Krüppel homolog 1 represses insect ecdysone biosynthesis by directly inhibiting the transcription of steroidogenic enzymes

Juvenile hormone-induced microRNA miR-iab-8 regulates lipid homeostasis and metamorphosis in Drosophila melanogaster

Metamorphosis plays an important role in the evolutionary success of insects. Accumulating evidence indicated that microRNAs (miRNAs) are involved in the regulation of processes associated with insect metamorphosis. However, the miRNAs coordinated with juvenile hormone (JH)-regulated metamorphosis remain poorly reported. In the present study, using high-throughput miRNA sequencing combined with Drosophila genetic approaches, we demonstrated that miR-iab-8, which primarily targets homeotic genes to modulate haltere-wing transformation and sterility was up-regulated by JH and involved in JH-mediated metamorphosis. Overexpression of miR-iab-8 in the fat body resulted in delayed development and failure of larval-pupal transition. Furthermore, metabolomic analysis results revealed that overexpression of miR-iab-8 caused severe energy metabolism defects especially the lipid metabolism, resulting in significantly reduced triacylglycerol (TG) content and glycerophospholipids but enhanced accumulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). In line with this, Nile red staining demonstrated that during the third larval development, the TG content in the miR-iab-8 overexpression larvae was continuously decreased, which is opposite to the control. Additionally, the transcription levels of genes committed to TG synthesis and breakdown were found to be significantly increased and the expression of genes responsible for glycerophospholipids metabolism were also altered. Overall, we proposed that JH induced miR-iab-8 expression to perturb the lipid metabolism homeostasis especially the TG storage in the fat body, which in turn affected larval growth and metamorphosis.

Pyriproxyfen enhances germline stem cell proliferation and reduces reproduction in Drosophila by up-regulating juvenile hormone signaling

BACKGROUND

Pyriproxyfen is an insect growth regulator (IGR) that is effective against various types of insect pests. However, the molecular mechanism underlying pyriproxyfen effects on insect reproduction remains unclear. Thus, in this study, we attempted to uncover the mechanisms underlying the impact of pyriproxyfen on the reproductive system of the model organism Drosophila melanogaster.

RESULTS

A significant decrease in Drosophila reproduction was observed after pyriproxyfen treatment. The juvenile hormone (JH) titer was significantly increased (120.4%) in the ovary samples of pyriproxyfen-treated flies. Likewise, the concentrations of key enzymes and the expression of key genes related to the JH signaling pathway were also increased in the pyriproxyfen-treated group compared with the control group. Furthermore, pyriproxyfen treatment significantly increased (15.6%) the number of germline stem cells (GSCs) and significantly decreased (17%) the number of cystoblasts (CBs). However, no significant differences were observed in the number of somatic cells. We performed RNA interference (RNAi) on five key genes (Met, Tai, gce, ftz-f1, and hairy) related to the JH signaling pathway in germ cells using the germ cell-specific Gal4 driver. Interestingly, RNAi of the selected genes significantly decreased the number of both GSCs and CBs in pyriproxyfen-treated transgenic flies. These results further validate that pyriproxyfen enhances GSC proliferation by up-regulating JH signaling.

CONCLUSION

Our results indicate that pyriproxyfen significantly decreases reproduction by affecting germ cells in female adult ovaries. The effect of pyriproxyfen on germ cell proliferation and differentiation is mediated by an increase in JH signaling. This study has significant implications for optimizing pest control strategies, developing sustainable agriculture practices, and understanding the mechanism of insecticide action. © 2024 Society of Chemical Industry.

Juvenile hormone signal transducer hairy inhibits Krüppel homolog1 expression

Hairy and Krüppel homolog 1 (Kr-h1) are transcriptional repressors that act synergistically to mediate the gene-repressive action of juvenile hormone (JH). However, whether a regulatory relationship exists between Hairy and Kr-h1 remains unclear. In this study, an inhibitory effect of Hairy on Kr-h1 expression was found. Genetic studies in Drosophila have shown that the simultaneous overexpression of Hairy and Kr-h1 can rescue the defective phenotypes caused by the overexpression of a single factor. Reduced expression of Kr-h1 was observed in Hairy-overexpressing flies and cells, whereas the expression levels of Hairy were unaffected in cells with ectopic expression of Kr-h1. The inhibitory effect of Hairy on Kr-h1 expression was found to occur at the transcriptional level, as Hairy bound directly to the B-box within the Kr-h1 promoter via the bHLH motif and recruited the corepressors C-terminal binding protein (CtBP) and Groucho (Gro) through the PLSLV and WRPW motifs, respectively. Our findings revealed a regulatory relationship between two JH response factors, which advances our understanding of the molecular mechanism of JH signaling.

Schlank orchestrates insect developmental transition by switching H3K27 acetylation to trimethylation in the prothoracic gland

Insect developmental transitions are precisely coordinated by ecdysone and juvenile hormone (JH). We previously revealed that accumulated H3K27 trimethylation (H3K27me3) at the locus encoding JH signal transducer Hairy is involved in the larval-pupal transition in insects, but the underlying mechanism remains to be fully defined. Here, we show in Drosophila and Bombyx that Rpd3-mediated H3K27 deacetylation in the prothoracic gland during the last larval instar promotes ecdysone biosynthesis and the larval-pupal transition by enabling H3K27me3 accumulation at the Hairy locus to induce its transcriptional repression. Importantly, we find that the homeodomain transcription factor Schlank acts to switch active H3K27 acetylation (H3K27ac) to repressive H3K27me3 at the Hairy locus by directly binding to the Hairy promoter and then recruiting the histone deacetylase Rpd3 and the histone methyltransferase PRC2 component Su(z)12 through physical interactions. Moreover, Schlank inhibits Hairy transcription to facilitate the larval-pupal transition, and the Schlank signaling cascade is suppressed by JH but regulated in a positive feedback manner by ecdysone. Together, our data uncover that Schlank mediates epigenetic reprogramming of H3K27 modifications in hormone actions during insect developmental transition.

Untangling the gordian knot: The intertwining interactions between developmental hormone signaling and epigenetic mechanisms in insects

Hormones control developmental and physiological processes, often by regulating the expression of multiple genes simultaneously or sequentially. Crosstalk between hormones and epigenetics is pivotal to dynamically coordinate this process. Hormonal signals can guide the addition and removal of epigenetic marks, steering gene expression. Conversely, DNA methylation, histone modifications and non-coding RNAs can modulate regional chromatin structure and accessibility and regulate the expression of numerous (hormone-related) genes. Here, we provide a review of the interplay between the classical insect hormones, ecdysteroids and juvenile hormones, and epigenetics. We summarize the mode-of-action and roles of these hormones in post-embryonic development, and provide a general overview of epigenetic mechanisms. We then highlight recent advances on the interactions between these hormonal pathways and epigenetics, and their involvement in development. Furthermore, we give an overview of several 'omics techniques employed in the field. Finally, we discuss which questions remain unanswered and possible avenues for future research.

Juvenile hormone inhibits adult cuticle formation in Drosophila melanogaster through Kr-h1/Dnmt2-mediated DNA methylation of Acp65A promoter

Differentiation of imaginal epidermal cells of Drosophila melanogaster to form adult cuticles occurs at approximately 40-93 h after puparium formation. Juvenile hormone (JH) given at pupariation results in formation of a second pupal cuticle in the abdomen instead of the adult cuticle. Although the adult cuticle gene Acp65A has been reported to be down-regulated following JH treatment, the regulatory mechanism remains unclear. Here, we found that the JH primary response gene Krüppel homologue 1 (Kr-h1) plays a vital role in the repression of adult cuticle formation through the mediation of JH action. Overexpression of Kr-h1 mimicked-while knocking down of Kr-h1 attenuated-the inhibitory action of JH on the formation of the adult abdominal cuticle. Further, we found that Kr-h1 inhibited the transcription of Acp65A by directly binding to the consensus Kr-h1 binding site (KBS) within the Acp65A promoter region. Moreover, the DNA methyltransferase Dnmt2 was shown to interact with Kr-h1, combined with the KBS to promote the DNA methylation of sequences around the KBS, in turn inhibiting the transcription of Acp65A. This study advances our understanding of the molecular basis of the "status quo" action of JH on the Drosophila adult metamorphosis.

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.

