Juvenile hormone differentially regulates two Grp78 genes encoding protein chaperones required for insect fat body cell homeostasis and vitellogenesis

Differential regulation of reproduction and molting by juvenile hormone in aphids

Insects rely on juvenile hormones to regulate various physiological processes, including reproduction and molting; currently eight forms of this hormone are known. In most insects, only JH Ⅲ is synthesized. Meanwhile, aphids produce JH Ⅲ and JH Ⅲ skipped bisepoxide (JHSB3). However, it remains unclear whether these compounds play distinct roles in functional regulation. In this study, we demonstrated that the tested concentrations of JH Ⅲ effectively increased the number of aphid offspring, whereas high concentrations of JHSB3 affected the molting process. Drip experiments showed that 10 mg/L JH Ⅲ increased the number of offspring from 39.38 ± 8.03 to 56.50 ± 13.17, whereas 10 mg/L JHSB3 resulted in a 60.00 %± 5.77 % failure rate in molting before adulthood. Transcriptomic analysis also revealed that in the JH Ⅲ treatment group, 9 genes and 7 pathways associated with reproduction were expressed, but not genes or pathways associated with molting. In addition, 16 genes and 9 pathways associated with molting as well as 5 genes and 4 pathways associated with reproduction were identified in the JHSB3 treatment group. JH Ⅲ promotes reproduction in aphids by enhancing Vg expression, whereas JHSB3 affects molting by inhibiting the synthesis of molting hormone-related enzymes. The results indicate that JH Ⅲ and JHSB3 exhibit diverse functions in Aphis craccivora. The findings have significant implications for further studies on the physiological functions of different JHs.

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.

Identification and Functions of JHE 6 Specifically Expressed in Bombyx mori Silk Gland

Juvenile hormone esterase (JHE) is the specific enzyme that degrades juvenile hormone (JH) and regulates the JH titer in insects. JH also regulates the development of the silk gland and the synthesis and secretion of silk proteins in Bombyx mori. Here, we identified nine possible JHE family members, Bmjhe1-9. Notably, Bmjhe6 is specifically expressed in the silk gland. Using semi-quantitative, quantitative real-time RT-PCR and Western blot, it was confirmed that Bmjhe6 was specifically expressed in the middle silk gland (MSG) with high levels in the anterior region of the MSG (A-MSG). The immunofluorescence localization analysis revealed that Bmjhe6 is produced within cells, secreted into the gland lumen, and co-transported with silk proteins into the anterior silk gland (ASG). In vitro hormone induction experiments demonstrated that Bmjhe6 responds to a JH analog, increasing its expression after 12-24 h, whereas 20-hydroxyecdysone inhibited it. In addition, Bmjhe6 knockdown using dsBmjhe6 injections accelerated larval development, resulting in increased larval body and silk gland weight. This induced disordered sericin genes (Ser2, Ser3) expression, and key genes in the JH synthesis pathway (BmKr-h1 and BmMet1) were significantly upregulated along with the transcription factors (SGF-1 and Sage). These results indicate that Bmjhe6 plays an important role in silk gland growth and silk protein synthesis by modulating JH signal.

The chaperone BiP promotes dengue virus replication and mosquito vitellogenesis in Aedes aegypti

Binding immunoglobulin protein (BiP, also known as GRP78), a chaperone and master regulator of the unfolded protein response (UPR) pathway, plays an essential role in several flavivirus infections, but its functional role in regulating dengue virus replication in the mosquito remains largely unknown. We here demonstrated the interaction between a dengue virus serotype 2 (DENV2) and BiP in Aedes aegypti and report the discovery of a novel functional role of BiP in mosquito vitellogenesis. Silencing Ae. aegypti BiP (AaBiP) expression resulted in the significant inhibition of DENV2 viral genome replication, viral protein production, and infectious viral particle biogenesis. Co-immunoprecipitation assays showed that the DENV2 non-structural protein 1 (NS1) interacts with the AaBiP protein, and silencing AaBiP expression led to enhanced DENV2 NS1 aggregation, indicating that AaBiP plays a role in viral protein stability. A kinetic study focusing on pulse treatment of MG132, a proteasome inhibitor, in AaBiP-silenced mosquitoes showed that DENV2 NS1 was drastically elevated, which further suggests that AaBiP-mediated viral protein degradation is mediated by proteasomal machinery. Silencing of AaBiP also resulted in a reduction in mosquito fertility and fecundity. Depletion of AaBiP inhibited mosquito vitellogenesis due to the reduction of vitellogenin mRNA and elevated aggregation of vitellogenin protein post blood meal, further suppressing ovary development and fecundity. Overall, our results suggest that AaBiP is a dual-function protein with roles in both the regulation of dengue virus replication and mosquito reproduction. Our findings will be useful in the establishment of more efficient strategies for vector-borne disease control.

