Methoprene-tolerant (Met) knockdown in the adult female cockroach, Diploptera punctata completely inhibits ovarian development

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.

Live-bearing cockroach genome reveals convergent evolutionary mechanisms linked to viviparity in insects and beyond

Live birth (viviparity) has arisen repeatedly and independently among animals. We sequenced the genome and transcriptome of the viviparous Pacific beetle-mimic cockroach and performed comparative analyses with two other viviparous insect lineages, tsetse flies and aphids, to unravel the basis underlying the transition to viviparity in insects. We identified pathways undergoing adaptive evolution for insects, involved in urogenital remodeling, tracheal system, heart development, and nutrient metabolism. Transcriptomic analysis of cockroach and tsetse flies revealed that uterine remodeling and nutrient production are increased and the immune response is altered during pregnancy, facilitating structural and physiological changes to accommodate and nourish the progeny. These patterns of convergent evolution of viviparity among insects, together with similar adaptive mechanisms identified among vertebrates, highlight that the transition to viviparity requires changes in urogenital remodeling, enhanced tracheal and heart development (corresponding to angiogenesis in vertebrates), altered nutrient metabolism, and shifted immunity in animal systems.

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.

Impact of JH Signaling on Reproductive Physiology of the Classical Insect Model, Rhodnius prolixus

In adult females of several insect species, juvenile hormones (JHs) act as gonadotrophic hormones, regulating egg production. JH binds to its nuclear receptor, Methoprene tolerant (Met), triggering its dimerization with the protein Taiman (Tai). The resulting active complex induces transcription of JH response genes, such as Krüppel homolog 1 (Kr-h1). In this study we report for the first time the participation of the isoform JH III skipped bisepoxide (JHSB₃) and its signaling pathway in the reproductive fitness of the classical insect model Rhodnius prolixus. The topical application of synthetic JHSB₃ increases transcript and protein expression of yolk protein precursors (YPPs), mainly by the fat body but also by the ovaries, the second source of YPPs. These results are also confirmed by ex vivo assays. In contrast, when the JH signaling cascade is impaired via RNA interference by downregulating RhoprMet and RhoprTai mRNA, egg production is inhibited. Although RhoprKr-h1 transcript expression is highly dependent on JHSB₃ signaling, it is not involved in egg production but rather in successful hatching. This research contributes missing pieces of JH action in the insect model in which JH was first postulated almost 100 years ago.

Role of Methoprene-tolerant in the regulation of oogenesis in Dipetalogaster maxima

Juvenile hormone (JH) signalling, via its receptor Methoprene-tolerant (Met), controls metamorphosis and reproduction in insects. Met belongs to a superfamily of transcription factors containing the basic Helix Loop Helix (bHLH) and Per Arnt Sim (PAS) domains. Since its discovery in 1986, Met has been characterized in several insect species. However, in spite of the importance as vectors of Chagas disease, our knowledge on the role of Met in JH signalling in Triatominae is limited. In this study, we cloned and sequenced the Dipetalogaster maxima Met transcript (DmaxMet). Molecular modelling was used to build the structure of Met and identify the JH binding site. To further understand the role of the JH receptor during oogenesis, transcript levels were evaluated in two main target organs of JH, fat body and ovary. Functional studies using Met RNAi revealed significant decreases of transcripts for vitellogenin (Vg) and lipophorin (Lp), as well as their receptors. Lp and Vg protein amounts in fat body, as well as Vg in hemolymph were also decreased, and ovarian development was impaired. Overall, these studies provide additional molecular insights on the roles of JH signalling in oogenesis in Triatominae; and therefore are relevant for the epidemiology of Chagas´ disease.

S6K1 acts through FOXO to regulate juvenile hormone biosynthesis in the red flour beetle, Tribolium castaneum