The CsmiR1579-CsKr-h1 module mediates rice stem borer development and reproduction: An effective target for transgenic insect-resistant rice

The rice stem borer (RSB, Chilo suppressalis) is a significant agricultural pest that mainly depends on chemical control. However, it has grown to varied degrees of pesticide resistance, which poses a severe threat to rice production and emphasizes the need for safer, more efficient alternative pest management strategies. Here, in vitro and in vivo experiments analyses reveal miR-1579 binds to the critical transcription factor Krüppel homologue 1 (Kr-h1) and negatively regulates its expression. Overexpression of miR-1579 in larvae with significantly lower levels of Kr-h1 was associated with a decline in larval growth and survival. Furthermore, in female pupae, miR-1579 overexpression led to abnormalities in ovarian development, suggesting that targeting miR-1579 could be a potential management strategy against C. suppressalis. Therefore, we generated transgenic rice expressing miR-1579 and screened three lines that had a single copy of highly abundant mature miR-1579 transcripts. Expectedly, fed with transgenic miR-1579 rice lines were significantly lower survival rates in larvae and high levels of resistance to damage caused by C. suppressalis infestation. These findings suggest that miRNA-mediated RNAi could provide an effective and species-specific strategy for C. suppressalis control.

TGFß/activin-dependent activation of Torso controls the timing of the metamorphic transition in the red flour beetle Tribolium castaneum

Understanding the mechanisms governing body size attainment during animal development is of paramount importance in biology. In insects, a crucial phase in determining body size occurs at the larva-pupa transition, marking the end of the larval growth period. Central to this process is the attainment of the threshold size (TS), a critical developmental checkpoint that must be reached before the larva can undergo metamorphosis. However, the intricate molecular mechanisms by which the TS orchestrates this transition remain poor understood. In this study, we investigate the role of the interaction between the Torso and TGFß/activin signaling pathways in regulating metamorphic timing in the red flour beetle, Tribolium castaneum. Our results show that Torso signaling is required specifically during the last larval instar and that its activation is mediated not only by the prothoracicotropic hormone (Tc-Ptth) but also by Trunk (Tc-Trk), another ligand of the Tc-Torso receptor. Interestingly, we show that while Tc-Torso activation by Tc-Ptth determines the onset of metamorphosis, Tc-Trk promotes growth during the last larval stage. In addition, we found that the expression of Tc-torso correlates with the attainment of the TS and the decay of juvenile hormone (JH) levels, at the onset of the last larval instar. Notably, our data reveal that activation of TGFß/activin signaling pathway at the TS is responsible for repressing the JH synthesis and inducing Tc-torso expression, initiating metamorphosis. Altogether, these findings shed light on the pivotal involvement of the Ptth/Trunk/Torso and TGFß/activin signaling pathways as critical regulatory components orchestrating the TS-driven metamorphic initiation, offering valuable insights into the mechanisms underlying body size determination in insects.

An Overview of Ovarian Calyx Fluid Proteins of Toxoneuron nigriceps (Viereck) (Hymenoptera: Braconidae): An Integrated Transcriptomic and Proteomic Approach

The larval stages of the tobacco budworm, Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae), are parasitized by the endophagous parasitoid wasp, Toxoneuron nigriceps (Viereck) (Hymenoptera: Braconidae). During the injections of eggs, this parasitoid wasp also injects into the host body the secretion of the venom gland and the calyx fluid, which contains a polydnavirus (T. nigriceps BracoVirus: TnBV) and the Ovarian calyx fluid Proteins (OPs). The effects of the OPs on the host immune system have recently been described. In particular, it has been demonstrated that the OPs cause hemocytes to undergo a number of changes, such as cellular oxidative stress, actin cytoskeleton modifications, vacuolization, and the inhibition of hemocyte encapsulation capacity, which results in both a loss of hemocyte functionality and cell death. In this study, by using a combined transcriptomic and proteomic analysis, the main components of T. nigriceps ovarian calyx fluid proteins were identified and their possible role in the parasitic syndrome was discussed. This study provides useful information to support the analysis of the function of ovarian calyx fluid proteins, to better understand T. nigriceps parasitization success and for a more thorough understanding of the components of ovarian calyx fluid proteins and their potential function in combination with other parasitoid factors.

Kruppel homolog 1 modulates ROS production and antimicrobial peptides expression in shrimp hemocytes during infection by the Vibrio parahaemolyticus strain that causes AHPND

Shrimp aquaculture has been seriously affected by acute hepatopancreatic necrosis disease (AHPND), caused by a strain of Vibrio parahaemolyticus that carries the Pir toxin plasmids (V. parahaemolyticus (AHPND)). In this study, the transcription factor, Kruppel homolog 1-like of Peneaus vannamei (PvKr-h1), was significantly induced in shrimp hemocytes after V. parahaemolyticus (AHPND) challenge, suggesting that PvKr-h1 is involved in shrimp immune response. Knockdown of PvKr-h1 followed by V. parahaemolyticus (AHPND) challenge increased bacterial abundance in shrimp hemolymph coupled with high shrimp mortality. Moreover, transcriptome and immunofluorescence analyses revealed that PvKr-h1 silencing followed by V. parahaemolyticus (AHPND) challenge dysregulated the expression of several antioxidant-related enzyme genes, such as Cu-Zu SOD, GPX, and GST, and antimicrobial peptide genes, i.e., CRUs and PENs, and reduced ROS activity and nuclear translocation of Relish. These data reveal that PvKr-h1 regulates shrimps' immune response to V. parahaemolyticus (AHPND) infection by suppressing antioxidant-related enzymes, enhancing ROS production and promoting nuclei import of PvRelish to stimulate antimicrobial peptide genes expression.

Insect growth-regulating activity of 1-benzyl-2-methylbenzimidazole derivatives on silkworms

Derivatives of 1-benzyl-2-methylbenzimidazoles (BMBIs) were synthesized to evaluate their biological activities against Bombyx mori, a lepidopteran model insect. Synthesized BMBIs exhibited two different biological activities: inhibition of development and acute lethality. From a structural perspective, the activity varied with the position of the substitutions on the 1-benzyl moiety; BMBIs with substitutions on the 2 and/or 4 positions had comparatively high activity in comparison with those with substitutions on the 3-position. There was more activity for the inhibition of development with low doses, and more for acute lethality with high doses. The activity was also affected by the applied stage, that is, application in the 4th instar mostly interfered the larval molting or pupation, whereas that in the 3rd instar caused more acute mortality. Taken together, these results suggest that BMBIs have multiple modes of action.

Overexpression of BmJHBPd2 Repressed Silk Synthesis by Inhibiting the JH/Kr-h1 Signaling Pathway in Bombyx mori

The efficient production of silkworm silk is crucial to the silk industry. Silk protein synthesis is regulated by the juvenile hormone (JH) and 20-Hydroxyecdysone (20E). Therefore, the genetic regulation of silk production is a priority. JH binding protein (JHBP) transports JH from the hemolymph to target organs and cells and protects it. In a previous study, we identified 41 genes containing a JHBP domain in the Bombyx mori genome. Only one JHBP gene, BmJHBPd2, is highly expressed in the posterior silk gland (PSG), and its function remains unknown. In the present study, we investigated the expression levels of BmJHBPd2 and the major silk protein genes in the high-silk-producing practical strain 872 (S872) and the low-silk-producing local strain Dazao. We found that BmJHBPd2 was more highly expressed in S872 than in the Dazao strain, which is consistent with the expression pattern of fibroin genes. A subcellular localization assay indicated that BmJHBPd2 is located in the cytoplasm. In vitro hormone induction experiments showed that BmJHBPd2 was upregulated by juvenile hormone analogue (JHA) treatment. BmKr-h1 upregulation was significantly inhibited by the overexpression of BmJHBPd2 (BmJHBPd2OE) at the cell level when induced by JHA. However, overexpression of BmJHBPd2 in the PSG by transgenic methods led to the inhibition of silk fibroin gene expression, resulting in a reduction in silk yield. Further investigation showed that in the transgenic BmJHBPd2OE silkworm, the key transcription factor of the JH signaling pathway, Krüppel homolog 1 (Kr-h1), was inhibited, and 20E signaling pathway genes, such as broad complex (Brc), E74A, and ultraspiracle protein (USP), were upregulated. Our results indicate that BmJHBPd2 plays an important role in the JH signaling pathway and is important for silk protein synthesis. Furthermore, our findings help to elucidate the mechanisms by which JH regulates silk protein synthesis.