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.

Functional Analysis of a Multiple-Domain CTL15 in the Innate Immunity, Eclosion, and Reproduction of Tribolium castaneum

C-type lectin X (CTL-X) plays critical roles in immune defense, cell adhesion, and developmental regulation. Here, a transmembrane CTL-X of Tribolium castaneum, TcCTL15, with multiple domains was characterized. It was highly expressed in the early and late pupae and early adults and was distributed in all examined tissues. In addition, its expression levels were significantly induced after being challenged with pathogen-associated molecular patterns (PAMPs) and bacteria. In vitro, the recombinant TcCTL15 could recognize bacteria through binding PAMPs and exhibit agglutinating activity against a narrow range of bacteria in the presence of Ca²⁺. RNAi-mediated TcCTL15-knockdown-larvae infected with Escherichia coli and Staphylococcus aureus showed less survival, had activated immune signaling pathways, and induced the expression of antimicrobial peptide genes. Moreover, silencing TcCTL15 caused eclosion defects by impairing ecdysone and crustacean cardioactive peptide receptors (CCAPRs). Suppression of TcCTL15 in female adults led to defects in ovary development and fecundity, accompanied by concomitant reductions in the mRNA levels of vitellogenin (TcVg) and farnesol dehydrogenase (TcFDH). These findings imply that TcCTL15 has extensive functions in developmental regulation and antibacterial immunity. Uncovering the function of TcCTL15 will enrich the understanding of CTL-X in invertebrates. Its multiple biological functions endow the potential to be an attractive target for pest control.

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.

A chromosome-level genome assembly provides insights into the environmental adaptability and outbreaks of Chlorops oryzae

Chlorops oryzae is a pest of rice that has caused severe damage to crops in major rice-growing areas in recent years. We generated a 447.60 Mb high-quality chromosome-level genome with contig and scaffold N50 values of 1.17 Mb and 117.57 Mb, respectively. Hi-C analysis anchored 93.22% scaffolds to 4 chromosomes. The relatively high expression level of Heat Shock Proteins (HSPs) and antioxidant genes in response to thermal stress suggests these genes may play a role in the environmental adaptability of C. oryzae. The identification of multiple pathways that regulate reproductive development (juvenile hormone, 20-hydroxyecdsone, and insulin signaling pathways) provides evidence that these pathways also play an important role in vitellogenesis and thus insect population maintenance. These findings identify possible reasons for the increased frequency of outbreaks of C. oryzae in recent years. Our chromosome-level genome assembly may provide a basis for further genetic studies of C. oryzae, and promote the development of novel, sustainable strategies to control this pest.

A chromosome-level genome assembly for the rice pest, Chlorops oryzae, pinpoints molecular pathways that might contribute toward increased outbreaks for this important crop pest.

Juvenile hormone promotes paracellular transport of yolk proteins via remodeling zonula adherens at tricellular junctions in the follicular epithelium

Juvenile hormone (JH) acts as a gonadotrophic hormone stimulating insect vitellogenesis and oogenesis. Paracellular transport of yolk proteins through intercellular channels (patency) in the follicular epithelium is a developmentally regulated and evolutionarily conserved process during vitellogenesis. However, the mechanisms underlying patency opening are poorly understood. Using the migratory locust Locusta migratoria as a model system, we report here that JH-regulated remodeling of zonula adherens (ZA), the belt-like adherens junction maintaining physical linking between follicle cells controlled the opening of patency. JH triggered phosphorylation of Partitioning defective protein 3 (Par3) via a signaling cascade including G protein-coupled receptor (GPCR), small GTPase Cell division cycle 42 (Cdc42) and atypical Protein kinase C (aPKC). Par3 phosphorylation resulted in its disassociation from β-Catenin, the cytoplasmic partner of ZA core component E-Cadherin. Release of Par3 from the β-Catenin/E-Cadherin complex caused ZA disassembly at tricellular contacts, consequently leading to patency enlargement. This study provides new insight into how JH stimulates insect vitellogenesis and egg production via inducing the opening of paracellular route for vitellogenin transport crossing the follicular epithelium barrier.