As the downstream effector of the target of rapamycin complex 1 (TORC1) signaling pathway, the ribosomal protein S6 kinase (S6K) is an important regulator of insect reproduction, however, the underlying mechanism remains obscure. In this study, a S6K gene, named TcS6K1, was isolated from the red flour beetle, Tribolium castaneum. Analysis of temporal and spatial expression patterns revealed that TcS6K1 is expressed at the highest level in the one-day-old first instar larvae and head of 7-day-old females, respectively. RNAi-mediated knockdown of TcS6K1 in either female or male adults decreased the number of eggs laid, with a concomitant reduction of mRNA levelsof vitellogenin genes, TcVg1 and TcVg2, two male accessory gland secretory proteins, as well as the juvenile hormone (JH) biosynthesis-related gene, farnesol dehydrogenase (TcFDH). While the mRNA and protein levels of the transcription factor forkhead box O (TcFOXO) were not affected, suppression of TcS6K1 expression promoted TcFOXO nuclear translocation to exert its transcriptional action. Further RNAi and EMSA analysis revealed that TcFOXO negatively regulated the expression of TcFDH. These results indicate that S6K might regulate beetles' reproduction through FOXO/JH signaling pathway.

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.

Molecular Identification and Functional Characterization of Methoprene-Tolerant (Met) and Krüppel-Homolog 1 (Kr-h1) in Harmonia axyridis (Coleoptera: Coccinellidae)

Juvenile hormone (JH) plays a key role in regulating insect reproductive processes. Methoprene-tolerant (Met), as a putative JH receptor, transduces JH signals by activating the transcription factor krüppel homolog 1 (Kr-h1). To understand the effects of Met and Kr-h1 genes on female reproduction of natural enemy insects, the Met and Kr-h1 were identified and analyzed from Harmonia axyridis Pallas (HmMet and HmKr-h1). The HmMet protein belonged to the bHLH-PAS family with bHLH domain, PAS domains, and PAC domain. HmMet mRNA was detected in all developmental stages, and the highest expression was found in the ovaries of female adults. The HmKr-h1 protein had eight C2H2-type zinc finger domains. HmKr-h1 mRNA was highly expressed from day 7 to day 9 of female adults. The tissue expression showed that HmKr-h1 was highly expressed in its wing, leg, and fat body. Knockdown of HmMet and HmKr-h1 substantially reduced the transcription of HmVg1 and HmVg2, inhibited yolk protein deposition, and reduced fecundity using RNA interference. In addition, the preoviposition period was significantly prolonged after dsMet-injection, but there was no significant difference after dsKr-h1-silencing. However, the effect on hatchability results was the opposite. Therefore, we infer that both HmMet and HmKr-h1 are involved in female reproduction of H. axyridis, and their specific functions are different in certain physiological processes. In several continents, H. axyridis are not only beneficial insects, but also invasive pests. This report will provide basis for applying or controlling the H. axyridis.

Binding of de novo synthesized radiolabeled juvenile hormone (JH III) by JH receptors from the Cuban subterranean termite Prorhinotermes simplex and the German cockroach Blattella germanica

Juvenile hormone (JH) controls insect reproduction and development through an intracellular receptor complex comprising two bHLH-PAS proteins, the JH-binding Methoprene-tolerant (Met) and its partner Taiman (Tai). Many hemimetabolous insects including cockroaches strictly depend on JH for stimulation of vitellogenesis. In termites, the eusocial hemimetabolans, JH also regulates the development of caste polyphenism. Studies addressing the agonist ligand binding to recombinant JH receptors currently include three species belonging to two holometabolous insect orders, but none that would represent any of the hemimetabolous orders. Here, we examined JH receptors in two representatives of Blattodea, the cockroach Blattella germanica and the termite Prorhinotermes simplex. To test the JH-binding capacity of Met proteins from these species, we performed chemical synthesis and tritium labeling of the natural blattodean JH homolog, JH III. Our improved protocol increased the yield and specific activity of [10-³H]JH III relative to formerly available preparations. Met proteins from both species specifically bound [³H]JH III with high affinity, whereas Met variants mutated at a critical position within the ligand-binding domain were incapable of such binding. Furthermore, JH III and the synthetic JH mimic fenoxycarb stimulated dimerization between Met and Tai components of the respective JH receptors of both species. These data present primary evidence for agonist binding by JH receptors in any hemimetabolous species and provide a molecular basis for JH action in cockroaches and termites.

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.

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.