Possible Regulation of Larval Juvenile Hormone Titers in Bombyx mori by BmFAMeT6

Juvenile hormone (JH) plays a vital role in the growth, development, and reproduction of insects and other arthropods. Previous experiments have suggested that BmFAMeT6 could affect the duration of the silk moth's larval stage. In this study, we established the BmFAMeT6 overexpression strain and BmFAMeT6 knockout strain using the GAL4/UAS binary hybrid system and CRISPR/Cas 9 system, respectively, and found that the larval stage of the overexpression strain was shorter, while the knockout strain was longer. Our results exhibited that both the JH titers and BmKr-h1 levels in the larvae of the third instar were reduced significantly by BmFAMeT6 overexpression, but were increased obviously by BmFAMeT6 knockout. In addition, injection of farnesoic acid induced changes in the JH I and JH II levels in the hemolymphs of larvae. This study is the first to directly reveal the role of BmFAMeT6 in the regulation of insect JH titers and the relationship between farnesoic acid and JH (JH I and JH II). This provides a new perspective on regulating the growth and development of insects such as Bombyx mori.

Genome-wide analysis of the chromatin sites targeted by HEX 70a storage protein in the honeybee brain and fat body

Hexamerins, the proteins massively stored in the larval haemolymph of insects, are gradually used throughout metamorphosis as a source of raw material and energy for the development of adult tissues. Such behaviour defined hexamerins as storage proteins. Immunofluorescence experiments coupled with confocal microscopy show a hexamerin, HEX 70a, in the nucleus of the brain and fat body cells from honeybee workers, an unexpected localization for a storage protein. HEX 70a colocalizes with fibrillarin, a nucleolar-specific protein and H3 histone, thus suggesting a potential role as a chromatin-binding protein. This was investigated through chromatin immunoprecipitation and high-throughput DNA sequencing (ChIP-seq). The significant HEX 70a-DNA binding sites were mainly localized at the intergenic, promoter and intronic regions. HEX 70a targeted DNA stretches mapped to the genomic regions encompassing genes with relevant functional attributes. Several HEX 70a targeted genes were associated with H3K27ac or/and H3K27me3, known as active and repressive histone marks. Brain and fat body tissues shared a fraction of the HEX 70 targeted genes, and tissue-specific targets were also detected. The presence of overrepresented DNA motifs in the binding sites is consistent with specific HEX 70a-chromatin association. In addition, a search for HEX 70a targets in RNA-seq public libraries of fat bodies from nurses and foragers revealed differentially expressed targets displaying hex 70a-correlated developmental expression, thus supporting a regulatory activity for HEX 70a. Our results support the premise that HEX 70a is a moonlighting protein that binds chromatin and has roles in the brain and fat body cell nuclei, apart from its canonical role as a storage protein.

Juvenile hormone suppresses sensory organ precursor determination to block Drosophila adult abdomen morphogenesis

Juvenile hormone (JH) has a classic "status quo" action at both the pupal and adult molts when administrated exogenously. In Drosophila, treatment with JH at pupariation inhibits the formation of abdominal bristles, which are derived from the histoblasts. However, the mechanism via which JH exerts this effect remains poorly understood. In this study, we analyzed the effect of JH on histoblast proliferation, migration, and differentiation. Our results indicated that whereas the proliferation and migration of histoblasts remained unaffected following treatment with a JH mimic (JHM), their differentiation, particularly the specification of sensor organ precursor (SOP) cells, was inhibited. This effect was attributable to downregulated proneural genes achaete (ac) and Scute (sc) expression levels, which prevented the specification of SOP cells in proneural clusters. Moreover, Kr-h1 was found to mediate this effect of JHM. Histoblast-specific overexpression or knockdown of Kr-h1, respectively mimicked or attenuated the effects exerted by JHM on abdominal bristle formation, SOP determination, and transcriptional regulation of ac and sc. These results indicated that the defective SOP determination was responsible for the inhibition of abdominal bristle formation by JHM, which, in turn, was mainly mediated via the transducing action of Kr-h1.

m6A-dependent mevalonate kinase in juvenile hormone synthesis pathway regulates the diapause process of bivoltine silkworm (Bombyx mori)

BACKGROUND

Research has shown that epigenetic modification are involved the regulation of diapause in bivoltine silkworms (Bombyx mori), but it remains unclear how epigenetic modification in response to environmental signals precisely to regulate the diapause processing of bivoltine B. mori.

METHODS AND RESULTS

In this study, the diapause terminated eggs of bivoltine B. mori, Qiufeng (QF) were divided into two groups: a QFHT group incubated at 25 °C with a natural day/night cycle to produce diapause eggs, and a QFLT group incubated at 16.5 °C in darkness to produce non-diapause eggs. On the 3rd day of the pupal stage, the total RNAs of the eggs were extracted and their N6-adenosine methylation (m⁶A) abundances were analyzed to explore the effects of m⁶A methylation on diapause in the silkworm. The results showed that 1984 m⁶A peaks are shared, 1563 in QFLT and 659 in QFHT. The m⁶A methylation level of the QFLT group was higher than that of the QFHT one in various signaling pathways. The m⁶A methylation rate of mevalonate kinase (MK) in the insect hormone synthesis pathway was significantly different between the two groups. The knockdown of MK by RNA interference in the pupae of QFLT resulted in females laying diapause eggs rather than non-diapause eggs after mating.

CONCLUSIONS

m⁶A methylation involves in the diapause regulation of bivoltine B. mori by changing the expression levels of MK. This result provides a clearer image of the environmental signals on the regulation of diapause in bivoltine silkworms.

Decapentaplegic retards lipolysis during metamorphosis in Bombyx mori and Drosophila melanogaster

Insect morphogen decapentaplegic (Dpp) functions as one of the key extracellular ligands of the Bone Morphogenetic Protein (BMP) signaling pathway. Previous studies in insects mainly focused on the roles of Dpp during embryonic development and the formation of adult wings. In this study, we demonstrate a new role for Dpp in retarding lipolysis during metamorphosis in both Bombyx mori and Drosophila melanogaster. CRISPR/Cas9-mediated mutation of Bombyx dpp causes pupal lethality, induces an excessive and premature breakdown of lipids in the fat body, and upregulates the expressions of several lipolytic enzyme genes, including brummer (bmm), lipase 3 (lip3), and hormone-sensitive lipase (hsl), and lipid storage droplet 1 (lsd1), a lipid droplets (LD)-associated protein gene. Further investigation in Drosophila reveals that salivary gland-specific knockdown of the dpp gene and fat body-specific knockdown of Mad involved in Dpp signaling phenocopy the effects of Bombyx dpp mutation on pupal development and lipolysis. Taken together, our data indicate that the Dpp-mediated BMP signaling in the fat body maintains lipid homeostasis by retarding lipolysis, which is necessary for pupa-adult transition during insect metamorphosis.

Compounds Inhibiting Noppera-bo, a Glutathione S-transferase Involved in Insect Ecdysteroid Biosynthesis: Novel Insect Growth Regulators

Glutathione S-transferases (GSTs) are conserved in a wide range of organisms, including insects. In 2014, an epsilon GST, known as Noppera-bo (Nobo), was shown to regulate the biosynthesis of ecdysteroid, the principal steroid hormone in insects. Studies on fruit flies, Drosophila melanogaster, and silkworms, Bombyx mori, demonstrated that loss-of-function mutants of nobo fail to synthesize ecdysteroid and die during development, consistent with the essential function of ecdysteroids in insect molting and metamorphosis. This genetic evidence suggests that chemical compounds that inhibit activity of Nobo could be insect growth regulators (IGRs) that kill insects by disrupting their molting and metamorphosis. In addition, because nobo is conserved only in Diptera and Lepidoptera, a Nobo inhibitor could be used to target IGRs in a narrow spectrum of insect taxa. Dipterans include mosquitoes, some of which are vectors of diseases such as malaria and dengue fever. Given that mosquito control is essential to reduce mosquito-borne diseases, new IGRs that specifically kill mosquito vectors are always in demand. We have addressed this issue by identifying and characterizing several chemical compounds that inhibit Nobo protein in both D. melanogaster and the yellow fever mosquito, Aedes aegypti. In this review, we summarize our findings from the search for Nobo inhibitors.

