Juvenile hormone-receptor complex acts on mcm4 and mcm7 to promote polyploidy and vitellogenesis in the migratory locust

Post-eclosion growth in the Drosophila Ejaculatory Duct is driven by Juvenile Hormone signaling and is essential for male fertility

The Drosophila Ejaculatory duct (ED) is a secretory tissue of the somatic male reproductive system. The ED is involved in the secretion of seminal fluid components and ED-specific antimicrobial peptides that aid in fertility and the female post-mating response. The ED is composed of secretory epithelial cells surrounded by a layer of innervated contractile muscle. The ED grows in young adult males during the first 24h post-eclosion, but the cell cycle status of the ED secretory cells and the role of post-eclosion ED growth have been unexplored. Here, we show that secretory cells of the adult Drosophila ED undergo variant cell cycles lacking mitosis called the endocycle, that lead to an increase in the cell and organ size of the ED post eclosion. The cells largely exit the endocycle by day 3 of adulthood, when the growth of the ED ceases, resulting in a tissue containing cells of ploidies ranging from 8C-32C. The size of the ED directly correlates with the ploidy of the secretory cells, with additional ectopic endocycles increasing organ size. When endoreplication is compromised in ED secretory cells, it leads to reduced organ size, reduced protein synthesis and compromised fertility. We provide evidence that the growth and endocycling in the young adult male ED is dependent on Juvenile hormone (JH) signaling and we suggest that hormone-induced early adult endocycling is required for optimal fertility and function of the ED tissue. We propose to use the ED as a post-mitotic tissue model to study the role of polyploidy in regulating secretory tissue growth and function.

RNAi-mediated glucose transporter 4 (Glut4) silencing inhibits ovarian development and enhances deltamethrin-treated energy depletion in Locusta migratoria

Energy metabolism is essential for insect development, reproduction and detoxification. Insects often reallocate energy and resources to manage external stress, balancing the demands of detoxification and reproduction. Glucose transport 4 (Glut4), a glucose transporter, is involved in glucose and lipid metabolism. However, the specific molecular mechanism of Glut4 in insect reproduction, and its role in the response to insecticide-induced oxidative stress remain unclear. In this study, LmGlut4 was identified and analyzed in Locusta migratoria. Silencing of LmGlut4 significantly reduced vitellogenin (Vg) biosynthesis in the fat body and Vg absorption by oocytes, ultimately hindering ovarian development and oocyte maturation. Knockdown of LmGlut4 also inhibited the biosynthesis of key insect hormones, such as juvenile hormone (JH), 20-hydroxyecdysone (20E) and insulin. Furthermore, LmGlut4 knockdown led to reduced triglyceride (TG) and glycogen content in the fat body and ovary, as well as decreased capacity for trehalose biosynthesis in adipocytes. Additionally, dsLmGlut4-treated locusts showed heightened sensitivity to deltamethrin, leading to increased triglyceride depletion during detoxification. This study sheds light on the biological function of LmGlut4 in the ovary and provides potential target genes for exploring biological pest management strategies.

Cell cycle variants during Drosophila male accessory gland development

The Drosophila melanogaster male accessory gland (AG) is a functional analog of the mammalian prostate and seminal vesicles containing two secretory epithelial cell types, termed main and secondary cells. This tissue is responsible for making and secreting seminal fluid proteins and other molecules that contribute to successful reproduction. The cells of this tissue are binucleate and polyploid, due to variant cell cycles that include endomitosis and endocycling during metamorphosis. Here, we provide evidence of additional cell cycle variants in this tissue. We show that main cells of the gland are connected by ring canals that form after the penultimate mitosis, and we describe an additional post-eclosion endocycle required for gland maturation that is dependent on juvenile hormone signaling. We present evidence that the main cells of the D. melanogaster AG undergo a unique cell cycle reprogramming throughout organ development that results in step-wise cell cycle truncations culminating in cells containing two octoploid nuclei with under-replicated heterochromatin in the mature gland. We propose this tissue as a model to study developmental and hormonal temporal control of cell cycle variants in terminally differentiating tissues.

Cloning and functional analysis of the juvenile hormone receptor gene CsMet in Coccinella septempunctata

The potential role of the juvenile hormone receptor gene (methoprene-tolerant, Met) in reproduction of Coccinella septempunctata L. (Coleoptera: Coccinellidae)(Coleoptera: Coccinellidae), was investigated by cloning, analyzing expression profiles by quantitative real-time PCR, and via RNA interference (RNAi). CsMet encoded a 1518-bp open reading frames with a predicted protein product of 505 amino acids; the latter contained 2 Per-Arnt-Sim repeat profile at amino acid residues 30-83 and 102-175. CsMet was expressed in different C. septempunctata larvae developmental stages and was most highly expressed in third instar. CsMet expression in female adults gradually increased from 20 to 30 d, and expression levels at 25 and 30 d were significantly higher than levels at 1-15 d. CsMet expression in 20-d-old male adults was significantly higher than in males aged 1-15 d. CsMet expression levels in fat body tissues of male and female adults were significantly higher than expression in the head, thorax, and reproductive system. At 5 and 10 d after CsMet-dsRNA injection, CsMet expression was significantly lower than the controls by 75.05% and 58.38%, respectively. Ovary development and vitellogenesis in C. septempunctata injected with CsMet-dsRNA were significantly delayed and fewer mature eggs were produced. This study provides valuable information for the large-scale rearing of C. septempunctata.

Endocrine factors modulating vitellogenesis and oogenesis in insects: An update

The endocrine system plays a pivotal role in shaping the mechanisms that ensure successful reproduction. With over a million known insect species, understanding the endocrine control of reproduction has become increasingly complex. Some of the key players include the classic insect lipid hormones juvenile hormone (JH) and ecdysteroids, and neuropeptides such as insulin-like peptides (ILPs). Individual endocrine factors not only modulate their own target tissue but also play crucial roles in crosstalk among themselves, ensuring successful vitellogenesis and oogenesis. Recent advances in omics, gene silencing, and genome editing approaches have accelerated research, offering both fundamental insights and practical applications for studying in-depth endocrine signaling pathways. This review provides an updated and integrated view of endocrine factors modulating vitellogenesis and oogenesis in insect females.

Cell type specific polyploidization in the royal fat body of termite queens

Tissue-specific endopolyploidy is widespread among plants and animals and its role in organ development and function has long been investigated. In insects, the fat body cells of sexually mature females produce substantial amounts of egg yolk precursor proteins (vitellogenins) and exhibit high polyploid levels, which is considered crucial for boosting egg production. Termites are social insects with a reproductive division of labor, and the fat bodies of mature termite queens exhibit higher ploidy levels than those of other females. The fat bodies of mature termite queens are known to be histologically and cytologically specialized in protein synthesis. However, the relationship between such modifications and polyploidization remains unknown. In this study, we investigated the relationship among cell type, queen maturation, and ploidy levels in the fat body of the termite Reticulitermes speratus. We first confirmed that the termite fat body consists of two types of cells, that is, adipocytes, metabolically active cells, and urocytes, urate-storing cells. Our ploidy analysis using flow cytometry has shown that the fat bodies of actively reproducing queens had more polyploid cells than those of newly emerged and pre-reproductive queens, regardless of the queen phenotype (adult or neotenic type). Using image-based analysis, we found that not urocytes, but adipocytes became polyploid during queen differentiation and subsequent sexual maturation. These results suggest that polyploidization in the termite queen fat body is associated with sexual maturation and is regulated in a cell type-specific manner. Our study findings have provided novel insights into the development of insect fat bodies and provide a basis for future studies to understand the functional importance of polyploidy in the fat bodies of termite queens.

