × 10⁴ PIB/mL). Meanwhile, the LT50 of recombinant baculovirus expressing dsRNA of HaJHBP was only 54.2% of the control value, which means that larval death was accelerated. Furthermore, the mRNA level of target genes was reduced in recombinant baculovirus-treated cotton bollworm larvae. Transcription of several key genes involved in hormone signaling pathways-for example, ecdysone receptor gene (HaEcR)-was also altered. This study establishes a new strategy for pest management by interfering with insect hormone-related gene expression via baculoviruses, and the engineered baculoviruses have great potential application in cotton production.

Intraluminal Farnesol and Farnesal in the Mealworm's Alimentary Canal: An Unusual Storage Site Uncovering Hidden Eukaryote Ca2+-Homeostasis-Dependent "Golgicrine" Activities

Farnesol, the sesquiterpenoid precursor of the six presently known insect juvenile hormones (JHs) was for the first time chemically identified in 1961, not in JH synthesizing glands or whole body extracts, but in excrements of the mealworm Tenebrio molitor. This finding was thought to be irrelevant and remained unexplored. In 1970, it was reported that the fall to zero of the JH titer in both prediapausing adults and in last instar larvae of the Colorado potato beetle causes severe malfunctioning of the Golgi system in the fat body, among various other effects. This endomembrane system in the cytoplasm resides at the intersection of the secretory, lysosomal, and endocytic pathways and is required for the processing of secretory proteins. Why the Golgi needs farnesol-like endogenous sesquiterpenoids (FLS) for its proper functioning has also never been further investigated. In 1999, farnesol was found to be a natural endogenous ligand for particular types of voltage-gated Ca²⁺ channels in mammalian cells, a finding that also remained undervalued. Only since 2014 more attention has been paid to the functional research of the "noble unknown" farnesol, in particular to its Ca²⁺-homeostasis-related juvenilizing and anti-apoptotic activities. Here, we introduce the term "Golgicrine activity" that addresses the secretory activity of the RER-Golgi system from its role in Ca²⁺-homeostasis rather than from its conventional role in mere protein secretion. Golgicrine activity attributes the so far forgotten role of farnesol-like sesquiterpenoids in proper Golgi functioning, and unites the endocrine, exocrine and enterocrine functions of these sesquiterpenoids. This out of the box view may open novel perspectives for the better understanding of particular inflammatory bowel diseases and of neurodegenerative diseases as well, because the early initiation of Alzheimer's disease may possibly result from malfunctioning of the mevalonate-farnesol-cholesterol biosynthetic pathway and thus might be a farnesol- and Ca²⁺-homeostasis-dependent Golgicrine issue.

Methoprene-Tolerant (Met) Is Indispensable for Larval Metamorphosis and Female Reproduction in the Cotton Bollworm Helicoverpa armigera

Juvenile hormone (JH) represses larval metamorphosis and induces adult reproduction in insects. Methoprene-tolerant (Met) is identified as an intranuclear receptor that mediates JH actions. In the present study, we characterized a Met from the severe agricultural pest, Helicoverpa armigera, namely HaMet. In the larval stage, HaMet is predominantly expressed in the epidermis and midgut, and is upregulated before each molting, whereas in adults HaMet is maximally expressed in the ovary, testis, and fat body. The immunofluorescence assay revealed that HaMet was distributed in the longitudinal and circular muscle layers of midgut in larvae, whereas in the ovary of female adults, HaMet was localized in the nucleus of the oolemma. Knockdown of HaMet in final-instar larvae shortened the time of pupation, induced abnormal pupation, and dampened pupation rate. In female adults, HaMet depletion severely suppressed the transcription of Vitellogenin (Vg) and Vitellogenin Receptor (VgR), disrupted the Vg accumulation in fat body and the yolk protein uptake in oocytes, and finally led to an impaired fecundity. Our findings therefore confirmed that HaMet acted as a nuclear receptor of JH and played an essential role in larval metamorphosis, vitellogenesis, and oocyte maturation.