Circadian clock regulates developmental time through ecdysone and juvenile hormones in Bombyx mori

The circadian clock plays an integral role in hormone biosynthesis and secretion. However, how the circadian clock precisely coordinates hormonal homeostasis to maintain normal animal development remains unclear. Here, we show that knocking out the core clock gene Cryptochrome 1 (Cry1) significantly delays the developmental time in Bombyx mori. This study focuses on the ecdysone and juvenile hormone signalling pathways of fifth instar larvae with the longest developmental time delay. We found that the mutant reduced prothoracicotropic hormone synthesis in the brain, and could not produce sufficient ecdysone in the prothoracic gland, resulting in a delayed peak of 20-hydroxyecdysone titre in the hemolymph of fifth instar larvae, prolonging developmental time. Moreover, further investigation revealed that the mutant enhanced juvenile hormone biosynthesis and signalling pathway and that this higher juvenile hormone titre also resulted in prolonged developmental time in fifth instar larvae. Our results provide insights into the molecular mechanisms by which the circadian clock regulates animal development by maintaining hormonal homeostasis.

Dimmed gene knockout shortens larval growth and reduces silk yield in the silkworm, Bombyx mori

The bHLH domain transcription factor, Bombyx mori-derived dimmed (Bmdimm), is directly regulated by the JH-BmMet/BmSRC-BmKr-h1 pathway and plays a key role in regulating the expression of FibH, which codes the main component of silk protein. However, the other roles of Bmdimm in silk protein synthesis remain unclear. Here, we established a Bmdimm knockout (KO) line containing a 7-bp deletion via CRISPR/Cas9 system, which led to the absence of the bHLH domain. The expression level of silk protein genes and silk yield decreased significantly in the Bmdimm KO line. Moreover, knocking out Bmdimm led to shortened larval stages and significant weight loss in larvae and adults. Bmdimm was found to be highly expressed in the silk gland, but it was also expressed in the fat body. The expression level of Bmkr-h1 in the fat body was significantly downregulated in the Bmdimm KO line. Exogenous JHA treatment upregulated Bmkr-h1 and rescued the phenotype of larval growth in the Bmdimm KO line. In conclusion, knocking out Bmdimm led to a shortened larval stage via the inhibition of Bmkr-h1 expression, then reduced silk yield. These findings help to elucidate the regulatory mechanism of fibroin synthesis and larval development in silkworms.

Rethinking the ecdysteroid source during Drosophila pupal-adult development

Ecdysteroids, typified by 20-hydroxyecdysone (20E), are essential hormones for the development, reproduction and physiology of insects and other arthropods. For over half a century, the vinegar fly Drosophila melanogaster (Ephydroidea: Diptera) has been used as a model of ecdysteroid biology. Many aspects of the biosynthesis and regulation of ecdysteroids in this species are understood at the molecular level, particularly with respect to their secretion from the prothoracic gland (PG) cells of the ring gland, widely considered the dominant biosynthetic tissue during development. Discrete pulses of 20E orchestrate transitions during the D. melanogaster life cycle, the sources of which are generally well understood, apart from the large 20E pulse at the onset of pharate adult development, which has received little recent attention. As the source of this pharate adult pulse (PAP) is a curious blind spot in Drosophila endocrinology, we evaluate published biochemical and genetic data as they pertain to three hypotheses for the source of PAP 20E: the PG; an alternative biosynthetic tissue; or the recycling of stored 20E. Based on multiple lines of evidence, we contend the PAP cannot be derived from biosynthesis, with other data consistent with D. melanogaster able to recycle ecdysteroids before and during metamorphosis. Published data also suggest the PAP is conserved across Diptera, with evidence for pupal-adult ecdysteroid recycling occurring in other cyclorrhaphan flies. Further experimental work is required to test the ecdysteroid recycling hypothesis, which would establish fundamental knowledge of the function, regulation, and evolution of metamorphic hormones in dipterans and other insects.

Mammalian puberty: a fly perspective

Mammalian puberty and Drosophila metamorphosis, despite their evolutionary distance, exhibit similar design principles and conservation of molecular components. In this Viewpoint, we review recent advances in this area and the similarities between both processes in terms of the signaling pathways and neuroendocrine circuits involved. We argue that the detection and uptake of peripheral fat by Drosophila prothoracic endocrine cells induces endomembrane remodeling and ribosomal maturation, leading to the acquisition of high biosynthetic and secretory capacity. The absence of this fat-neuroendocrine interorgan communication leads to giant, obese, non-pupating larvae. Importantly, human leptin is capable of signaling the pupariation process in Drosophila, and its expression prevents obesity and triggers maturation in mutants that do not pupate. This implies that insect metamorphosis can be used to address issues related to the biology of leptin and puberty.

Interorgan communication through peripherally derived peptide hormones in Drosophila

In multicellular organisms, endocrine factors such as hormones and cytokines regulate development and homoeostasis through communication between different organs. For understanding such interorgan communications through endocrine factors, the fruit fly Drosophila melanogaster serves as an excellent model system due to conservation of essential endocrine systems between flies and mammals and availability of powerful genetic tools. In Drosophila and other insects, functions of neuropeptides or peptide hormones from the central nervous system have been extensively studied. However, a series of recent studies conducted in Drosophila revealed that peptide hormones derived from peripheral tissues also play critical roles in regulating multiple biological processes, including growth, metabolism, reproduction, and behaviour. Here, we summarise recent advances in understanding target organs/tissues and functions of peripherally derived peptide hormones in Drosophila and describe how these hormones contribute to various biological events through interorgan communications.

Knockdown of Ecdysone-Induced Protein 93F Causes Abnormal Pupae and Adults in the Eggplant Lady Beetle

Ecdysone-induced protein 93F (E93) plays triple roles during post-embryonic development in insects whose juvenile instars are more than four. However, it only acts as a specifier of adult structures in Drosophila flies whose larval instars are fixed at three. In this study, we determined the functions of E93 in the eggplant lady beetle (Henosepilachna vigintioctopunctata), which has four larval instars. We uncovered that E93 was abundantly expressed at the prepupal and pupal stages. A precocious inhibition of the juvenile hormone signal by RNA interference (RNAi) of HvKr-h1 or HvHairy, two vital downstream developmental effectors, at the penultimate instar larval stage increased the expression of E93, Conversely, ingestion of JH by the third-instar larvae stimulated the expression of HvKr-h1 but repressed the transcription of either HvE93X1 or HvE93X2. However, disturbance of the JH signal neither drove premature metamorphosis nor caused supernumerary instars. In contrast, depletion of E93 at the third- and fourth-instar larval and prepupal stages severely impaired pupation and caused a larval-pupal mixed phenotype: pupal spines and larval scoli were simultaneously presented on the cuticle. RNAi of E93 at the pupal stage affected adult eclosion. When the beetles had suffered from a dsE93 injection at the fourth-instar larval and pupal stages, a few resultant adults emerged, with separated elytra, abnormally folded hindwings, a small body size and short appendages. Taken together, our results suggest the larval instars are fixed in H. vigintioctopunctata; E93 serves as a repressor of larval characters and a specifier of adult structures during the larval–pupal–adult transition.

The TGF-β Receptor Gene Saxophone Influences Larval-Pupal-Adult Development in Tribolium castaneum

The transforming growth factor-β (TGF-β) superfamily encodes a large group of proteins, including TGF-β isoforms, bone morphogenetic proteins and activins that act through conserved cell-surface receptors and signaling co-receptors. TGF-β signaling in insects controls physiological events, including growth, development, diapause, caste determination and metamorphosis. In this study, we used the red flour beetle, Tribolium castaneum, as a model species to investigate the role of the type I TGF-β receptor, saxophone (Sax), in mediating development. Developmental and tissue-specific expression profiles indicated Sax is constitutively expressed during development with lower expression in 19- and 20-day (6th instar) larvae. RNAi knockdown of Sax in 19-day larvae prolonged developmental duration from larvae to pupae and significantly decreased pupation and adult eclosion in a dose-dependent manner. At 50 ng dsSax/larva, Sax knockdown led to an 84.4% pupation rate and 46.3% adult emergence rate. At 100 ng and 200 ng dsSax/larva, pupation was down to 75.6% and 50%, respectively, with 0% adult emergence following treatments with both doses. These phenotypes were similar to those following knockdowns of 20-hydroxyecdysone (20E) receptor genes, ecdysone receptor (EcR) or ultraspiracle protein (USP). Expression of 20E biosynthesis genes disembodied and spookier, 20E receptor genes EcR and USP, and 20E downstream genes BrC and E75, were suppressed after the Sax knockdown. Topical application of 20E on larvae treated with dsSax partially rescued the dsSax-driven defects. We can infer that the TGF-β receptor gene Sax influences larval-pupal-adult development via 20E signaling in T. castaneum.