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.

TDRD5 Is Required for Spermatogenesis and Oogenesis in Locusta migratoria

Tudor family proteins exist in all eukaryotic organisms and play a role in many cellular processes by recognizing and binding to proteins with methylated arginine or lysine residues. TDRD5, a member of Tudor domain-containing proteins (TDRDs), has been implicated in the P-element-induced wimpy testis-interacting RNA (piRNA) pathway and germ cell development in some model species, but little is known about its function in other species. Therefore, we identified and characterized LmTDRD5, the TDRD5 ortholog in Locusta migratoria, a hemimetabolous pest. The LmTdrd5 gene has 19 exons that encode a protein possessing a single copy of the Tudor domain and three LOTUS domains at its N-terminus. qRT-PCR analysis revealed a high LmTdrd5 expression level in genital glands. Using RNA interference, LmTdrd5 knockdown in males led to a lag in meiosis phase transition, decreased spermatid elongation and sperm production, and downregulated the expression of the two germ cell-specific transcription factors, LmCREM and LmACT, as well as the sperm tail marker gene LmQrich2.LmTdrd5 knockdown in females reduced the expression levels of vitellogenin (Vg) and Vg receptor (VgR) and impaired ovarian development and oocyte maturation, thus decreasing the hatchability rate. These results demonstrate that LmTdrd5 is essential for germ cell development and fertility in locusts, indicating a conserved function for TDRD5.

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.

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.

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.

Transcriptome analysis reveals TOR signalling-mediated plant flush shoots governing Diaphorina citri Kuwayama oviposition

Asian Citrus Psyllid (ACP), Diaphorina citri, is a key vector transmitting the causative agent of Huanglongbing (HLB) disease. Population growth of ACP is evident after feeding on plant flush shoots, as they only oviposit here. However, the underlying mechanism as to why flush shoots govern oviposition is unclear. This study compares the fecundity and ovarian morphology of ACP between young flush and mature leaves. Furthermore, the transcriptome of mated females infesting Murraya paniculata was analysed. Finally, the gene of the key Target of Rapamycin (TOR) signalling pathway was silenced by RNAi. Results indicated that flush shoot feeding activated the development of the psyllids ovary and therefore induced oviposition. A total of 126 and 2794 differentially expressed genes were detected at 1 and 5 days, respectively, after pest infestation of flush shoots compared to mature leaves. Many genes are involved in protein metabolism, Mitogen-Activated Protein Kinase (MAPK) signalling pathway, hormone synthesis, and TOR signalling pathway: all thought to activate reproduction. Silencing of the positive regulator gene DcRheb in the TOR pathway resulted in lower levels of ecdysone and juvenile hormone and decreased vitellogenin synthesis, further disrupting reproductive ability. This study enhances understanding of the molecular mechanism underlying ACP's reproductive strategy.

The RNA helicase DDX3 is required for ovarian development and oocyte maturation in Locusta migratoria

DDX3 represents a well-defined subfamily of DEAD-box RNA helicase and exerts multiple functions in RNA metabolism, cell cycle, tumorigenesis, signal pathway, and fertility. Our previous study has shown that LmDDX3, the ortholog of DDX3 in Locusta migratoria, is ubiquitously expressed, and with a high abundance in testis and ovary. Knockdown of LmDDX3 results in a lethal phenotype in nymph, but it still remains unclear for its role in reproductive process. In this study, we therefore characterized LmDDX3 expression in female adult locust and analyzed its function in oocyte development. LmDDX3 was expressed in all tissues examined with significant more transcripts in ovary and hindgut. In ovary, a strong expression level was detected at the day just after adult eclosion, and a dramatic reduction then occurred during the oocyte development. LmDDX3 RNAi led to a reduced vitellogenin (Vg) expression in fat body via partially at least, the JH signaling pathway, and caused an upregulation of vitellogenin receptor (VgR) in ovary, and thus blocked the ovarian development and oocyte maturation. Sequence and phylogenetic analysis indicated that LmDDX3 was closely related to termite DDX3. Taken together, these data reveal a critical role for LmDDX3 in regulating the transcription of Vg and VgR, two major factors in vitellogenesis that is a key process required for ovary development and oocyte maturation in locust, and contribute thereof a new putative target for locust biological control.