Gene silencing of cathepsins B and L using CTV-based, plant-mediated RNAi interferes with ovarial development in Asian citrus psyllid (ACP), Diaphorina citri

Diaphorina citri Kuwayama (Hemiptera: Liviidae) is a vector of the bacteria Candidatus Liberibacter americanus (CLam) and Candidatus Liberibacter asiaticus (CLas), which are phloem-restricted and associated with the most important and destructive worldwide citrus disease, Huanglongbing (HLB). Currently, no cure for HLB has been described. Therefore, measures have focused on reducing D. citri populations. In these insects, cathepsin B (DCcathB) and L (DCcathL) enzymes play an important role in digestion, and are involved in embryogenesis, immune defense, and ecdysis. In this study, we used a CTV-based vector to deliver dsRNA (CTV-dsRNA) into Citrus macrophylla plants targeting DCcathB and DCcathL genes in D. citri that fed on the phloem of these CTV-RNAi infected plants. Subsequently, we evaluated expression of DCcathB and DCcathL genes as well as the Vitellogenin (Vg) gene by RT-qPCR in D. citri fed on CTV-dsRNA occurring in plant phloem. It was found that a defective phenotype in D. citri females as a result of knockdown of DCcathB and DCcathL genes mediated by CTV dsRNA. These results showed that Psyllids fed on plants treated with the CTV-dsRNA exhibited downregulation of the Vg gene, one of the most important genes associated with embryogenic and female development, which was associated with dsRNA-mediated silencing of the two cathepsin genes. Based on our findings, a CTV-based strategy for delivering RNAi via plants that targets DCcathB and DCcathL genes may represent a suitable avenue for development of dsRNA-based tools to manage D. citri that limits the spread of HLB.

RNAi-mediated CrebA silencing inhibits reproduction and immunity in Locusta migratoria manilensis

Locusta migratoria manilensis is a major agricultural pest that causes severe direct and indirect damage to several crops. Thus, to provide a theoretical foundation for pest control, the role of CrebA in the reproduction and immune regulation of L. migratoria was investigated. CrebA is a bZIP transcription factor that critically regulates intracellular protein secretion. In this study, CrebA was widely expressed in the brain, fat body, integument, midgut, and reproductive tissues of different maturity stages of adult locusts, especially in the female fat body. RNA interfering (RNAi)-mediated silencing of CrebA inhibited locusts ovarian development, and key reproduction gene expressions, Vgs, VgRs, Chico, and JHAMT were downregulated. After the locusts were injected with Micrococcus luteus or Escherichia coli, M. luteus activated lysozyme expression, while the E. coli activated both phenol oxidase cascade and lysozyme expression. Furthermore, both bacteria stimulated the upregulation of the antimicrobial peptide genes DEF3 and DEF4. However, CrebA silencing is fatal to locusts infection with E. coli, with a mortality rate of up to 96.3%, and resulted in a significant decrease in the expression of DEF3 and DEF4 and changes in the activities of phenol oxidase and lysozyme of locusts infected by bacteria. Collectively, CrebA may be involved in diverse biological processes, including reproduction and immunity. CrebA inhibited locusts reproduction by regulating JH signaling pathway and inhibits the expression of immune genes TLR6, IMD, and AMPs. These results demonstrate CrebA seems to play a crucial role in reproduction and innate immunity.

Juvenile hormone receptor Methoprene tolerant: Functions and applications

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

Integrated analysis of miRNA and mRNA expression profiles in response to gut microbiota depletion in the abdomens of female Bactrocera dorsalis

Insect gut microbiota has been reported to participate in regulating host multiple biological processes including metabolism and reproduction. However, the corresponding molecular mechanisms remain largely unknown. Recent studies suggest that microRNAs (miRNAs) are involved in complex interactions between the gut microbiota and the host. Here, we used next-generation sequencing technology to characterize miRNA and mRNA expression profiles and construct the miRNA-gene regulatory network in response to gut microbiota depletion in the abdomens of female Bactrocera dorsalis. A total of 3016 differentially expressed genes (DEGs) and 18 differentially expressed miRNAs (DEMs) were identified. Based on the integrated analysis of miRNA and mRNA sequencing data, 229 negatively correlated miRNA-gene pairs were identified from the miRNA-mRNA network. Gene ontology enrichment analysis indicated that DEMs could target several genes involved in the metabolic process, oxidation-reduction process, oogenesis, and insulin signaling pathway. Finally, real-time quantitative polymerase chain reaction further verified the accuracy of RNA sequencing results. In conclusion, our study provides the profiles of miRNA and mRNA expressions under antibiotics treatment and provides an insight into the roles of miRNAs and their target genes in the interaction between the gut microbiota and its host.

Biological Characteristics and Energy Metabolism of Migrating Insects

Through long-distance migration, insects not only find suitable breeding locations and increase the survival space and opportunities for the population but also facilitate large-scale material, energy, and information flow between regions, which is important in maintaining the stability of agricultural ecosystems and wider natural ecosystems. In this study, we summarize the changes in biological characteristics such as morphology, ovarian development, reproduction, and flight capability during the seasonal migration of the insect. In consideration of global research work, the interaction between flight and reproduction, the influence and regulation of the insulin-like and juvenile hormone on the flight and reproductive activities of migrating insects, and the types of energy substances, metabolic processes, and hormone regulation processes during insect flight are elaborated. This systematic review of the latest advances in the studies on insect migration biology and energy metabolism will help readers to better understand the biological behavior and regulation mechanism of the energy metabolism of insect migration.

Variability of polyteny of giant chromosomes in Drosophila melanogaster salivary glands

Polyteny is an effective mechanism for accelerating growth and enhancing gene expression in eukaryotes. The purpose of investigation was to study the genetic variability of polyteny degree of giant chromosomes in the salivary glands of Drosophila melanogaster Meig. in relation to the differential fitness of different genotypes. 16 strains, lines and hybrids of fruit flies were studied. This study demonstrates the significant influence of hereditary factors on the level of polytenization of giant chromosomes in Drosophila. This is manifested in the differences between strains and lines, the effect of inbreeding, chromosome isogenization, hybridization, adaptively significant selection, sexual differences, and varying degrees of individual variability of a trait in different strains, lines, and hybrids. The genetic component in the variability of the degree of chromosome polyteny in Drosophila salivary glands was 45.3%, the effect of sex was 9.5%. It has been shown that genetic distances during inbreeding, outbreeding or hybridization, which largely determine the selective value of different genotypes, also affect polyteny patterns. Genetic, humoral, and epigenetic aspects of endocycle regulation, which may underlie the variations in the degree of chromosome polyteny, as well as the biological significance of the phenomenon of endopolyploidy, are discussed.

Juvenile hormone acts on male accessory gland function via regulating l-asparaginase expression and triacylglycerol mobilization in Aedes aegypti

Hormones control the reproductive development of Aedes aegypti mosquitoes. The adult male reproductive process and mating behavior require adequate nutrients and energy. Understanding the molecular mechanism linking hormones, energy metabolism, and reproduction in male mosquitoes is important. In this study, we found that the size of the male accessory gland, an essential part of the male reproductive system, gradually increased after eclosion. However, it was significantly reduced in male mosquitoes deficient in methoprene-tolerant (Met), the receptor of juvenile hormone. Likewise, egg hatchability of females that mated with Met-depleted males showed the same downward trend. The mRNA level of the gene encoding accessory gland protein, l-asparaginase (ASNase), was reduced in Met dsRNA-treated males. Electrophoretic mobility shift assay and quantitative reverse transcription-PCR results revealed that Met was capable of binding directly to the promoter of ASNase and activated its transcription. RNA interference of ASNase in males resulted in the reduction of egg hatchability of the females with which they mated. These results showed that Met influenced the fecundity of male mosquitoes by directly upregulating the expression of the ASNase gene. Moreover, the levels of triacylglycerol and the sizes of lipid droplets were decreased by 72-78 h after eclosion in the fat body cells, whereas both of them increased in Met-depleted male mosquitoes, indicating that Met knockdown reduced lipid catabolism. These data demonstrate that Met might influence the egg hatchability of females by regulating lipid metabolism and the development of the male accessory gland in male mosquitoes.