Role of Endocrine System in the Regulation of Female Insect Reproduction

The proper synthesis and functioning of ecdysteroids and juvenile hormones (JHs) are very important for the regulation of vitellogenesis and oogenesis. However, their role and function contrast among different orders, and even in the same insect order. For example, the JH is the main hormone that regulates vitellogenesis in hemimetabolous insect orders, which include Orthoptera, Blattodea, and Hemiptera, while ecdysteroids regulate the vitellogenesis among the insect orders of Diptera, some Hymenoptera and Lepidoptera. These endocrine hormones also regulate each other. Even at some specific stage of insect life, they positively regulate each other, while at other stages of insect life, they negatively control each other. Such positive and negative interaction of 20-hydroxyecdysone (20E) and JH is also discussed in this review article to better understand the role of these hormones in regulating the reproduction. Therefore, the purpose of the present review is to deeply understand the complex interaction of endocrine hormones with each other and with the insulin signaling pathway. The role of microbiomes in the regulation of the insect endocrine system is also reviewed, as the endocrine hormones are significantly affected by the compounds produced by the microbiota.

Dynamic transcriptome analysis and Methoprene-tolerant gene knockdown reveal that juvenile hormone regulates oogenesis and vitellogenin synthesis in Propylea Japonica

Propylea japonica has been regarded as one of the most remarkable natural enemies against aphid in China. However, the mechanism of juvenile hormone (JH) regulation of reproduction in P. japonica is still unclear. In this study, we investigated the JH titers of P. japonica and the development of the ovaries. We selected the six different developmental stages of ladybeetle females for transcriptome sequencing. We identified 583 genes involved in insect reproduction regulation, including 107 insect hormone synthesis signaling pathway-related genes and 476 nutrition-sensing signaling pathway-related genes. Transcriptome analysis indicated that a large number JH synthesis- and metabolism-related enzyme genes and some potential nutrient signal sensing- and transduction-related genes were significantly differentially expressed during P. japonica development. We investigated the effects of Met gene silencing on the reproduction of female adults and found that the ovarian maturation, vitellogenesis, and follicular epithelium development in the dsMet treatment group were significantly inhibited.

Juvenile hormone mediates lipid storage in the oocytes of Dipetalogaster maxima

Triatomines are vectors of Chagas disease and important model organisms in insect physiology. "Kissing bugs" are obligatory hematophagous insects. A blood meal is required to successfully complete oogenesis, a process primarily controlled by juvenile hormone (JH). We used Dipetalogaster maxima as an experimental model to further understand the roles of JH in the regulation of vitellogenesis and oogenesis. A particular focus was set on the role of JH controlling lipid and protein recruitment by the oocytes. The hemolymph titer of JH III skipped bisepoxide increased after a blood meal. Following a blood meal there were increased levels of mRNAs in the fat body for the yolk protein precursors, vitellogenin (Vg) and lipophorin (Lp), as well as of their protein products in the hemolymph; mRNAs of the Vg and Lp receptors (VgR and LpR) were concomitantly up-regulated in the ovaries. Topical administration of JH induced the expression of Lp/LpR and Vg/VgR genes, and prompted the uptake of Lp and Vg in pre-vitellogenic females. Knockdown of the expression of LpR by RNA interference in fed females did not impair the Lp-mediated lipid transfer to oocytes, suggesting that the bulk of lipid acquisition by oocytes occurred by other pathways rather than by the endocytic Lp/LpR pathway. In conclusion, our results strongly suggest that JH signaling is critical for lipid storage in oocytes, by regulating Vg and Lp gene expression in the fat body as well as by modulating the expression of LpR and VgR genes in ovaries.

Identification of juvenile hormone-induced posttranslational modifications of methoprene tolerant and Krüppel homolog 1 in the yellow fever mosquito, Aedes aegypti