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.

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.

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.

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.

What happens after a blood meal? A transcriptome analysis of the main tissues involved in egg production in Rhodnius prolixus, an insect vector of Chagas disease

The blood-sucking hemipteran Rhodnius prolixus is a vector of Chagas disease, one of the most neglected tropical diseases affecting several million people, mostly in Latin America. The blood meal is an event with a high epidemiological impact since adult mated females feed several times, with each meal resulting in a bout of egg laying, and thereby the production of hundreds of offspring. By means of RNA-Sequencing (RNA-Seq) we have examined how a blood meal influences mRNA expression in the central nervous system (CNS), fat body and ovaries in order to promote egg production, focusing on tissue-specific responses under controlled nutritional conditions. We illustrate the cross talk between reproduction and a) lipids, proteins and trehalose metabolism, b) neuropeptide and neurohormonal signaling, and c) the immune system. Overall, our molecular evaluation confirms and supports previous studies and provides an invaluable molecular resource for future investigations on different tissues involved in successful reproductive events. These analyses serve as a starting point for new investigations, increasing the chances of developing novel strategies for vector population control by translational research, with less impact on the environment and more specificity for a particular organism.

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.

Identification and Characterization of Three Heat Shock Protein 90 (Hsp90) Homologs in the Brown Planthopper

Hsp90 (heat shock protein 90) chaperone machinery is considered to be a key regulator of proteostasis under both physiological and stress growth conditions in eukaryotic cells. The high conservation of both the sequence and function of Hsp90 allows for the utilization of various species to explore new phenotypes and mechanisms. In this study, three Hsp90 homologs were identified in the brown planthopper (BPH), Nilaparvata lugens: cytosolic NlHsp90, endoplasmic reticulum (ER) NlGRP94 and mitochondrial NlTRAP1. Sequence analysis and phylogenetic construction showed that these proteins belonged to distinct classes consistent with the predicted localization and suggested an evolutionary relationship between NlTRAP1 and bacterial HtpG (high-temperature protein G). Temporospatial expression analyses showed that NlHsp90 was inducible under heat stress throughout the developmental stage, while NlGRP94 was only induced at the egg stage. All three genes had a significantly high transcript level in the ovary. The RNA interference-mediated knockdown of NlHsp90 its essential role in nymph development and oogenesis under physiological conditions. NlGRP94 was also required during the early developmental stage and played a crucial role in oogenesis, fecundity and late embryogenesis. Notably, we first found that NlHsp90 and NlGRP94 were likely involved in the cuticle structure of female BPH. Together, our research revealed multifunctional roles of Hsp90s in the BPH.

The effect of E93 knockdown on female reproduction in the red flour beetle, Tribolium castaneum

The E93 transcription factor is a member of helix-turn-helix transcription factor family containing a Pip-squeak motif. This ecdysone primary response gene was identified as a regulator of cell death in Drosophila melanogaster where it is involved in ecdysone-induced autophagy and caspase activity that mediate degeneration of larval tissues during metamorphosis from larva to pupa. However, its function in adult insects is not well studied. To study E93 function in the red flour beetle, Tribolium castaneum, double-stranded RNA (dsRNA) targeting E93 (dsE93) was injected into newly emerged adults. Knockdown of E93 caused a decrease in the synthesis of vitellogenin (Vg), oocyte development, and egg-laying. Sequencing of RNA isolated from adults injected with dsE93 and control dsmalE (dsRNA targeting Escherichia coli malE gene) followed by differential gene expression analysis showed upregulation of genes involved in the metabolism of reserved nutrients. E93 knockdown induced changes in gene expression resulted in a decrease in Vg synthesis in the fat body and oocyte maturation in ovaries. Mating experiments showed that females injected with dsE93 did not lay eggs. Knockdown of E93 caused a reduction in the number and size of lipid droplets in the fat body when compared with that in control beetles injected with dsmalE. These data suggest that during the first 2-3 days after the emergence of adult females, E93 suppresses genes coding for enzymes that metabolize reserved nutrients until initiation of vitellogenesis and oogenesis.