Conceptual framework for the insect metamorphosis from larvae to pupae by transcriptomic profiling, a case study of Helicoverpa armigera (Lepidoptera: Noctuidae)

BACKGROUND

Insect metamorphosis from larvae to pupae is one of the most important stages of insect life history. Relatively comprehensive information related to gene transcription profiles during lepidopteran metamorphosis is required to understand the molecular mechanism underlying this important stage. We conducted transcriptional profiling of the brain and fat body of the cotton bollworm Helicoverpa armigera (Lepidoptera: Noctuidae) during its transition from last instar larva into pupa to explore the physiological processes associated with different phases of metamorphosis.

RESULTS

During metamorphosis, the differences in gene expression patterns and the number of differentially expressed genes in the fat body were found to be greater than those in the brain. Each stage had a specific gene expression pattern, which contributed to different physiological changes. A decrease in juvenile hormone levels at the feeding stage is associated with increased expression levels of two genes (juvenile hormone esterase, juvenile hormone epoxide hydrolase). The expression levels of neuropeptides were highly expressed at the feeding stage and the initiation of the wandering stage and less expressed at the prepupal stage and the initiation of the pupal stage. The transcription levels of many hormone (or neuropeptide) receptors were specifically increased at the initiation of the wandering stage in comparison with other stages. The expression levels of many autophagy-related genes in the fat body were found to be gradually upregulated during metamorphosis. The activation of apoptosis was probably related to enhanced expression of many key genes (Apaf1, IAP-binding motif 1 like, cathepsins, caspases). Active proliferation might be associated with enhanced expression levels in several factors (JNK pathway: jun-D; TGF-β pathway: decapentaplegic, glass bottom boat; insulin pathway: insulin-like peptides from the fat body; Wnt pathway: wntless, TCF/Pangolin).

CONCLUSIONS

This study revealed several vital physiological processes and molecular events of metamorphosis and provided valuable information for illustrating the process of insect metamorphosis from larvae to pupae.

Kr-h1, a Cornerstone Gene in Insect Life History

Insect life cycle is coordinated by hormones and their downstream effectors. Krüppel homolog1 (Kr-h1) is one of the crucial effectors which mediates the actions of the two critical hormones of insects, the juvenile hormone (JH) and 20-hydroxyecdysone (20E). It is a transcription factor with a DNA-binding motif of eight C₂H₂ zinc fingers which is found to be conserved among insect orders. The expression of Kr-h1 is fluctuant during insect development with high abundance in juvenile instars and lower levels in the final instar and pupal stage, and reappearance in adults, which is governed by the coordination of JH, 20E, and miRNAs. The dynamic expression pattern of Kr-h1 is closely linked to its function in the entire life of insects. Over the past several years, accumulating studies have advanced our understanding of the role of Kr-h1 during insect development. It acts as a universal antimetamorphic factor in both hemimetabolous and holometabolous species by directly inhibiting the transcription of 20E signaling genes Broad-Complex (Br-C) and Ecdysone induced protein 93F (E93), and steroidogenic enzyme genes involved in ecdysone biosynthesis. Meanwhile, it promotes vitellogenesis and ovarian development in the majority of studied insects. In addition, Kr-h1 regulates insect behavioral plasticity and caste identity, neuronal morphogenesis, maturation of sexual behavior, as well as embryogenesis and metabolic homeostasis. Hence, Kr-h1 acts as a cornerstone regulator in insect life.

Cyromazine Effects the Reproduction of Drosophila by Decreasing the Number of Germ Cells in the Female Adult Ovary

Simple Summary

Cyromazine, an insect growth regulator, is used to control the Dipteran pest population. Previous findings observed that treatment with cyromazine increased the larval mortality, by interfering with the ecdysone signaling. In addition, the application of exogenous 20E significantly reduced the mortality caused by cyromazine. Many studies have also supported the role of ecdysone signaling in the maintenance of germline stem cells (GSCs), where mutations in ecdysone signaling-related genes significantly decreased the number of GSCs. However, to date, no study has reported the effect of cyromazine on the GSCs of Drosophila melanogaster. In the present study, we observed that cyromazine significantly reduced the number of both GSCs and cystoblasts (CBs) in the ovary of adult female. To further understand the effect of cyromazine on germ cells, we selected some key genes related to the ecdysone signaling pathway and evaluated their expression through RT-qPCR. Additionally, we measured the ecdysone titer from the cyromazine-treated ovaries. Our results indicated a significant decrease in the expression of ecdysone signaling-related genes and also in the ecdysone titer. These results further supported our findings that cyromazine reduced the number of germ cells by interfering with the ecdysone signaling pathway.

Abstract

In the present study, we observed a 58% decrease in the fecundity of Drosophila melanogaster, after treatment with the cyromazine. To further elucidate the effects of cyromazine on reproduction, we counted the number of both germline stem cells (GSCs) and cystoblasts (CBs) in the ovary of a 3-day-old adult female. The results showed a significant decrease in the number of GSCs and CBs as compared to the control group. The mode of action of cyromazine is believed to be through the ecdysone signaling pathway. To further support this postulate, we observed the expression of key genes involved in the ecdysone signaling pathway and also determined the ecdysone titer from cyromazine-treated ovaries. Results indicated a significant decrease in the expression of ecdysone signaling-related genes as compared to the control group. Furthermore, the titer of the ecdysone hormone was also markedly reduced (90%) in cyromazine-treated adult ovaries, suggesting that ecdysone signaling was directly related to the decrease in the number of GSCs and CBs. However, further studies are required to understand the mechanism by which cyromazine affects the GSCs and CBs in female adult ovaries.

FKBP39 Controls the Larval Stage JH Activity and Development in Drosophila melanogaster

Simple Summary

Two endocrine hormones, ecdysone and juvenile hormone (JH), control insect development and reproduction. Some studies in the literature have suggested that FKBP39 functions as a transcriptional factor and regulates the JH pathway in Drosophila. However, the physiological roles of FKBP39 are still elusive. To determine the FKBP39 roles in vivo, we first developed an antibody to check the FKBP39 expression pattern and then detected JH activity-related phenotypes in fkbp39 mutants, such as pupariation, reproduction, and Kr-h1 expression. We found that FKBP39 expresses at a high level and controls JH activity at the larval stage. Moreover, we found that rp49, the most widely used reference gene for Real-time quantitative PCR (qRT-PCR), significantly decreased in the fkbp39 mutant. This work will provide valuable information for studies on JH activity and insect development.

Abstract

FK506-binding protein 39kD (FKBP39) localizes in the nucleus and contains multiple functional domains. Structural analysis suggests that FKBP39 might function as a transcriptional factor and control juvenile hormone (JH) activity. Here, we show that FKBP39 expresses at a high level and localizes in the nucleolus of fat body cells during the first two larval stages and early third larval stage. The fkbp39 mutant displays delayed larval-pupal transition and an increased expression of Kr-h1, the main mediator of the JH pathway, at the early third larval stage. Moreover, the fkbp39 mutant has a fertility defect that is independent of JH activity. Interestingly, the expression of rp49, the most widely used reference gene for qRT-PCR in Drosophila, significantly decreased in the fkbp39 mutant, suggesting that FKBP39 might regulate ribosome assembly. Taken together, our data demonstrate the expression pattern and physiological roles of FKBP39 in Drosophila.

Transcriptional Regulators of Ecdysteroid Biosynthetic Enzymes and Their Roles in Insect Development

Steroid hormones are responsible for coordinating many aspects of biological processes in most multicellular organisms, including insects. Ecdysteroid, the principal insect steroid hormone, is biosynthesized from dietary cholesterol or plant sterols. In the last 20 years, a number of ecdysteroidogenic enzymes, including Noppera-bo, Neverland, Shroud, Spook/Spookier, Cyp6t3, Phantom, Disembodied, Shadow, and Shade, have been identified and characterized in molecular genetic studies using the fruit fly Drosophila melanogaster. These enzymes are encoded by genes collectively called the Halloween genes. The transcriptional regulatory network, governed by multiple regulators of transcription, chromatin remodeling, and endoreplication, has been shown to be essential for the spatiotemporal expression control of Halloween genes in D. melanogaster. In this review, we summarize the latest information on transcriptional regulators that are crucial for controlling the expression of ecdysteroid biosynthetic enzymes and their roles in insect development.