Recent studies reported that JH-regulated phosphorylation status of the JH-receptor complex contributes to its transcription activity in Aedes aegypti. However, phosphorylation sites of these proteins have not yet been identified. In this study, we found that the fusion of an EGFP tag to Ae. aegypti Kr-h1 (AaKr-h1) and Met (AaMet) improved their stability in mosquito Aag-2 cells, which allowed their purification. The liquid chromatography and tandem mass spectrometry analysis of the purified AaKr-h1 showed that the phosphoserine residue at position 694, located in the evolutionarily conserved SVIQ motif, is dephosphorylated when the cells are exposed to JH. The AaKr-h1 dephosphorylation mutant (S694V) showed significantly higher activity in inducing the luciferase gene regulated by JH response elements. The phosphorylation profile of Met also changed after exposing Aag-2 cells to JH III. The Ser-77 and Ser-710 residues of Met were phosphorylated after JH III treatment. In contrast, the two phosphoserine residues at positions 73 and 747 were dephosphorylated after JH III treatment. JH exposure also induced transient and reversible phosphorylation of Thr-664 and Ser-723 residues. Overall, these data show that JH induces changes in post-translational modifications of AaMet and AaKr-h1. SIGNIFICANCE: Female Aedes aegypti mosquitoes are known to vector many disease agents, including Zika virus, dengue virus chikungunya virus, and Mayaro and yellow fever virus. In the present study, we developed an efficient method to prepare Ae. aegypti Met and Kr-h1, which are typically difficult to produce and purify, using a mosquito cell line expression system. A liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based approaches were utilized to map the phosphorylation profiles of the isolated proteins. We then monitored the changes induced by JH activation in the phosphorylation profiles to check if the JH modulates post-translation modification of its key transcription factors. We found that the JH induced alterations in the phosphorylation profiles of the multiple residues of AaMet. In contrast, activation of the JH signaling pathway was accompanied by dephosphorylation of AaKr-h1 at phosphoserine-694, increasing its transcriptional activity. In addition, S694 of AaKr-h1 was located in the RMSSVIQYA motif highly conserved in orthologous proteins from other insect species. These results can help us further understand how JH modulates its key transcription factors and provide a basis for the development of novel insect control strategies.

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.

Crucial Role of Juvenile Hormone Receptor Components Methoprene-Tolerant and Taiman in Sexual Maturation of Adult Male Desert Locusts

Currently (2020), Africa and Asia are experiencing the worst desert locust (Schistocerca gregaria) plague in decades. Exceptionally high rainfall in different regions caused favorable environmental conditions for very successful reproduction and population growth. To better understand the molecular mechanisms responsible for this remarkable reproductive capacity, as well as to fill existing knowledge gaps regarding the regulation of male reproductive physiology, we investigated the role of methoprene-tolerant (Scg-Met) and Taiman (Scg-Tai), responsible for transducing the juvenile hormone (JH) signal, in adult male locusts. We demonstrated that knockdown of these components by RNA interference strongly inhibits male sexual maturation, severely disrupting reproduction. This was evidenced by the inability to show mating behavior, the absence of a yellow-colored cuticle, the reduction of relative testes weight, and the drastically reduced phenylacetonitrile (PAN) pheromone levels of the treated males. We also observed a reduced relative weight, as well as relative protein content, of the male accessory glands in Scg-Met knockdown locusts. Interestingly, in these animals the size of the corpora allata (CA), the endocrine glands where JH is synthesized, was significantly increased, as well as the transcript level of JH acid methyltransferase (JHAMT), a rate-limiting enzyme in the JH biosynthesis pathway. Moreover, other endocrine pathways appeared to be affected by the knockdown, as evidenced by changes in the expression levels of the insulin-related peptide and two neuroparsins in the fat body. Our results demonstrate that JH signaling pathway components play a crucial role in male reproductive physiology, illustrating their potential as molecular targets for pest control.

Transcriptomic analysis reveals the regulation network of BmKrüppel homolog 1 in the oocyte development of Bombyx mori

Krüppel homolog 1 (Kr-h1), a zinc finger transcription factor, is involved in the metamorphosis and adult reproduction of insects. However, the role of Kr-h1 in reproduction of holometabolic insects remains to be elucidated. The regulation network of Kr-h1-associated genes in the reproduction in Bombyx mori was investigated in this study. The higher expression level of BmKr-h1 in the ovaries was detected during the late pupal stage and adults. RNA interference (RNAi)-mediated depletion of BmKr-h1 in the female at day 6 of pupae resulted in abnormal oocytes at 48 h post-double-stranded RNA treatment, which showed less yolk protein deposition and partially transparent chorion. RNA-seq and subsequent differentially expressed transcripts analysis showed that knockdown of BmKr-h1 caused a decrease in the expression of 2882 genes and an increase in the expression of 2565 genes in the oocytes at day 8 of pupae. Totally, 27 genes coding for transcription factors were down-regulated, while six genes coding for other transcription factors were up-regulated. BmKr-h1 bound to the Kr-h1 binding site of the transcription factors AP-1 (activating protein-1) and FOXG1 to increase their messenger RNA transcripts in the BmN cells, respectively. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses of that positively co-expressed with AP-1 and FOXG1 transcripts showed mainly enrichment in the metabolic-related pathways, the nutrient absorption and the yolk protein absorption processes. These data suggested that BmKr-h1 might directly regulate the metabolic-related pathways, the nutrient absorption and the yolk protein absorption processes or probably through AP-1 and /or FOXG1 to regulate oocyte development.