Involvement of Methoprene-tolerant and Krüppel homolog 1 in juvenile hormone-mediated vitellogenesis of female Liposcelis entomophila (End.) (Psocoptera: Liposcelididae)

Methoprene-tolerant (Met) as an intracellular receptor of juvenile hormone (JH) and the Krüppel-homolog 1 (Kr-h1) as a JH-inducible transcription factor had been proved to contribute to insect reproduction. Their functions vary in different insect orders, however, they are not clear in Psocoptera. In this study, LeMet and LeKr-h1 were identified and their roles in vitellogenesis and ovarian development were investigated in Liposcelis entomophila (Enderlein). Treatment with exogenous JH III significantly induced the expression of LeKr-h1, LeVg, and LeVgR. Furthermore, silencing LeMet and LeKr-h1 remarkably reduced the transcription of LeVg and LeVgR, disrupted the production of Vg in fat body and the uptake of Vg by oocytes, and ultimately led to a decline in fecundity. The results indicated that the JH signaling pathway was essential to the reproductive process of this species. Interestingly, knockdown of LeMet or LeKr-h1 also resulted in fluctuations in the expression of FoxO, indicating the complex regulatory interactions between different hormone factors. Besides, knockdown of both LeMet and LeKr-h1 significantly increased L. entomophila mortality. Our study provides initial insight into the roles of JH signaling in the female reproduction of psocids and provided evidence that RNAi-mediated knockdown of Met or Kr-h1 is a potential pest control strategy.

Comparative proteomic analysis reveals insights into the response of Cryptolaemus montrouzieri to bottom-up transfer of cadmium and lead across a multi-trophic food chain

Contamination of agro-ecosystems with heavy metals can affect the development and reproduction of insect natural enemies. This study reports a detailed Tandem Mass Tag based quantitative proteomic analysis of underlying mechanisms responsible for stress response of Cryptolaemus montrouzieri against heavy metals (cadmium (Cd) and lead (Pb)) transported across a multi-trophic food chain. A total of 6639 proteins were detected under Cd as well as Pb stress. In Pb versus the control cluster, 69 proteins (28 up-regulated and 41 down-regulated) were differentially expressed whereas 268 proteins were differentially expressed under Cd versus the control cluster, having 198 proteins up-regulated and 70 down-regulated proteins. The analysis of differentially expressed proteins showed that 27 proteins overlapped in both clusters representing the core proteome to Pb and Cd stress. The bioinformatics analysis demonstrated that these proteins were mapped to 57 and 99 pathways in Pb versus control and Cd versus control clusters, respectively. The functional classification by COG, GO and KEGG databases showed significant changes in protein expression by C. montrouzieri under Pb and Cd stress. The heavy metal stress (Pb and Cd) induced significant changes in expression of proteins like hexokinase (HK), succinyl-CoA, trypsin like proteins, cysteine proteases, cell division cycle proteins, and yellow gene proteins. The results provide detailed information on the protein expression levels of C. montrouzieri and will serve as basic information for future proteomic studies on heavy metal responses of insect predators within a multi-trophic food chain.

Aggregation pheromone 4-vinylanisole promotes the synchrony of sexual maturation in female locusts

Reproductive synchrony generally occurs in various group-living animals. However, the underlying mechanisms remain largely unexplored. The migratory locust, Locusta migratoria, a worldwide agricultural pest species, displays synchronous maturation and oviposition when forms huge swarm. The reproductive synchrony among group members is critical for the maintenance of locust swarms and population density of next generation. Here, we showed that gregarious female locusts displayed more synchronous sexual maturation and oviposition than solitarious females and olfactory deficiency mutants. Only the presence of gregarious male adults can stimulate sexual maturation synchrony of female adults. Of the volatiles emitted abundantly by gregarious male adults, the aggregation pheromone, 4-vinylanisole, was identified to play key role in inducing female sexual maturation synchrony. This maturation-accelerating effect of 4-vinylanisole disappeared in the females of Or35^-/- lines, the mutants of 4-vinylanisole receptor. Interestingly, 4-vinylanisole displayed a time window action by which mainly accelerates oocyte maturation of young females aged at middle developmental stages (3–4 days post adult eclosion). We further revealed that juvenile hormone/vitellogenin pathway mediated female sexual maturation triggered by 4-vinylanisole. Our results highlight a ‘catch-up’ strategy by which gregarious females synchronize their oocyte maturation and oviposition by time-dependent endocrinal response to 4-vinylanisole, and provide insight into reproductive synchrony induced by olfactory signal released by heterosexual conspecifics in a given group.

Since 2019, a plague of flying insects known as migratory locusts has been causing extensive damage to crops in East Africa. Migratory locusts sometimes live a solitary lifestyle but, if environmental conditions allow, they form large groups containing millions of individuals known as swarms that are responsible for causing locust plagues.Locusts are able to maintain such large swarms because they can aggregate and synchronize.

When they live in swarms, individual locusts produce odors that are sensed by other individuals in the group. For example, an aggregation pheromone, called 4-vinylanisole, is known to help keep large groups of locusts together. However, it is less clear how odors synchronize the reproductive cycles of the females in a swarm so that they are ready to mate with males and lay their eggs at the same time.

To address this question, Chen et al. examined when female locusts reached sexual maturity after they were exposed to odors produced by other locusts living alone or in groups. The experiments found that only 4-vinylanisole, which was abundantly released by adult male locusts living in groups, stimulated female locusts to reach sexual maturity at the same time. This odor increased the levels of a hormone known as juvenile hormone in less-developed females to help them reach sexual maturity sooner.

These findings demonstrate that when migratory locusts are living in swarms, male locusts promote the female locusts to reach sexual maturity at the same time by promoting less-developed females to ‘catch up’ with other females in the group. A next step will be to investigate the neural and molecular mechanisms underlying the ‘catch up’ effect induced by 4-vinylanisole.

Sexual dimorphism of diapause regulation in the hemipteran bug Pyrrhocoris apterus

Diapause is one of the major strategies for insects to prepare for and survive harsh seasons. In females, the absence of juvenile hormone (JH) is a hallmark of adult reproductive diapause, a developmental arrest, which is much less characterized in males. Here we show that juvenile hormone III skipped bisepoxide (JHSB₃) titers in hemolymph remarkably differ between reproductive males and females of the linden bug Pyrrhocoris apterus, whereas no JH was detected in diapausing adults of both sexes. Like in females, ectopic application of JH mimic effectively terminated male diapause through the canonical JH receptor components, Methoprene-tolerant and Taiman. In contrast to females, long photoperiod induced reproduction even in males with silenced JH reception or in males with removed corpus allatum (CA), the JH-producing gland. JHSB₃ was detected in the accessory glands (MAG) of reproductive males, unexpectedly, even in males without CA. If there is a source of JHSB₃ outside CA or a long-term storage of JHSB₃ in MAGs remains to be elucidated. These sex-related idiosyncrasies are further manifested in different dynamics of diapause termination in P. apterus by low temperature. We would like to propose that this sexual dimorphism of diapause regulation might be explained by the different reproductive costs for each sex.

Key role of juvenile hormone in controlling reproductive diapause in females of the Asian lady beetle Harmonia axyridis

BACKGROUND

The Asian lady beetle Harmonia axyridis is an important predator of several agricultural pests, including aphids and whiteflies, and thus can contribute to pest management. Commercial viability as a pest control method requires that the beetle can be mass-reared, and that workable conditions for extended shelf-life can be guaranteed. One of the features of Harmonia's life cycle is that it enters diapause in the adult stage when the length of the photophase starts shortening in late summer. Reduction of juvenile hormone (JH) titer has been demonstrated to be the common endocrine mechanism inducing reproductive diapause in insects. However, whether H. axyridis enters diapause dependent on JH shutdown and how the JH level is regulated before diapause remains unknown.

RESULTS

Like in other insects, the absence of JH triggers the induction and maintenance of reproductive diapause in H. axyridis, indicated by JH measurements and the knockdown of an intracellular JH receptor methoprene-tolerant (Met). Methoprene, a JH analog, significantly reversed diapause into reproduction via Met. Combined with RNA-sequencing and RNA interference, we also demonstrated that JH biosynthesis rather than the JH degradation pathway determines the reduction of JH titer in diapausing females.

CONCLUSION

Our results reveal the vital role of JH in regulating reproductive diapause in female H. axyridis. Harmonia axyridis diapause could thus be manipulated by targeting JH production and JH signaling. © 2021 Society of Chemical Industry.