A salivary gland-secreted peptide regulates insect systemic growth

Insect salivary glands have been previously shown to function in pupal attachment and food lubrication by secreting factors into the lumen via an exocrine way. Here, we find in Drosophila that a salivary gland-derived secreted factor (Sgsf) peptide regulates systemic growth via an endocrine way. Sgsf is specifically expressed in salivary glands and secreted into the hemolymph. Sgsf knockout or salivary gland-specific Sgsf knockdown decrease the size of both the body and organs, phenocopying the effects of genetic ablation of salivary glands, while salivary gland-specific Sgsf overexpression increases their size. Sgsf promotes systemic growth by modulating the secretion of the insulin-like peptide Dilp2 from the brain insulin-producing cells (IPCs) and affecting mechanistic target of rapamycin (mTOR) signaling in the fat body. Altogether, our study demonstrates that Sgsf mediates the roles of salivary glands in Drosophila systemic growth, establishing an endocrine function of salivary glands.

Juvenile Hormone Studies in Drosophila melanogaster

In the field of insect endocrinology, juvenile hormone (JH) is one of the most wondrous entomological terms. As a unique sesquiterpenoid hormone produced and released by the endocrine gland, corpus allatum (CA), JH is a critical regulator in multiple developmental and physiological processes, such as metamorphosis, reproduction, and behavior. Benefited from the precise genetic interventions and simplicity, the fruit fly, Drosophila melanogaster, is an indispensable model in JH studies. This review is aimed to present the regulatory factors on JH biosynthesis and an overview of the regulatory roles of JH in Drosophila. The future directions of JH studies are also discussed, and a few hot spots are highlighted.

Juvenile hormone membrane signaling phosphorylates USP and thus potentiates 20-hydroxyecdysone action in Drosophila

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) coordinately regulate development and metamorphosis in insects. Two JH intracellular receptors, methoprene-tolerant (Met) and germ-cell expressed (Gce), have been identified in the fruit fly Drosophila melanogaster. To investigate JH membrane signaling pathway without the interference from JH intracellular signaling, we characterized phosphoproteome profiles of the Met gce double mutant in the absence or presence of JH in both chronic and acute phases. Functioning through a potential receptor tyrosine kinase and phospholipase C pathway, JH membrane signaling activated protein kinase C (PKC) which phosphorylated ultraspiracle (USP) at Ser35, the PKC phosphorylation site required for the maximal action of 20E through its nuclear receptor complex EcR-USP. The usp^S35A mutant, in which Ser was replaced with Ala at position 35 by genome editing, showed decreased expression of Halloween genes that are responsible for ecdysone biosynthesis and thus attenuated 20E signaling that delayed developmental timing. The usp^S35A mutant also showed lower Yorkie activity that reduced body size. Altogether, JH membrane signaling phosphorylates USP at Ser35 and thus potentiates 20E action that regulates the normal fly development. This study helps better understand the complex JH signaling network.

Phenotypic Plasticity: What Has DNA Methylation Got to Do with It?

How does one genome give rise to multiple, often markedly different, phenotypes in response to an environmental cue? This phenomenon, known as phenotypic plasticity, is common amongst plants and animals, but arguably the most striking examples are seen in insects. Well-known insect examples include seasonal morphs of butterfly wing patterns, sexual and asexual reproduction in aphids, and queen and worker castes of eusocial insects. Ultimately, we need to understand how phenotypic plasticity works at a mechanistic level; how do environmental signals alter gene expression, and how are changes in gene expression translated into novel morphology, physiology and behaviour? Understanding how plasticity works is of major interest in evolutionary-developmental biology and may have implications for understanding how insects respond to global change. It has been proposed that epigenetic mechanisms, specifically DNA methylation, are the key link between environmental cues and changes in gene expression. Here, we review the available evidence on the function of DNA methylation of insects, the possible role(s) for DNA methylation in phenotypic plasticity and also highlight key outstanding questions in this field as well as new experimental approaches to address these questions.

Intrinsic and damage-induced JAK/STAT signaling regulate developmental timing by the Drosophila prothoracic gland

Development involves tightly paced, reproducible sequences of events, yet it must adjust to conditions external to it, such as resource availability and organismal damage. A major mediator of damage-induced immune responses in vertebrates and insects is JAK/STAT signaling. At the same time, JAK/STAT activation by the Drosophila Upd cytokines is pleiotropically involved in normal development of multiple organs. Whether inflammatory and developmental JAK/STAT roles intersect is unknown. Here, we show that JAK/STAT is active during development of the prothoracic gland (PG), which controls metamorphosis onset through ecdysone production. Reducing JAK/STAT signaling decreased PG size and advanced metamorphosis. Conversely, JAK/STAT hyperactivation by overexpression of pathway components or SUMOylation loss caused PG hypertrophy and metamorphosis delay. Tissue damage and tumors, known to secrete Upd cytokines, also activated JAK/STAT in the PG and delayed metamorphosis, at least in part by inducing expression of the JAK/STAT target Apontic. JAK/STAT damage signaling, therefore, regulates metamorphosis onset by co-opting its developmental role in the PG. Our findings in Drosophila provide insights on how systemic effects of damage and cancer can interfere with hormonally controlled development and developmental transitions.

Summary: Damage signaling from tumors mediated by JAK/STAT-activating Upd cytokines delays the Drosophila larva–pupa transition through co-option of a JAK/STAT developmental role in the prothoracic gland.

The transcription factor Sox30 is involved in Nile tilapia spermatogenesis

Spermatogenesis is a complex process in which spermatogonial stem cells differentiate and develop into mature spermatozoa. The transcriptional regulatory network involved in fish spermatogenesis remains poorly understood. Here, we demonstrate in Nile tilapia that the Sox transcription factor family member Sox30 is specifically expressed in the testes and mainly localizes to spermatocytes and spermatids. CRISPR/Cas9-mediated sox30 mutation results in abnormal spermiogenesis, reduction of sperm motility, and male subfertility. Comparative transcriptome analysis shows that sox30 mutation alters the expression of genes involved in spermatogenesis. Further chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq), ChIP-PCR, and luciferase reporter assays reveal that Sox30 positively regulates the transcription of ift140 and ptprb, two genes involved in spermiogenesis, by directly binding to their promoters. Taken together, our data indicate that Sox30 plays essential roles in Nile tilapia spermatogenesis by directly regulating the transcription of the spermiogenesis-related genes ift140 and ptprb.

Krüppel-homologue 1 Mediates Hormonally Regulated Dominance Rank in a Social Bee

Dominance hierarchies are ubiquitous in invertebrates and vertebrates, but little is known on how genes influence dominance rank. Our gaps in knowledge are specifically significant concerning female hierarchies, particularly in insects. To start filling these gaps, we studied the social bumble bee Bombus terrestris, in which social hierarchies among females are common and functionally significant. Dominance rank in this bee is influenced by multiple factors, including juvenile hormone (JH) that is a major gonadotropin in this species. We tested the hypothesis that the JH responsive transcription factor Krüppel homologue 1 (Kr-h1) mediates hormonal influences on dominance behavior. We first developed and validated a perfluorocarbon nanoparticles-based RNA interference protocol for knocking down Kr-h1 expression. We then used this procedure to show that Kr-h1 mediates the influence of JH, not only on oogenesis and wax production, but also on aggression and dominance rank. To the best of our knowledge, this is the first study causally linking a gene to dominance rank in social insects, and one of only a few such studies on insects or on female hierarchies. These findings are important for determining whether there are general molecular principles governing dominance rank across gender and taxa.

Krüppel-homolog 1 exerts anti-metamorphic and vitellogenic functions in insects via phosphorylation-mediated recruitment of specific cofactors

Background

The zinc-finger transcription factor Krüppel-homolog 1 (Kr-h1) exerts a dual regulatory role during insect development by preventing precocious larval/nymphal metamorphosis and in stimulating aspects of adult reproduction such as vitellogenesis. However, how Kr-h1 functions both as a transcriptional repressor in juvenile metamorphosis and an activator in adult reproduction remains elusive. Here, we use the insect Locusta migratoria to dissect the molecular mechanism by which Kr-h1 functions as activator and repressor at these distinct developmental stages.

Results

We report that the kinase PKCα triggers Kr-h1 phosphorylation at the amino acid residue Ser¹⁵⁴, a step essential for its dual functions. During juvenile stage, phosphorylated Kr-h1 recruits a corepressor, C-terminal binding protein (CtBP). The complex of phosphorylated Kr-h1 and CtBP represses the transcription of Ecdysone induced protein 93F (E93) and consequently prevents the juvenile-to-adult transition. In adult insects, phosphorylated Kr-h1 recruits a coactivator, CREB-binding protein (CBP), and promotes vitellogenesis by inducing the expression of Ribosomal protein L36. Furthermore, Kr-h1 phosphorylation with the concomitant inhibition of E93 transcription is evolutionarily conserved across insect orders.