Ovarian morphological features and proteome reveal fecundity fitness disadvantages in β-cypermethrin-resistant strains of Blattella germanica (L.) (Blattodea: Blattellidae)

To evaluate whether the development of β-cypermethrin resistance in Blattella germanica (L.) (Blattaria: Blattellidae) affects the fecundity fitness of this insect and to determine the underlying mechanism, we compared fecundity differences between β-cypermethrin-resistant (R) and sensitive (S) strains of B. germanica, observed the physiological structural changes of ovaries from an visual perspective, and analyzed differences in the ovarian proteome using proteomic methods. The results showed that, compared with the S strain of B. germanica, the R strain of B. germanica had a significantly higher ootheca shedding rate, a significantly lower number of hatched and surviving nymphs, a significantly higher female proportion in the population and defective ovarian development. Ovarian proteomic analysis showed a total of 64 differentially expressed proteins in the R strain, including 18 upregulated proteins and 46 downregulated proteins. Twenty-four significantly differentially expressed proteins were further studied, and 14 were successfully identified, which were mainly classified into the following categories: immunity-related proteins, development-related proteins, structural proteins, energy metabolism-related proteins and proteins with unknown functions. The differential expression of these proteins reflects the overall changes in cell structure and metabolism associated with β-cypermethrin resistance and explains the possible molecular mechanism of fecundity fitness disadvantages. In summary, β-cypermethrin resistance can cause fecundity fitness disadvantages in B. germanica. The metabolic deviations needed to overcome the adverse effects of insecticides may result in an energy exchange that affects energy allocation and, ultimately, the basic needs of the insect. The fitness cost due to insecticide resistance is critical to the delay of the evolution of resistance.

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.

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.

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

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

Molecular mechanisms underlying milk production and viviparity in the cockroach, Diploptera punctata

Viviparous reproduction is characterized by maternal retention of developing offspring within the reproductive tract during gestation, culminating in live birth. In some cases, a mother will provide nutrition beyond that present in the yolk; this is known as matrotrophic viviparity. While this phenomenon is best associated with mammals, it is observed in insects such as the viviparous cockroach, Diploptera punctata. Female D. punctata carry developing embryos in the brood sac, a reproductive organ that acts as both a uterus and a placenta by protecting and providing a nutritive secretion to the intrauterine developing progeny. While the basic physiology of D. punctata pregnancy has been characterized, little is known about the molecular mechanisms underlying this phenomenon. This study combined RNA-seq analysis, RNA interference, and other assays to characterize molecular and physiological changes associated with D. punctata reproduction. A comparison of four stages of the female reproductive cycle and males revealed unique gene expression profiles corresponding to each stage and between sexes. Differentially regulated transcripts of interest include the previously identified family of milk proteins and transcripts associated with juvenile hormone metabolism. RNA interference and methoprene application experiments established the potential impacts of bothbreakdown and synthesis reduction of juvenile hormone in maintaining pregnancy in D. punctata. These studies provide the comprehensive molecular mechanisms associated with cockroach viviparity, which will be a critical resource for comparative purposes among viviparity in insect systems.

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

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

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

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

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

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

Molecular Characterization of Primary Juvenile Hormone Responders Methoprene-Tolerant (Met) and Krüppel Homolog 1 (Kr-h1) in Grapholita molesta (Lepidoptera: Tortricidae) with Clarification of Their Roles in Metamorphosis and Reproduction