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.

Rhythmic change of adipokinetic hormones diurnally regulates locust vitellogenesis and egg development

Adipokinetic hormones (AKHs), the neurohormones synthesized in the insect corpora cardiaca are known to mobilize lipids and carbohydrates for energy-consuming activities including reproduction. However, both inhibitory and stimulatory effects of AKHs on insect reproduction have been reported, and the underlying mechanisms remain elusive. Using the migratory locust, Locusta migratoria, as a model system, we report here that AKHs are expressed in response to rhythmic diel change, and AKH III expression increases markedly at photophase. Diurnal injection of AKH III but not AKH I or AKH II in adult females stimulates vitellogenesis and egg development. In contrast, AKH treatment at scotophase represses female reproduction. RNA interference-mediated knockdown of AKH receptor (AKHR) results in significantly reduced vitellogenin (Vg) expression in the fat body at photophase along with reduced Vg deposition in the ovary. AKHR knockdown also leads to decreased expression of Brummer, triacylglycerol lipase and trehalose transporter, accompanied by suppressed mobilization of triacylglycerol and trehalose. We propose that in addition to stimulating Vg expression at photophase, AKH/AKHR signalling is likely to regulate ovarian uptake of Vg via triacylglycerol mobilization and trehalose homeostasis. This study provides new insights into the understanding of AKH/AKHR signalling in the regulation of insect reproduction.

Effects of lipopolysaccharide and juvenile hormone III treatments on cell growth and gene expression in the Ceratitis capitata (Diptera: Tephritidae) CCE/CC128 cell line

The Mediterranean fruit fly Ceratitis capitata is one of the most important insect pest species in the world. It has a high colonization capacity and population variety, giving it considerable genetic diversity. Strategies for its control have included the sterile insect technique and insect growth regulators. Many studies have analyzed the medfly transcriptome, and along with the medfly genome sequence, the sequences of multiple genes related to sex determination, mating, development, pheromone detection, immunity, or stress have been identified. In this study, the medfly CCE/CC128 cell line was used to assess cell growth variation and changes in the expression of genes covering different functions, after lipopolysaccharide (LPS) and juvenile hormone III (JHIII) treatments. No significant effects on cell growth and gene expression were observed in the cells treated with LPS. In the cells treated with JHIII, the results showed significant effects on cell growth, and an overexpression was found of the Shade gene, one of the Halloween gene members of the cytochrome p450 family, which is involved in development and the synthesis of 20-hydroxyecdysone. This study shows preliminary results on the insect cell line in combination with whole-genome sequencing, which can facilitate studies regarding growth, toxicity, immunity, and transcriptome regulations as a response to different compounds and environmental alterations.

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 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.

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.

Juvenile hormone signaling in insect oogenesis

Juvenile hormone (JH) plays a crucial role in insect reproduction, but its molecular mode of action only became clear within the last decade. We here review recent findings revealing the intricate crosstalk between JH and ecdysone signaling with nutrient sensing pathways in Drosophila melanogaster, Aedes aegypti, Tribolium castaneum and Locusta migratoria. The finding for a critical role of ecdysis triggering hormone (ETH) in both molting and ooogenesis now also highlights the importance of an integrated view of development and reproduction. Furthermore, insights from non-model insects, especially so social Hymenoptera and termites, where JH function gradually becomes decoupled from reproduction and plays a role in division of labor, emphasize the need to consider life cycle and life history strategies when studying insect reproductive physiology.

The microRNAs let-7 and miR-278 regulate insect metamorphosis and oogenesis by targeting the juvenile hormone early-response gene Krüppel-homolog 1

Krüppel-homolog 1 (Kr-h1), a zinc-finger transcription factor, inhibits larval metamorphosis and promotes adult reproduction by transducing juvenile hormone (JH). Although the transcriptional regulation of Kr-h1 has been extensively studied, little is known about its regulation at the post-transcriptional level. Using the migratory locust Locusta migratoria as a model system, we report here that the microRNAs let-7 and miR-278 bound to the Kr-h1 coding sequence and downregulated its expression. Application of let-7 and miR-278 mimics (agomiRs) significantly reduced the level of Kr-h1 transcripts, resulting in partially precocious metamorphosis in nymphs as well as markedly decreased yolk protein precursors, arrested ovarian development and blocked oocyte maturation in adults. Moreover, the expression of let-7 and miR-278 was repressed by JH, constituting a regulatory loop of JH signaling. This study thus reveals a previously unknown regulatory mechanism whereby JH suppresses the expression of let-7 and miR-278, which, together with JH induction of Kr-h1 transcription, prevents the precocious metamorphosis of nymphs and stimulates the reproduction of adult females. These results advance our understanding of the coordination of JH and miRNA regulation in insect development.