Conclusion

Our results suggest that Kr-h1 phosphorylation is indispensable for the recruitment of transcriptional cofactors, and for its anti-metamorphic and vitellogenic actions in insects. Our data shed new light on the understanding of Kr-h1 regulation and function in JH-regulated insect metamorphosis and reproduction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01157-3.

Histone H3K27 methylation-mediated repression of Hairy regulates insect developmental transition by modulating ecdysone biosynthesis

Insect development is cooperatively orchestrated by the steroid hormone ecdysone and juvenile hormone (JH). The polycomb repressive complex 2 (PRC2)-mediated histone H3K27 trimethylation (H3K27me3) epigenetically silences gene transcription and is essential for a range of biological processes, but the functions of H3K27 methylation in insect hormone action are poorly understood. Here, we demonstrate that H3K27 methylation-mediated repression of Hairy transcription in the larval prothoracic gland (PG) is required for ecdysone biosynthesis in Bombyx and Drosophila H3K27me3 levels in the PG are dynamically increased during the last larval instar. H3K27me3 reduction induced by the down-regulation of PRC2 activity via inhibitor treatment in Bombyx or PG-specific knockdown of the PRC2 component Su(z)12 in Drosophila diminishes ecdysone biosynthesis and disturbs the larval-pupal transition. Mechanistically, H3K27 methylation targets the JH signal transducer Hairy to repress its transcription in the PG; PG-specific knockdown or overexpression of the Hairy gene disrupts ecdysone biosynthesis and developmental transition; and developmental defects caused by PG-specific Su(z)12 knockdown can be partially rescued by Hairy down-regulation. The application of JH mimic to the PG decreases both H3K27me3 levels and Su(z)12 expression. Altogether, our study reveals that PRC2-mediated H3K27 methylation at Hairy in the PG during the larval period is required for ecdysone biosynthesis and the larval-pupal transition and provides insights into epigenetic regulation of the crosstalk between JH and ecdysone during insect development.

Transcriptional identification of differentially expressed genes during the prepupal-pupal transition in the oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae)

The oriental armyworm, Mythimna separata (Walker) is a serious pest of agriculture that does particular damage to Gramineae crops in Asia, Europe, and Oceania. Metamorphosis is a key developmental stage in insects, although the genes underlying the metamorphic transition in M. separata remain largely unknown. Here, we sequenced the transcriptomes of five stages; mature larvae (ML), wandering (W), and pupation (1, 5, and 10 days after pupation, designated P1, P5, and P10) to identify transition-associated genes. Four libraries were generated, with 22,884, 23,534, 26,643, and 33,238 differentially expressed genes (DEGs) for the ML-vs-W, W-vs-P1, P1-vs-P5, and P5-vs-P10, respectively. Gene ontology enrichment analysis of DEGs showed that genes regulating the biosynthesis of the membrane and integral components of the membrane, which includes the cuticular protein (CP), 20-hydroxyecdysone (20E), and juvenile hormone (JH) biosynthesis, were enriched. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that DEGs were enriched in the metabolic pathways. Of these DEGs, thirty CP, seventeen 20E, and seven JH genes were differentially expressed across the developmental stages. For transcriptome validation, ten CP, 20E, and JH-related genes were selected and verified by real-time PCR quantitative. Collectively, our results provided a basis for further studies of the molecular mechanism of metamorphosis in M. separata.

Signaling Pathways That Regulate the Crustacean Molting Gland

A pair of Y-organs (YOs) are the molting glands of decapod crustaceans. They synthesize and secrete steroid molting hormones (ecdysteroids) and their activity is controlled by external and internal signals. The YO transitions through four physiological states over the molt cycle, which are mediated by molt-inhibiting hormone (MIH; basal state), mechanistic Target of Rapamycin Complex 1 (mTORC1; activated state), Transforming Growth Factor-β (TGFβ)/Activin (committed state), and ecdysteroid (repressed state) signaling pathways. MIH, produced in the eyestalk X-organ/sinus gland complex, inhibits the synthesis of ecdysteroids. A model for MIH signaling is organized into a cAMP/Ca2+-dependent triggering phase and a nitric oxide/cGMP-dependent summation phase, which maintains the YO in the basal state during intermolt. A reduction in MIH release triggers YO activation, which requires mTORC1-dependent protein synthesis, followed by mTORC1-dependent gene expression. TGFβ/Activin signaling is required for YO commitment in mid-premolt. The YO transcriptome has 878 unique contigs assigned to 23 KEGG signaling pathways, 478 of which are differentially expressed over the molt cycle. Ninety-nine contigs encode G protein-coupled receptors (GPCRs), 65 of which bind a variety of neuropeptides and biogenic amines. Among these are putative receptors for MIH/crustacean hyperglycemic hormone neuropeptides, corazonin, relaxin, serotonin, octopamine, dopamine, allatostatins, Bursicon, ecdysis-triggering hormone (ETH), CCHamide, FMRFamide, and proctolin. Contigs encoding receptor tyrosine kinase insulin-like receptor, epidermal growth factor (EGF) receptor, and fibroblast growth factor (FGF) receptor and ligands EGF and FGF suggest that the YO is positively regulated by insulin-like peptides and growth factors. Future research should focus on the interactions of signaling pathways that integrate physiological status with environmental cues for molt control.

Autophagy triggers CTSD (cathepsin D) maturation and localization inside cells to promote apoptosis

CTSD/CathD/CATD (cathepsin D) is a lysosomal aspartic protease. A distinguishing characteristic of CTSD is its dual functions of promoting cell proliferation via secreting a pro-enzyme outside the cells as a ligand, and promoting apoptosis via the mature form of this enzyme inside cells; however, the regulation of its secretion, expression, and maturation is undetermined. Using the lepidopteran insect Helicoverpa armigera, a serious agricultural pest, as a model, we revealed the dual functions and regulatory mechanisms of CTSD secretion, expression, and maturation. Glycosylation of asparagine 233 (N233) determined pro-CTSD secretion. The steroid hormone 20-hydroxyecdysone (20E) promoted CTSD expression. Macroautophagy/autophagy triggered CTSD maturation and localization inside midgut cells to activate CASP3 (caspase 3) and promote apoptosis. Pro-CTSD was expressed in the pupal epidermis and was secreted into the hemolymph to promote adult fat body endoreplication/endoreduplication, cell proliferation, and association. Our study revealed that the differential expression and autophagy-mediated maturation of CTSD in tissues determine its roles in apoptosis and cell proliferation, thereby determining the cell fates of tissues during lepidopteran metamorphosis.Abbreviations: 20E: 20-hydroxyecdysone; 3-MA: 3-methyladenine; ACTB/β-actin: actin beta; AKT: protein kinase B; ATG1: autophagy-related 1; ATG4: autophagy-related 4; ATG5: autophagy-related 5; ATG7: autophagy-related 7; ATG14: autophagy-related 14; BSA: bovine serum albumin; CASP3: caspase 3; CQ: choroquine; CTSD: cathepsin D; DAPI: 4',6-diamidino-2-phenylindole; DMSO: dimethyl sulfoxide; DPBS: dulbecco's phosphate-buffered saline; DsRNA: double-stranded RNA; EcR: ecdysone receptor; EcRE: ecdysone response element; EdU: 5-ethynyl-2´-deoxyuridine; G-m-CTSD: glycosylated-mautre-CTSD; G-pro-CTSD: glycosylated-pro-CTSD; HaEpi: Helicoverpa armigera epidermal cell line; HE staining: hematoxylin and eosin staining; IgG: immunoglobin G; IM: imaginal midgut; JH: juvenile hormone; Kr-h1: krueppel homologous protein 1; LM: larval midgut; M6P: mannose-6-phosphate; PBS: phosphate-buffered saline; PCD: programmed cell death; PNGase: peptide-N-glycosidase F; RFP: red fluorescent protein; RNAi: RNA interference; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SYX17: syntaxin 17; USP1: ultraspiracle isoform 1.