Methoprene-tolerant (Met) is a putative JH intracellular receptor that transduces JH signal by activation of the inducible Krüppel homolog 1 (Kr-h1). We analyzed the gene sequences of Met and Kr-h1 and their patterns of expression in Grapholita molesta (Busck) immature and adult stages in order to better understand the roles of these primary JH responders in regulating the metamorphosis and reproduction of this global pest of fruit crops. The deduced amino acid sequences of both GmMet and GmKr-h1 were highly homologous to those of other Lepidoptera, especially the cotton bollworm, Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae). Peak expression of GmMet occurred during the last 3 to 5 d of the final instar, followed by that of GmKr-h1, in the last 3 d of final instar. Similar patterns of GmMet and GmKr-h1 expression were detected across various tissue types in the fifth-instar larvae, with the highest expression observed in the head, followed by the epidermis, and the fat body. When expression of GmMet and GmKr-h1 was knocked down via dsRNA injection in the fifth instar, the results were increased larval mortality, abnormal pupation, delayed pupal duration, reduced adult emergence, extended preoviposition period, and reduced fecundity. We infer that both GmMet and GmKr-h1 participated in regulation of metamorphosis and reproduction in G. molesta, the former acting upstream of the latter, and could present biorational targets for novel pest control compounds.

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.

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

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

Juvenile hormone and sesquiterpenoids in arthropods: Biosynthesis, signaling, and role of MicroRNA

Arthropod molting and reproduction are precisely controlled by the levels of sesquiterpenoids, a class of C15 hormones derived from three isoprene units. The two major functional arthropod sesquiterpenoids are juvenile hormone (JH) and methyl farnesoate (MF). In hemimetabolous insects (such as the aphids, bugs, and cockroaches) and holometabolous insects (such as beetles, bees, butterflies, and flies), dramatic decrease in the titers of JH and/or MF promote metamorphosis from larvae to adults either directly or through an intermediate pupal stage, respectively. JH is absent in crustaceans (lobster, shrimp, crab) and other arthropods (chelicerates such as ticks, mites, spiders, scorpions and myriapods such as millipede and centipedes). In some crustaceans, molting and reproduction is dependent on changing levels of MF. The regulation of sesquiterpenoid production is thus crucial in the life cycle of arthropods. Dynamic and complex mechanisms have evolved to regulate sesquiterpenoid production. Noncoding RNAs such as the microRNAs are primary regulators. This article provides an overview of microRNAs that are known to regulate sesquiterpenoid production in arthropods.

Dissecting the role of Krüppel homolog 1 in the metamorphosis and female reproduction of the cotton bollworm, Helicoverpa armigera

In insects, metamorphosis and reproduction are controlled by juvenile hormone (JH) and 20-hydroxyecdysone (20E). Krüppel homolog 1 (Kr-h1), a transcription factor, is regarded as a JH-early inducible gene responsible for the repression of metamorphosis. However, the role of Kr-h1 in reproduction of holometabolic insects is relatively less understood. In this study, we studied the role of Kr-h1 in larvae-pupae transition and female reproduction in the major agricultural pest Helicoverpa armigera. Two HaKr-h1 isoforms (HaKr-h1α and HaKr-h1ß) were identified, with HaKr-h1α predominant in the cotton bollworm. In larvae, HaKr-h1 was predominately expressed in the epidermis and markedly up-regulated during the moult stage, whereas in adults HaKr-h1 was mainly expressed in females and the highest transcription was detected in the ovaries. Considering the function of hormones in larval metamorphosis, we examined the modulation of gene expression in response to hormones, which showed that HaKr-h1 was significantly induced by both JH analogue (JHA) and 20E. Knockdown of HaKr-h1 in fifth-instar larvae resulted in precocious metamorphosis from larvae to pupae. Moreover, a fluorescence immunoassay coupled with heterologous expression revealed that HaKr-h1 was localized in the nucleus of oocyte membrane. In female adults, depletion of HaKr-h1 severely repressed the transcription of vitellogenin, disrupted oocyte maturation and reduced the number of eggs laid, suggesting that HaKr-h1 is required for vitellogenesis and egg production in H. armigera. The present study provides insight into the roles of HaKr-h1 in JH-mediated reproduction and highlights HaKr-h1 as a target for suppression of lepidopteran pests.