Replication protein A is required for juvenile hormone-dependent vitellogenesis and oocyte maturation in locusts

Aside from inhibiting insect metamorphosis, juvenile hormone (JH) has a well-known role in stimulating various aspects of insect reproduction. Replication protein A (RPA), a heterotrimeric complex comprised of RPA1, RPA2 and RPA3 subunits plays an essential role in DNA replication and DNA repair. Here we report that RPAs are highly expressed in the fat body of adult female locust, Locusta migratoria. While RPA1 is upregulated by the JH receptor Methoprene-tolerant (Met), RPA2 and RPA3 expression appears to be primarily controlled by Forkhead box O transcription factor (FoxO). Knockdown of RPA1, RPA2 or RPA3 results in markedly reducd vitellogenin (Vg) expression in the fat body, accompanied by arrested ovarian growth and inhibited oocyte maturation. In addition, depletion of an RPA subunit leads to increased expression of other RPA subunits as well as a pro-apoptotic gene, Smac that is involved in DNA repair and apoptosis. The data indicate a crucial role of RPAs in JH-dependent vitellogenesis and oocyte maturation.

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.

Juvenile hormone signaling promotes ovulation and maintains egg shape by inducing expression of extracellular matrix genes

It is well documented that the juvenile hormone (JH) can function as a gonadotropic hormone that stimulates vitellogenesis by activating the production and uptake of vitellogenin in insects. Here, we describe a phenotype associated with mutations in the Drosophila JH receptor genes, Met and Gce: the accumulation of mature eggs with reduced egg length in the ovary. JH signaling is mainly activated in ovarian muscle cells and induces laminin gene expression in these cells. Meanwhile, JH signaling induces collagen IV gene expression in the adult fat body, from which collagen IV is secreted and deposited onto the ovarian muscles. Laminin locally and collagen IV remotely contribute to the assembly of ovarian muscle extracellular matrix (ECM); moreover, the ECM components are indispensable for ovarian muscle contraction. Furthermore, ovarian muscle contraction externally generates a mechanical force to promote ovulation and maintain egg shape. This work reveals an important mechanism for JH-regulated insect reproduction.

Involvement of Cis-Acting Elements in Molecular Regulation of JH-Mediated Vitellogenin Gene 2 of Female Periplaneta americana

Vitellogenins (Vgs) are yolk protein precursors that are regulated by juvenile hormone (JH) and/or 20-hydroxyecdysone (20E) in insects. JH acts as the principal gonadotropin that stimulates vitellogenesis in hemimetabolous insects. In this study, we cloned and characterized the Periplaneta americana Vitellogenin 2 (Vg2) promoter. Multiple sites for putative transcription factor binding were predicted for the 1,804 bp Vg2 promoter region, such as the Broad-Complex, ecdysone response element (EcRE), GATA, Hairy, JH response element (JHRE), and Methoprene (Met)-binding motif, among others. Luciferase reporter assay has identified that construct -177 bp is enough to support JH III induction but not 20E suppression. This 38 bp region (from -177 to -139 bp) contains two conserved response element half-sites separated by 2 nucleotides spacer (DR2) and is designated as Vg2RE (^-168GAGTCACGGAGTCGCCGCTG^-149). Mutation assay and luciferase assay data using mutated constructs verified the crucial role of G residues in Vg2RE for binding the isolated fat body nuclear protein. In Sf9 cells, a luciferase reporter placed under the control of a minimal promoter containing Vg2RE was induced by JH III in a dose- and time-dependent manner. Nuclear proteins isolated from previtellogenic female fat body cells bound to Vg2RE, and this binding was outcompeted by a 50-fold excess of cold Drosophila melanogaster DR4 and Galleria mellonella JH binding protein response elements (Chorion factor-I/Ultraspiracle). Affinity pull-down experiment with nuclear extracts of previtellogenic female fat body, using 31-bp probe Vg2RE as bait, yielded a 71 kDa candidate nuclear protein that may mediate the regulatory action of the JH III.

The vitellogenin receptor functionality of the migratory locust depends on its phosphorylation by juvenile hormone

Vitellogenin receptor (VgR) plays a pivotal role in ovarian vitellogenin (Vg) uptake and vertical transmission of pathogenic microbes and Wolbachia symbionts. However, the regulatory mechanisms of VgR action as an endocytic receptor and translocation from oocyte cytoplasm to the membrane remain poorly understood. Here, by using the migratory locust Locusta migratoria as a model system, we report that juvenile hormone (JH) promotes VgR phosphorylation at Ser¹³⁶¹ in the second EGF-precursor homology domain. A signaling cascade including GPCR, PLC, extracellular calcium, and PKC-ι is involved in JH-stimulated VgR phosphorylation. This posttranslational regulation is a prerequisite for VgR binding to Vg on the external surface of the oocyte membrane and subsequent VgR/Vg endocytosis. Acidification, a condition in endosomes, induces VgR dephosphorylation along with the dissociation of Vg from VgR. Phosphorylation modification is also required for VgR recycling from oocyte cytoplasm to the membrane. Additionally, VgR phosphorylation and its requirement for Vg uptake and VgR recycling are evolutionarily conserved in other representative insects including the cockroach Periplaneta americana and the cotton bollworm Helicoverpa armigera This study fills an important knowledge gap of low-density lipoprotein receptors in posttranslational regulation, endocytosis, and intracellular recycling.

Identification of G protein-coupled receptors required for vitellogenesis and egg development in an insect with panoistic ovary

G protein-coupled receptors (GPCRs), a superfamily of integral transmembrane proteins regulate a variety of physiological processes in insects. Juvenile hormone (JH) is known to stimulate Vitellogenin (Vg) synthesis in the fat body, secretion into the hemolymph and uptake by developing oocytes. However, the role of GPCRs in JH-dependent insect vitellogenesis and oocyte maturation remains elusive. In the present study, we performed transcriptomic analysis and RNA interference (RNAi) screening in vitellogenic females of the migratory locust Locusta migratoria. Of 22 GPCRs identified in ovarian transcriptome, LGR4, OR-A1, OR-A2, Mthl1, Mthl5 and Smo were most abundant in the ovary. By comparison, mAChR-C expressed at higher levels in the fat body, whereas Oct/TyrR, OARβ, AdoR and ADGRA3 were at higher expression levels in the brain. Our RNAi screening demonstrated that knockdown of six GPCRs resulted in defective phenotypes of Vg accumulation in developing oocytes, accompanied by blocked ovarian development and impaired oocyte maturation. While LGR4 and Oct/TyrR appeared to control Vg synthesis in the fat body, OR-A1, OR-A2, mAChR-C and CirlL regulated Vg transportation and uptake. The findings provide fundamental evidence for deciphering the regulatory mechanisms of GPCRs in JH-stimulated insect reproduction.

Juvenile hormone regulates the reproductive diapause through Methoprene-tolerant gene in Galeruca daurica

Juvenile hormone (JH) signalling plays an important role in regulation of reproductive diapause in insects. However, its underlying molecular mechanism has been unclear. Methoprene-tolerant (Met), as a universal JH receptor, is involved in JH action. To gain some insight into its function in the reproductive diapause of Galeruca daurica, a serious pest on the Inner Mongolia grasslands undergoing obligatory summer diapause at the adult stage, we cloned the complete open-reading frame (ORF) sequences of Met and other 7 JH signalling-related genes, including JH acid methyltransferase (JHAMT), JH esterase (JHE), JH epoxide hydrolase (JHEH), Krüppel homologue 1 (Kr-h1), vitellogenin (Vg), forkhead box O (FOXO) and fatty acid synthase 2 (FAS2), from this species. GdMet encoded a putative protein, which contained three domains typical of the bHLH-PAS family. Expression patterns of these eight genes were developmentally regulated during adult development. Topical application of JH analogue (JHA) methoprene into the 3-day-old and 5-day-old adults induced the expression of GdMet. Silencing GdMet by RNAi inhibited the expression of JHBP, JHE, Kr-h1 and Vg, whereas promoted the FAS2 expression, which enhanced lipid accumulation and fat body development, and finally induced the adults into diapause ahead. Combining with our previous results, we conclude that JH may regulate reproductive diapause through a conserved Met-dependent pathway in G. daurica.