Early behavioral and molecular events leading to caste switching in the ant Harpegnathos

Ant societies show a division of labor in which a queen is in charge of reproduction while nonreproductive workers maintain the colony. In Harpegnathos saltator, workers retain reproductive ability, inhibited by the queen pheromones. Following the queen loss, the colony undergoes social unrest with an antennal dueling tournament. Most workers quickly abandon the tournament while a few workers continue the dueling for months and become gamergates (pseudoqueens). However, the temporal dynamics of the social behavior and molecular mechanisms underlining the caste transition and social dominance remain unclear. By tracking behaviors, we show that the gamergate fate is accurately determined 3 d after initiation of the tournament. To identify genetic factors responsible for this commitment, we compared transcriptomes of different tissues between dueling and nondueling workers. We found that juvenile hormone is globally repressed, whereas ecdysone biosynthesis in the ovary is increased in gamergates. We show that molecular changes in the brain serve as earliest caste predictors compared with other tissues. Thus, behavioral and molecular data indicate that despite the prolonged social upheaval, the gamergate fate is rapidly established, suggesting a robust re-establishment of social structure.

20E-mediated regulation of BmKr-h1 by BmKRP promotes oocyte maturation

BACKGROUND

Krüppel homolog 1 (Kr-h1) is a critical transcription factor for juvenile hormone (JH) signaling, known to play a key role in regulating metamorphosis and adult reproduction in insects. Kr-h1 can also be induced by molting hormone 20-hydroxyecdysone (20E), however, the underlying mechanism of 20E-induced Kr-h1 expression remains unclear. In the present study, we investigated the molecular mechanism of Kr-h1 induction by 20E in the reproductive system of a model lepidopteran insect, Bombyx mori.

RESULTS

Developmental and tissue-specific expression analysis revealed that BmKr-h1 was highly expressed in ovaries during the late pupal and adult stages and the expression was induced by 20E. RNA interference (RNAi)-mediated depletion of BmKr-h1 in female pupae severely repressed the transcription of vitellogenin receptor (VgR), resulting in the reduction in vitellogenin (Vg) deposition in oocytes. BmKr-h1 specifically bound the Kr-h1 binding site (KBS) between - 2818 and - 2805 nt upstream of BmVgR and enhanced the transcription of BmVgR. A 20E cis-regulatory element (CRE) was identified in the promoter of BmKr-h1 and functionally verified using luciferase reporter assay, EMSA and DNA-ChIP. Using pull-down assays, we identified a novel transcription factor B. mori Kr-h1 regulatory protein (BmKRP) that specifically bound the BmKr-h1 CRE and activated its transcription. CRISPR/Cas9-mediated knockout of BmKRP in female pupae suppressed the transcription of BmKr-h1 and BmVgR, resulting in arrested oogenesis.

CONCLUSION

We identified BmKRP as a new transcription factor mediating 20E regulation of B. mori oogenesis. Our data suggests that induction of BmKRP by 20E regulates BmKr-h1 expression, which in turn induces BmVgR expression to facilitate Vg uptake and oogenesis.

Vestigial mediates the effect of insulin signaling pathway on wing-morph switching in planthoppers

Wing polymorphism is an evolutionary feature found in a wide variety of insects, which offers a model system for studying the evolutionary significance of dispersal. In the wing-dimorphic planthopper Nilaparvata lugens, the insulin/insulin-like growth factor signaling (IIS) pathway acts as a ‘master signal’ that directs the development of either long-winged (LW) or short-winged (SW) morphs via regulation of the activity of Forkhead transcription factor subgroup O (NlFoxO). However, downstream effectors of the IIS–FoxO signaling cascade that mediate alternative wing morphs are unclear. Here we found that vestigial (Nlvg), a key wing-patterning gene, is selectively and temporally regulated by the IIS–FoxO signaling cascade during the wing-morph decision stage (fifth-instar stage). RNA interference (RNAi)-mediated silencing of Nlfoxo increase Nlvg expression in the fifth-instar stage (the last nymphal stage), thereby inducing LW development. Conversely, silencing of Nlvg can antagonize the effects of IIS activity on LW development, redirecting wing commitment from LW to the morph with intermediate wing size. In vitro and in vivo binding assays indicated that NlFoxO protein may suppress Nlvg expression by directly binding to the first intron region of the Nlvg locus. Our findings provide a first glimpse of the link connecting the IIS pathway to the wing-patterning network on the developmental plasticity of wings in insects, and help us understanding how phenotypic diversity is generated by the modification of a common set of pattern elements.

Many insects are capable of developing into either long-winged or short-winged adults, but the underlying molecular basis remains largely unknown. Pioneer studies showed that the insulin/insulin-like growth factor signaling pathway acts as a ‘master signal’ that directs wing buds to develop into long or short wings in the wing-dimorphic planthopper, Nilaparvata lugens. However, downstream effectors mediating the IIS pathway effects are unknown. Our findings highlight that vestigial, a key wing-patterning gene, is a main downstream effector that mediates the IIS activity on the development of alternative wing morphs during the wing-morph decision stage. The molecular mechanism of wing formation, including the function of vestigial, has been studied in great depth in the model insect Drosophila melanogaster. Our data provide a first glimpse of the link connecting the IIS pathway to the wing-patterning network in regulating developmental plasticity of wings in insects.

Regulation of ecdysone production in Drosophila by neuropeptides and peptide hormones

In both mammals and insects, steroid hormones play a major role in directing the animal's progression through developmental stages. To maximize fitness outcomes, steroid hormone production is regulated by the environmental conditions experienced by the animal. In insects, the steroid hormone ecdysone mediates transitions between developmental stages and is regulated in response to environmental factors such as nutrition. These environmental signals are communicated to the ecdysone-producing gland via the action of neuropeptide and peptide hormone signalling pathways. While some of these pathways have been well characterized, there is evidence to suggest more signalling pathways than has previously been thought function to control ecdysone production, potentially in response to a greater range of environmental conditions. Here, we review the neuropeptide and peptide hormone signalling pathways known to regulate the production of ecdysone in the model genetic insect Drosophila melanogaster, as well as what is known regarding the environmental signals that trigger these pathways. Areas for future research are highlighted that can further contribute to our overall understanding of the complex orchestration of environmental, physiological and developmental cues that together produce a functioning adult organism.

Intragenic DNA methylation regulates insect gene expression and reproduction through the MBD/Tip60 complex

DNA methylation is an important epigenetic modification. However, the regulations and functions of insect intragenic DNA methylation remain unknown. Here, we demonstrate that a regulatory mechanism involving intragenic DNA methylation controls ovarian and embryonic developmental processes in Bombyx mori. In B. mori, DNA methylation is found near the transcription start site (TSS) of ovarian genes. By promoter activity analysis, we observed that 5′ UTR methylation enhances gene expression. Moreover, methyl-DNA-binding domain protein 2/3 (MBD2/3) binds to the intragenic methyl-CpG fragment and recruits acetyltransferase Tip60 to promote histone H3K27 acetylation and gene expression. Additionally, genome-wide analyses showed that the peak of H3K27 acetylation appears near the TSS of methyl-modified genes, and DNA methylation is enriched in genes involved in protein synthesis in the B. mori ovary, with MBD2/3 knockdown resulting in decreased fecundity. These data uncover a mechanism of gene body methylation for regulating insect gene expression and reproduction.

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Insect intragenic 5mC enhances gene expression through histone H3K27 acetylation

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MBD2/3 binds the intragenic 5mC and recruits Tip60 to promote H3K27 acetylation

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Intragenic 5mCs modify protein synthesis-related genes in insect ovaries

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The intragenic 5mC plays a role in insect reproduction

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.

Metabolism and growth adaptation to environmental conditions in Drosophila

Organisms adapt to changing environments by adjusting their development, metabolism, and behavior to improve their chances of survival and reproduction. To achieve such flexibility, organisms must be able to sense and respond to changes in external environmental conditions and their internal state. Metabolic adaptation in response to altered nutrient availability is key to maintaining energy homeostasis and sustaining developmental growth. Furthermore, environmental variables exert major influences on growth and final adult body size in animals. This developmental plasticity depends on adaptive responses to internal state and external cues that are essential for developmental processes. Genetic studies have shown that the fruit fly Drosophila, similarly to mammals, regulates its metabolism, growth, and behavior in response to the environment through several key hormones including insulin, peptides with glucagon-like function, and steroid hormones. Here we review emerging evidence showing that various environmental cues and internal conditions are sensed in different organs that, via inter-organ communication, relay information to neuroendocrine centers that control insulin and steroid signaling. This review focuses on endocrine regulation of development, metabolism, and behavior in Drosophila, highlighting recent advances in the role of the neuroendocrine system as a signaling hub that integrates environmental inputs and drives adaptive responses.