Insect Transcription Factors: A Landscape of Their Structures and Biological Functions in Drosophila and beyond

Transcription factors (TFs) play essential roles in the transcriptional regulation of functional genes, and are involved in diverse physiological processes in living organisms. The fruit fly Drosophila melanogaster, a simple and easily manipulated organismal model, has been extensively applied to study the biological functions of TFs and their related transcriptional regulation mechanisms. It is noteworthy that with the development of genetic tools such as CRISPR/Cas9 and the next-generation genome sequencing techniques in recent years, identification and dissection the complex genetic regulatory networks of TFs have also made great progress in other insects beyond Drosophila. However, unfortunately, there is no comprehensive review that systematically summarizes the structures and biological functions of TFs in both model and non-model insects. Here, we spend extensive effort in collecting vast related studies, and attempt to provide an impartial overview of the progress of the structure and biological functions of current documented TFs in insects, as well as the classical and emerging research methods for studying their regulatory functions. Consequently, considering the importance of versatile TFs in orchestrating diverse insect physiological processes, this review will assist a growing number of entomologists to interrogate this understudied field, and to propel the progress of their contributions to pest control and even human health.

Protein kinase C mediates juvenile hormone-dependent phosphorylation of Na+/K+-ATPase to induce ovarian follicular patency for yolk protein uptake

In oviparous animals, vitellogenesis is prerequisite to egg production and embryonic growth after oviposition. For successful insect vitellogenesis and oogenesis, vitellogenin (Vg) synthesized in the fat body (homologue to vertebrate liver and adipose tissue) must pass through the intercellular channels, a condition known as patency in the follicular epithelium, to reach the surface of oocytes. This process is controlled by juvenile hormone (JH) in many insect species, but the underlying mechanisms remain elusive. Previous work has suggested the possible involvement of Na⁺/K⁺-ATPase in patency initiation, but again, the regulatory cascade of Na⁺/K⁺-ATPase for patency initiation has been lacking. Using the migratory locust Locusta migratoria as a model system, we report here that RNAi-mediated knockdown of gene coding for Na⁺/K⁺-ATPase, inhibition of its phosphorylation, or suppression of its activity causes loss of patency, resulting in blocked Vg uptake, arrested oocyte maturation, and impaired ovarian growth. JH triggers G protein-coupled receptor (GPCR), receptor tyrosine kinase (RTK), phospholipase C (PLC), inositol trisphosphate receptor (IP3R), and protein kinase C (PKC) to phosphorylate Na⁺/K⁺-ATPase α-subunit at amino acid residue Ser⁸, consequently activating Na⁺/K⁺-ATPase for the induction of patency in vitellogenic follicular epithelium. Our results thus point to a previously unidentified mechanism by which JH induces the phosphorylation and activation of Na⁺/K⁺-ATPase via a signaling cascade of GPCR, RTK, PLC, IP3R, and PKC. The findings advance our understanding of JH regulation in insect vitellogenesis and oogenesis.

Regulatory Pathways Controlling Female Insect Reproduction

The synthesis of vitellogenin and its uptake by maturing oocytes during egg maturation are essential for successful female reproduction. These events are regulated by the juvenile hormones and ecdysteroids and by the nutritional signaling pathway regulated by neuropeptides. Juvenile hormones act as gonadotropins, regulating vitellogenesis in most insects, but ecdysteroids control this process in Diptera and some Hymenoptera and Lepidoptera. The complex crosstalk between the juvenile hormones, ecdysteroids, and nutritional signaling pathways differs distinctly depending on the reproductive strategies adopted by various insects. Molecular studies within the past decade have revealed much about the relationships among, and the role of, these pathways with respect to regulation of insect reproduction. Here, we review the role of juvenile hormones, ecdysteroids, and nutritional signaling, along with that of microRNAs, in regulating female insect reproduction at the molecular level.