Krüppel homolog 1 regulates photoperiodic reproductive plasticity in the cabbage beetle Colaphellus bowringi

Many insects exhibit reproductive plasticity where the photoperiod determines whether the insect becomes reproductively active or enters diapause. Adult reproductive diapause is a strategy that allows insects to survive harsh environmental conditions. A deficiency in juvenile hormone (JH) leads to reproductive diapause. However, little is known about the molecular mechanisms by which JH signaling regulates reproductive diapause. In this study, we used the cabbage beetle Colaphellus bowringi, a serious pest, to investigate the role of Krüppel homolog 1 (Kr-h1) in controlling photoperiodic plasticity of female reproduction. We focused on Kr-h1, since it acts as a key mediator of JH signaling. We show here that JH-Methoprene-tolerant signaling upregulated the expression of Kr-h1 in reproductively active C. bowringi females when reared under short day conditions. In the long day-treated diapausing females, Kr-h1 transcripts decreased dramatically. Interfering with Kr-h1 function repressed reproductive development by blocking vitellogenesis and ovarian growth. Further, Kr-h1 depletion induced other diapause-like traits, including elevated lipid accumulation and high expression of diapause-related genes. RNA-Seq showed that Kr-h1 played both activating and repressive roles, depending on whether downstream genes were acting in reproduction- or diapause pathways, respectively. Finally, we identified the DNA replication gene mini-chromosome maintenance 4 and two triacylglycerol lipase genes as critical downstream factors of Kr-h1 that are critical for reproductive plasticity in C. bowringi. These results reveal that Kr-h1 is a key component of the regulatory pathway that coordinates reproduction and diapause in insects in response to photoperiodic input.

The direct regulation of Aalbdsx on AalVgR is indispensable for ovarian development in Aedes albopictus

BACKGROUND

Aedes albopictus is an important vector with an extensive worldwide distribution. Only female mosquitoes play a significant role in the transmission of pathogens. Doublesex (dsx) is a central nexus gene in the insect somatic sex determination hierarchy.

RESULTS

In this study, we characterized the full-length sex-specific splicing forms of the Ae. albopictus dsx (Aalbdsx) gene. Then, we identified 15 direct target genes of DSX in adult females using digital gene expression combined with quantitative real-time polymerase chain reaction (qPCR) by performing a chromatin immunoprecipitation (ChIP) assay with specific DSX antibodies. Knockdown of Aalbdsx suppressed ovarian development and decreased the transcript levels of the Aalbdsx target vitellogenin receptor (VgR) gene, whereas vitellogenin (Vg) expression showed an increase in the fat body. Genes in the major Vg regulatory pathway were also up-regulated.

CONCLUSION

Our results suggest that both Vg and VgR are direct target genes of Aalbdsx and that direct regulation of Aalbdsx on VgR is indispensable for ovarian development in Ae. albopictus, which not only provides a reference for the further elucidation of the evolutionarily conserved role of dsx in Ae. albopictus sexual differentiation but also reveals potential molecular targets for application to the development of sterile male mosquitoes to be released for vector control. © 2020 Society of Chemical Industry.

Regulatory Mechanisms of Vitellogenesis in Insects

Vitellogenesis is pre-requisite to insect egg production and embryonic development after oviposition. During insect vitellogenesis, the yolk protein precursor vitellogenin (Vg) is mainly synthesized in the fat body, transported by the hemolymph through the intercellular spaces (known as patency) in the follicular epithelium to reach the membrane of maturing oocytes, and sequestered into the maturing oocytes via receptor-mediated endocytosis. Insect vitellogenesis is governed by two critical hormones, the sesquiterpenoid juvenile hormone (JH) and the ecdysteriod 20-hydroxyecdysone (20E). JH acts as the principal gonadotropic hormone to stimulate vitellogenesis in basal hemimetabolous and most holometabolous insects. 20E is critical for vitellogenesis in some hymenopterans, lepidopterans and dipterans. Furthermore, microRNA (miRNA) and nutritional (amino acid/Target of Rapamycin and insulin) pathways interplay with JH and 20E signaling cascades to control insect vitellogenesis. Revealing the regulatory mechanisms underlying insect vitellogenesis is critical for understanding insect reproduction and helpful for developing new strategies of insect pest control. Here, we outline the recent research progress in the molecular action of gonadotropic JH and 20E along with the role of miRNA and nutritional sensor in regulating insect vitellogenesis. We highlight the advancements in the regulatory mechanisms of insect vitellogenesis by the coordination of hormone, miRNA and nutritional signaling pathways.

DDX6 Is Essential for Oocyte Development and Maturation in Locusta migratoria

DEAD-box protein 6 (DDX6) is a member of the DDX RNA helicase family that exists in all eukaryotes. It has been extensively studied in yeast and mammals and has been shown to be involved in messenger ribonucleoprotein assembly, mRNA storage, and decay, as well as in miRNA-mediated gene silencing. DDX6 participates in many developmental processes but the biological function of DDX6 in insects has not yet been adequately addressed. Herein, we characterized the LmDDX6 gene that encodes the LmDDX6 protein in Locusta migratoria, a global, destructive pest. LmDDX6 possesses five motifs unique to the DDX6 subfamily. In the phylogenetic tree, LmDDX6 was closely related to its orthologs in Apis dorsata and Zootermopsis nevadensis. RT-qPCR data revealed high expression of LmDDX6 in the ovary, muscle, and fat body, with a declining trend in the ovary after adult ecdysis. LmDDX6 knockdown downregulated the expression levels of the juvenile hormone receptor Met, and genes encoding Met downstream targeted Grp78-1 and Grp78-2, reduced LmVg expression, and impaired ovary development and oocyte maturation. These results demonstrate that LmDDX6 plays an essential role in locust female reproduction and, thus, could be a novel target for locust biological control.

Phase-related differences in egg production of the migratory locust regulated by differential oosorption through microRNA-34 targeting activinβ

Outbreaks of locust plagues result from the long-term accumulation of high-density egg production. The migratory locust, Locusta migratoria, displays dramatic differences in the egg-laid number with dependence on population density, while solitarious locusts lay more eggs compared to gregarious ones. However, the regulatory mechanism for the egg-laid number difference is unclear. Herein, we confirm that oosorption plays a crucial role in the regulation of egg number through the comparison of physiological and molecular biological profiles in gregarious and solitarious locusts. We find that gregarious oocytes display a 15% higher oosorption ratio than solitarious ones. Activinβ (Actβ) is the most highly upregulated gene in the gregarious terminal oocyte (GTO) compared to solitarious terminal oocyte (STO). Meanwhile, Actβ increases sharply from the normal oocyte (N) to resorption body 1 (RB1) stage during oosorption. The knockdown of Actβ significantly reduces the oosorption ratio by 13% in gregarious locusts, resulting in an increase in the egg-laid number. Based on bioinformatic prediction and experimental verification, microRNA-34 with three isoforms can target Actβ. The microRNAs display higher expression levels in STO than those in GTO and contrasting expression patterns of Actβ from the N to RB1 transition. Overexpression of each miR-34 isoform leads to decreased Actβ levels and significantly reduces the oosorption ratio in gregarious locusts. In contrast, inhibition of the miR-34 isoforms results in increased Actβ levels and eventually elevates the oosorption ratio of solitarious locusts. Our study reports an undescribed mechanism of oosorption through miRNA targeting of a TGFβ ligand and provides new insights into the mechanism of density-dependent reproductive adaption in insects.

The continuous accumulation of high-density eggs laid by flying swarms of adults results in huge populations of flightless juveniles, which contributes to the outbreaks of locust plagues. An interesting phenomenon is that locusts have the phenotypic plasticity of reproduction. The gregarious locusts lay fewer big eggs than do solitarious phase locusts. In contrast, the solitarious phase locusts lay more small eggs compared to the gregarious locusts. We find the egg-laid number is not only regulated by the phase status of parents but also controlled by oosorption, a type of oocyte death. Further studies confirmed the phase-related ratio of oocyte death in the mother is regulated by a microRNA, which posttranscriptionally influences the expression level of a TGFβ ligand. This maternal effect on progeny size is especially critical for gregarious locusts to control the population size and maintain population fitness, and for solitarious locusts to enhance chance for gregarization and further enlargement of population size. This is the first study to reveal the molecular mechanism underlying the regulation of a microRNA-gene circuit for locust oocyte death to determine the offspring number. These findings can provide some important clues to develop potential drugs to prevent vast locust reproduction from a plague upsurge.

Aedes aegypti post-emergence transcriptome: Unveiling the molecular basis for the hematophagic and gonotrophic capacitation

The adult females of Aedes aegypti mosquitoes are facultative hematophagous insects but they are unable to feed on blood right after pupae emergence. The maturation process that takes place during the first post-emergence days, hereafter named hematophagic and gonotrophic capacitation, comprises a set of molecular and physiological changes that prepare the females for the first gonotrophic cycle. Notwithstanding, the molecular bases underlying mosquito hematophagic and gonotrophic capacitation remain obscure. Here, we investigated the molecular and biochemical changes in adult Ae. aegypti along the first four days post-emergence, prior to a blood meal. We performed a RNA-Seq analysis of the head and body, comparing male and female gene expression time courses. A total of 811 and 203 genes were differentially expressed, respectively in the body and head, and both body parts showed early, mid, and late female-specific expression profiles. Female-specific up-regulation of genes involved in muscle development and the oxidative phosphorylation pathway were remarkable features observed in the head. Functional assessment of mitochondrial oxygen consumption in heads showed a gradual increase in respiratory capacity and ATP-linked respiration as a consequence of induced mitochondrial biogenesis and content over time. This pattern strongly suggests that boosting oxidative phosphorylation in heads is a required step towards blood sucking habit. Several salivary gland genes, proteases, and genes involved in DNA replication and repair, ribosome biogenesis, and juvenile hormone signaling were up-regulated specifically in the female body, which may reflect the gonotrophic capacitation. This comprehensive description of molecular and biochemical mechanisms of the hematophagic and gonotrophic capacitation in mosquitoes unravels potentially new targets for vector control.

Juvenile hormone upregulates sugarbabe for vitellogenesis and egg development in the migratory locust Locusta migratoria

Sugarbabe is a C₂ H₂ zinc-finger transcription factor that is sensitive to sugar and essential for lipid biosynthesis in larvae of Drosophila melanogaster. However, the role of Sugarbabe in adult insect development remains unexplored. Vitellogenesis is a nutrient-dependent process that is promoted by juvenile hormone (JH) in many insect species. Here, we cloned an ortholog gene of D. melanogaster Sugarbabe (DmSug) in the migratory locust Locusta migratoria. The locust Sugarbabe (LmSug) has five C₂ H₂ zinc-finger motifs similar to DmSug. LmSug was expressed at a low level in adult female locusts raised under poor nutrient conditions. JH treatment increased the expression level of LmSug. Knockdown of the JH receptor gene Met caused a reduction of LmSug expression. Depletion of the LmSug transcript level caused a significant reduction in vitellogenin expression in the fat body, resulting in impaired oocyte development and ovary growth. The results suggest that LmSug is expressed in response to JH, and plays an essential role in female insect reproduction.

Identification of Gonadulin and Insulin-Like Growth Factor From Migratory Locusts and Their Importance in Reproduction in Locusta migratoria

Many insect species have several genes coding for insulin-related peptides (IRPs), but so far only a single IRP gene has been identified in migratory locusts. Here, we report and characterize two other genes coding for peptides that are related to insulin, namely gonadulin and arthropod insulin-like growth factor (aIGF); peptides postulated to be orthologs of Drosophila melanogaster insulin-like peptides 8 and 6 respectively. In Locusta migratoria the aIGF transcript is expressed in multiple tissues as was previously reported for IRP in both L. migratoria and Schistocerca gregaria, but there are significant differences in expression patterns between the two species. The gonadulin transcript, however, seems specific to the ovary, whereas its putative receptor transcript is expressed most abundantly in the ovary, fat body and the central nervous system. Since the central nervous system-fat body-ovary axis is essential for successful reproduction, we studied the influence of gonadulin on vitellogenesis and oocyte growth. A reduction in the gonadulin transcript (via RNA interference) led to a significant reduction in vitellogenin mRNA levels in the fat body and a strong oocyte growth inhibition, thus suggesting an important role for gonadulin in reproduction in this species.

Insulin/IGF signaling and TORC1 promote vitellogenesis via inducing juvenile hormone biosynthesis in the American cockroach

Vitellogenesis, including vitellogenin (Vg) production in the fat body and Vg uptake by maturing oocytes, is of great importance for the successful reproduction of adult females. The endocrinal and nutritional regulation of vitellogenesis differs distinctly in insects. Here, the complex crosstalk between juvenile hormone (JH) and the two nutrient sensors insulin/IGF signaling (IIS) and target of rapamycin complex1 (TORC1), was investigated to elucidate the molecular mechanisms of vitellogenesis regulation in the American cockroach, Periplaneta americana Our data showed that a block of JH biosynthesis or JH action arrested vitellogenesis, in part by inhibiting the expression of doublesex (Dsx), a key transcription factor gene involved in the sex determination cascade. Depletion of IIS or TORC1 blocked both JH biosynthesis and vitellogenesis. Importantly, the JH analog methoprene, but not bovine insulin (to restore IIS) and amino acids (to restore TORC1 activity), restored vitellogenesis in the neck-ligated (IIS-, TORC1- and JH-deficient) and rapamycin-treated (TORC1- and JH-deficient) cockroaches. Combining classic physiology with modern molecular techniques, we have demonstrated that IIS and TORC1 promote vitellogenesis, mainly via inducing JH biosynthesis in the American cockroach.

Juvenile hormone acts through FoxO to promote Cdc2 and Orc5 transcription for polyploidy-dependent vitellogenesis

Vitellogenin (Vg) is a prerequisite for egg production and embryonic development after ovipositioning in oviparous animals. In many insects, juvenile hormone (JH) promotes fat body cell polyploidization for the massive Vg synthesis required for the maturation of multiple oocytes, but the underlying mechanisms remain poorly understood. Using the migratory locust Locusta migratoria as a model system, we report here that JH induces the dephosphorylation of Forkhead box O transcription factor (FoxO) through a signaling cascade including leucine carboxyl methyltransferase 1 (LCMT1) and protein phosphatase 2A (PP2A). JH promotes PP2A activity via LCMT1-mediated methylation, consequently triggering FoxO dephosphorylation. Dephosphorylated FoxO binds to the upstream region of two endocycle-related genes, cell-division-cycle 2 (Cdc2) and origin-recognition-complex subunit 5 (Orc5), and activates their transcription. Depletion of FoxO, Cdc2 or Orc5 results in blocked polyploidization of fat body cells, accompanied by markedly reduced Vg expression, impaired oocyte maturation and arrested ovarian development. The results suggest that JH acts via LCMT1-PP2A-FoxO to regulate Cdc2 and Orc5 expression, and to enhance ploidy of fat body cells in preparation for the large-scale Vg synthesis required for synchronous maturation of multiple eggs.

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.

A novel transcriptional cascade is involved in Fzr-mediated endoreplication

Endoreplication, known as endocycle, is a variant of the cell cycle that differs from mitosis and occurs in specific tissues of different organisms. Endoreplicating cells generally undergo multiple rounds of genome replication without chromosome segregation. Previous studies demonstrated that Drosophila fizzy-related protein (Fzr) and its mammalian homolog Cdh1 function as key regulators of endoreplication entrance by activating the anaphase-promoting complex/cyclosome to initiate the ubiquitination and subsequent degradation of cell cycle factors such as Cyclin B (CycB). However, the molecular mechanism underlying Fzr-mediated endoreplication is not completely understood. In this study, we demonstrated that the transcription factor Myc acts downstream of Fzr during endoreplication in Drosophila salivary gland. Mechanistically, Fzr interacts with chromatin-associated histone H2B to enhance H2B ubiquitination in the Myc promoter and promotes Myc transcription. In addition to negatively regulating CycB transcription, the Fzr-ubiquitinated H2B (H2Bub)-Myc signaling cascade also positively regulates the transcription of the MCM6 gene that is involved in DNA replication by directly binding to specific motifs within their promoters. We further found that the Fzr-H2Bub-Myc signaling cascade regulating endoreplication progression is conserved between insects and mammalian cells. Altogether, our work uncovers a novel transcriptional cascade that is involved in Fzr-mediated endoreplication.

Juvenile hormone regulates the shift from migrants to residents in adult oriental armyworm, Mythimna separata

In migratory insects, increasing evidence has demonstrated juvenile hormone (JH) is involved in regulating adult reproduction and flight. Our previous study demonstrated that the switch from migrants to residents in Mythimna separata could be induced by adverse environmental conditions during a sensitive period in adulthood (the first day post-emergence), but the role of JH in this switch is not clear. Here, we found a significantly different pattern of JH titers between migrants and residents, with migrants showing a slower release of JH during adulthood than residents. Application of JH analogue (JHA) in the 1-day-old adults, significantly accelerated adult reproduction and suppressed flight capacity. The pre-oviposition period and period of first oviposition of migrants treated with JHA were significantly shorter, while the total lifetime fecundity and mating percentage increased. The flight capacity and dorso-longitudinal muscle size of the migrants were decreased significantly when treated with JHA. The effect of JHA on reproduction and flight capacity indicate that JH titers during the sensitive period (first day post-emergence) regulates the shift from migrants to residents in M. separata.

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.

Post-transcriptional regulation of insect metamorphosis and oogenesis

Metamorphic transformation from larvae to adults along with the high fecundity is key to insect success. Insect metamorphosis and reproduction are governed by two critical endocrines, juvenile hormone (JH), and 20-hydroxyecdysone (20E). Recent studies have established a crucial role of microRNA (miRNA) in insect metamorphosis and oogenesis. While miRNAs target genes involved in JH and 20E-signaling pathways, these two hormones reciprocally regulate miRNA expression, forming regulatory loops of miRNA with JH and 20E-signaling cascades. Insect metamorphosis and oogenesis rely on the coordination of hormones, cognate genes, and miRNAs for precise regulation. In addition, the alternative splicing of genes in JH and 20E-signaling pathways has distinct functions in insect metamorphosis and oogenesis. We, therefore, focus in this review on recent advances in post-transcriptional regulation, with the emphasis on the regulatory role of miRNA and alternative splicing, in insect metamorphosis and oogenesis. We will highlight important new findings of miRNA interactions with hormonal signaling and alternative splicing of JH receptor heterodimer gene Taiman.

Juvenile hormone suppresses aggregation behavior through influencing antennal gene expression in locusts

Animals often exhibit dramatically behavioral plasticity depending on their internal physiological state, yet little is known about the underlying molecular mechanisms. The migratory locust, Locusta migratoria, provides an excellent model for addressing these questions because of their famous phase polyphenism involving remarkably behavioral plasticity between gregarious and solitarious phases. Here, we report that a major insect hormone, juvenile hormone, is involved in the regulation of this behavioral plasticity related to phase change by influencing the expression levels of olfactory-related genes in the migratory locust. We found that the treatment of juvenile hormone analog, methoprene, can significantly shift the olfactory responses of gregarious nymphs from attraction to repulsion to the volatiles released by gregarious nymphs. In contrast, the repulsion behavior of solitarious nymphs significantly decreased when they were treated with precocene or injected with double-stranded RNA of JHAMT, a juvenile hormone acid O-methyltransferase. Further, JH receptor Met or JH-response gene Kr-h1 knockdown phenocopied the JH-deprivation effects on olfactory behavior. RNA-seq analysis identified 122 differentially expressed genes in antennae after methoprene application on gregarious nymphs. Interestingly, several olfactory-related genes were especially enriched, including takeout (TO) and chemosensory protein (CSP) which have key roles in behavioral phase change of locusts. Furthermore, methoprene application and Met or Kr-h1 knockdown resulted in simultaneous changes of both TO1 and CSP3 expression to reverse pattern, which mediated the transition between repulsion and attraction responses to gregarious volatiles. Our results suggest the regulatory roles of a pleiotropic hormone in locust behavioral plasticity through modulating gene expression in the peripheral olfactory system.

A behavioral change from shy solitarious individuals to highly social gregarious individuals is critical to the formation of disastrous swarms of locusts. However, the underlying molecular mechanism of behavioral plasticity regulated by hormones is still largely unknown. Here, we investigated the effect of juvenile hormone (JH) on the behavioral transition in fourth-instar gregarious and solitarious locusts. We found that JH induced the behavioral shift of the gregarious locust from attraction to repulsion to the volatiles of gregarious locusts. The solitarious locust significantly decreased repulsion behavior after deprivation of JH by precocene or knockdown of JHAMT, a key enzyme to synthesize JH. JH application on gregarious locusts caused significant expression alteration of genes, especially the olfactory genes TO and CSP in the antennae. We further demonstrated that the JH signaling pathway suppressed aggregation behavior in gregarious locusts by increasing TO1 expression and decreasing CSP3 expression at the same time. Our results suggested that internal physiological factors can directly modulate periphery olfactory system to produce behavioral plasticity.

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.

Impact of insect growth regulators on biology and behavior of Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae)

This study investigated the effects of four insect growth regulators (IGRs) on biology and behavior of Chrysoperla carnea. IGRs were sprayed on eggs, larvae (~24-h old), and pupae at recommended doses along with their relatively low and high dose. Eggs, larval, and pupal survival were assessed along with effects on fecundity and fertility of C. carnea adults emerged when pupae were treated. IOBC (International Organization for Biological and Integrated Control) proposed toxicity scale was used to categorize the IGRs. Concerning the eggs lufenuron, pyriproxyfen, and diflubenzuron were categorized as slightly harmful (class 2), whereas buprofezin was categorized as moderately harmful (class 3). Lufenuron and diflubenzuron were classified as slightly harmful (class 2) to C. carnea larvae, while pyriproxyfen and buprofezin were categorized as harmless (class 1). Buprofezin did not affect the locomotion behavior of the larvae as time proportion spent in the treated and untreated zone was equal, while all others were affected significantly. Regarding the pupae, pyriproxyfen and buprofezin were found slightly harmful (class 2) and moderately harmful (class 3), respectively, and considerably reduced fecundity and fertility of the C. carnea adults. Lufenuron and diflubenzuron did not affect significantly when pupae were treated. Based on combined effect, the IGRs lufenuron and diflubenzuron did nott influence the significantly on population parameters comparatively. This could be helpful for the use of IGRs in integration with C. carnea towards their conservation in agroecosystem.

Structural and functional differentiation of a fat body-like tissue adhering to testis follicles facilitates spermatogenesis in locusts

The fat body is distributed throughout the body of insects, playing the essential role in intermediary metabolism and nutrient storage. However, the function of differentiation of fat bodies adhering to different tissues remains largely unknown. Here, we identified a fat body-like tissue (FLT) surrounding testis follicles and described its features at morphological, cellular and molecular levels. The FLT is morphologically distinguished with the abdominal fat body (FB) and dominated by diploid cells instead of polyploid cells. The transcriptomic analysis demonstrated that the FLT and FB have dramatically different gene expression profiles. Moreover, genes in the cell cycle pathway, which include both DNA replication- and cell division-related genes, were successively active during development of the FLT, suggesting that FLT cells possibly undergo a mitotic cycle rather than an endocycle. Deprivation of the FLT resulted in distortion of the testis follicles, disappearance of sperm bundles, reduction of total sperm number and increase of dead sperm, indicating a critical role of the FLT in the spermatogenesis in testis follicles. The special functional differentiation of the two similar tissues suggested that FLT-FB cells are able to establish a promising system to study mitotic-to-endocycle transition.

Transcriptome analysis of the molecular mechanism underlying immunity- and reproduction trade-off in Locusta migratoria infected by Micrococcus luteus

Immune response and reproductive success are two vital energy-consuming processes in living organisms. However, it is still unclear which process is prioritized when both are required. Therefore, the present study was designed to examine this question arising for one of the world’s most destructive agricultural pests, the migratory locust, Locusta migratoria. Transcripts from the ovaries and fat bodies of newly emerged locusts were analyzed, using RNA-seq based transcriptome and qualitative real-time PCR, at 4 h and 6 d after being infected with the gram-positive bacteria Micrococcus luteus. Changes in the main biological pathways involved in reproduction and immunization were analyzed using bioinformatics. After 4 h of infection, 348 and 133 transcripts were up- and down-regulated, respectively, whereas 5699 and 44 transcripts were up- and down-regulated, respectively, at 6 d after infection. Moreover, KEGG analysis indicated that vital pathways related with immunity and reproduction, such as Insulin resistance, FoxO signaling, Lysosome, mTOR signaling, and Toll-like receptor signaling pathways were up-regulated. Among the differentially expressed genes, 22 and 17 were related to immunity and reproduction, respectively. The expression levels of PPO1 and antimicrobial peptide defensin 3 were increased (log₂FC = 5.93 and 6.75, respectively), whereas those of VgA and VgB were reduced (log₂FC = -17.82 and -18.13, respectively). These results indicated that locust allocate energy and resources to maintain their own survival by increasing immune response when dealing with both immune and reproductive processes. The present study provides the first report of expression levels for genes related with reproduction and immunity in locusts, thereby providing a reference for future studies, as well as theoretical guidance for investigations of locust control.

Juvenile hormone signaling - a mini review

Since it was first postulated by Wigglesworth in 1934, juvenile hormone (JH) is considered a status quo hormone in insects because it prevents metamorphosis that is initiated by the molting hormone 20-hydroxyecdysone (20E). During the last decade, significant advances have been made regarding JH signaling. First, the bHLH-PAS transcription factor Met/Gce was identified as the JH intracellular receptor. In the presence of JH, with the assistance of Hsp83, and through physical association with a bHLH-PAS transcriptional co-activator, Met/Gce enters the nucleus and binds to E-box-like motifs in promoter regions of JH primary-response genes for inducing gene expression. Second, the zinc finger transcription factor Kr-h1 was identified as the anti-metamorphic factor which transduces JH signaling. Via Kr-h1 binding sites, Kr-h1 represses expression of 20E primary-response genes (i.e. Br, E93 and E75) to prevent 20E-induced metamorphosis. Third, through the intracellular signaling, JH promotes different aspects of female reproduction. Nevertheless, this action varies greatly from species to species. Last, a hypothetical JH membrane receptor has been predicted to be either a GPCR or a tyrosine kinase receptor. In future, it will be a great challenge to understand how the JH intracellular receptor Met/Gce and the yet unidentified JH membrane receptor coordinate to regulate metamorphosis and reproduction in insects.

Juvenile Hormone receptor Met is essential for ovarian maturation in the Desert Locust, Schistocerca gregaria

Juvenile hormones (JH) are key endocrine regulators produced by the corpora allata (CA) of insects. Together with ecdysteroids, as well as nutritional cues, JH coordinates different aspects of insect postembryonic development and reproduction. The function of the recently characterized JH receptor, Methoprene-tolerant (Met), appears to be conserved in different processes regulated by JH. However, its functional interactions with other hormonal signalling pathways seem highly dependent on the feeding habits and on the developmental and reproductive strategies employed by the insect species investigated. Here we report on the effects of RNA interference (RNAi) mediated SgMet knockdown during the first gonadotrophic cycle in female desert locusts (Schistocerca gregaria). This voracious, phytophagous pest species can form migrating swarms that devastate field crops and harvests in several of the world’s poorest countries. A better knowledge of the JH signalling pathway may contribute to the development of novel, more target-specific insecticides to combat this very harmful swarming pest. Using RNAi, we show that the JH receptor Met is essential for ovarian maturation, vitellogenesis and associated ecdysteroid biosynthesis in adult female S. gregaria. Interestingly, knockdown of SgMet also resulted in a significant decrease of insulin-related peptide (SgIRP) and increase of neuroparsin (SgNP) 3 and 4 transcript levels in the fat body, illustrating the existence of an intricate regulatory interplay between different hormonal factors. In addition, SgMet knockdown in females resulted in delayed display of copulation behaviour with virgin males, when compared with dsGFP injected control animals. Moreover, we observed an incapacity of adult dsSgMet injected female locusts to oviposit during the time of the experimental setup. As such, SgMet is an essential gene playing crucial roles in the endocrine communication necessary for successful reproduction of the desert locust.

Transcriptomic changes during caste development through social interactions in the termite Zootermopsis nevadensis

One of the most striking examples of phenotypic plasticity is the different phenotypes (i.e., castes) within a same nest of social insects. Castes usually derive from a single genotype initially by receiving social cues among individuals during development. Specific gene expression changes may be involved in caste differentiation, and thus, the regulatory mechanism of these changes should be clarified in order to understand social maintenance and evolution. The damp-wood termite Zootermopsis nevadensis is one of the most important model termite species, due to not only the availability of genomic, transcriptomic, and epigenomic information but also evidence that soldier- and worker-destined individuals can be identified in natural conditions. Given that the nutritional intakes via social interactions are crucial for caste differentiation in this species, there is a possibility that transcriptomic changes are influenced by the nutritional difference among these individuals. Here, whole body RNA-seq analysis of 3rd-instar larvae with biological replications and Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses were conducted. We found the drastic expression differences during caste developments between soldier- and worker-destined individuals. The results indicated that there are several key signaling pathways responsible for caste formations, which are involved in developments and social interactions. Particularly, the nutritional sensitive signaling was upregulated in soldier-destined individuals, while some metabolic pathways were identified in worker-destined individuals. These bioinformatic data obtained should be utilized to examine the molecular mechanisms of caste determination in social insects.

Clustered miR-2, miR-13a, miR-13b and miR-71 coordinately target Notch gene to regulate oogenesis of the migratory locust Locusta migratoria

MicroRNAs (miRNAs), ∼22-nt small noncoding RNAs with a crucial role in various biological processes of organisms, are usually clustered in the genome. However, little is known about the miRNA clusters involved in insect reproduction. By small RNA sequencing and quantification followed by qRT-PCR, we found that the expression of invertebrate-specific miR-2/13/71 cluster including miR-2, miR-13a, miR-13b and miR-71 significantly decreased after adult ecdysis of the migratory locust, Locusta migratoria. Luciferase reporter assay and RNA immunoprecipitation demonstrated that miR-2/13/71 bound to the protein coding sequence of Notch and downregulated its expression. Injection of miR-2/13/71 agomiRs led to significant decrease of Notch expression as well as markedly reduced levels of Vitellogenin mRNA, suppressed oocyte maturation and impaired ovarian growth. Moreover, the expression of miR-2/13/71 was repressed by juvenile hormone (JH). Our results thus point to a previously unidentified mechanism by which JH-repressed miR-2/13/71 coordinately downregulates Notch to modulate insect reproduction. The increase of JH and decrease of miR-2/13/71 expression in both previtellogenic and vitellogenic stages of adult females ensure a high level of Notch expression, critically contributing to JH-dependent vitellogenesis and oogenesis.

Fat Body Biology in the Last Decade

The insect fat body is analogous to vertebrate adipose tissue and liver. In this review, the new and exciting advancements made in fat body biology in the last decade are summarized. Controlled by hormonal and nutritional signals, insect fat body cells undergo mitosis during embryogenesis, endoreplication during the larval stages, and remodeling during metamorphosis and regulate reproduction in adults. Fat body tissues are major sites for nutrient storage, energy metabolism, innate immunity, and detoxification. Recent studies have revealed that the fat body plays a central role in the integration of hormonal and nutritional signals to regulate larval growth, body size, circadian clock, pupal diapause, longevity, feeding behavior, and courtship behavior, partially by releasing fat body signals to remotely control the brain. In addition, the fat body has emerged as a fascinating model for studying metabolic disorders and immune diseases. Potential future directions for fat body biology are also proposed herein.