Involvement of Met and Kr-h1 in JH-Mediated Reproduction of Female Bactrocera dorsalis (Hendel)

Juvenile hormone (JH) prevents metamorphosis during insect larval stages and promotes adult reproductive processes. Krüppel-homolog 1 (Kr-h1), a zinc finger transcription factor assumed to be induced by JH via the JH receptor methoprene-tolerant (Met), mediates the antimetamorphic effect of JH in insects, but its function in JH-mediated reproductive processes has not been fully explored. In this study, Met and Kr-h1 involved in the JH signaling pathway were first cloned and identified from the oriental fruit fly, Bactrocera dorsalis, an important pest infesting fruit and vegetables worldwide. Subsequent spatiotemporal expression analysis revealed that Met and Kr-h1 were both highly expressed in 7-day-old adults and fat body of female adults, respectively. Treatment with a JH analog (methoprene) significantly induced the expression of JH signaling and vitellogenin (Vg) genes and accelerated ovary development. RNA interference (RNAi) further revealed that either Met or Kr-h1 depletion at the adult stage of B. dorsalis impeded ovary development, with significantly lower egg production noted as well. In addition, rescue through methoprene application after RNAi stimulated the expression of JH signaling and Vg genes. Although there were still differences in ovary phenotype between rescued insects and the pre-RNAi control, ovary redevelopment with a larger surface area was observed, consistent with the spatiotemporal expression and phenotypes recorded in the original methoprene experiment. Our data reveal the involvement of Met and Kr-h1 in insect vitellogenesis and egg production, thus indicating the crucial role of the JH signaling pathway in insect reproduction.

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.

Role of the venus kinase receptor in the female reproductive physiology of the desert locust, Schistocerca gregaria

Venus kinase receptors (VKR) are a subfamily of invertebrate receptor tyrosine kinases, which have only recently been discovered. They contain an intracellular tyrosine kinase domain and an extracellular Venus FlyTrap domain. VKRs have been functionally and pharmacologically characterized in only two invertebrate species, namely the human parasite Schistosoma mansoni and the mosquito Aedes aegypti, where they play a crucial role in oogenesis. Here, we report the characterization of a VKR in the desert locust, Schistocerca gregaria. We performed an in-depth profiling study of the SgVKR transcript levels in different tissues throughout the female adult stage. Using the RNA interference technique, the possible role of SgVKR was investigated. SgVKR knockdown had significant effects on ovarian ecdysteroid levels and on the size of oocytes during the vitellogenic stage. SgVKR is probably involved in the complex cross-talk between several important pathways regulating female reproductive physiology. Contrary to A. aegypti and S. mansoni, we cannot conclude that this receptor is essential for reproduction, since silencing SgVKR did not affect fecundity or fertility. Considering the evolutionary distance between A. aegypti and S. gregaria, as well as the differences in regulation of their female reproductive physiology, this article constitutes a valuable asset in better understanding VKRs.

Juvenile hormone and hemimetabolan eusociality: a comparison of cockroaches with termites

Termites are social Dictyoptera that evolved eusociality independently from social Hymenoptera. They are characterized by unique developmental plasticity that is the basis of caste differentiation and social organization. As developmental plasticity is a result of endocrine regulation, in order to understand the evolution of termite sociality it is helpful to compare the endocrine underpinning of development between termites and cockroaches. Nijhout and Wheeler (1982) proposed that varying JH titers determine caste differentiation in termites. Based on current results, we extend this model by adding the importance of social interactions. High JH titers in the presence of soldiers lead to regressive development (decrease in body size, apparent regression in development), while an absence of soldiers induces (pre-)soldier differentiation. On the opposite side, low JH titers in colonies headed by reproductives result in progressive molts toward adults, while an absence of reproductives induces development of replacement reproductives. In cockroaches, transcription factors involved in JH signaling, including the adult specifier E93 (the co-called MEKRE93 pathway) regulate the morphogenetic transition between the nymph and the adult. In termites, we speculate that castes might be determined by social effects playing a modulatory action of JH in the MEKRE93 pathway.

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

Juvenile hormone (JH) has a well known role in stimulating insect vitellogenesis (i.e. yolk deposition) and oocyte maturation, but the molecular mechanisms of JH action in insect reproduction are unclear. The 78-kDa glucose-regulated protein (Grp78) is a heat shock protein 70-kDa family member and one of the most abundant chaperones in the endoplasmic reticulum (ER) where it helps fold newly synthesized peptides. Because of its prominent role in protein folding, and also ER stress, we hypothesized that Grp78 might be involved in fat body cell homeostasis and vitellogenesis and a regulatory target of JH. We report here that the migratory locust Locusta migratoria possesses two Grp78 genes that are differentially regulated by JH. We found that Grp78-1 is regulated by JH through Mcm4/7-dependent DNA replication and polyploidization, whereas Grp78-2 expression is directly activated by the JH-receptor complex comprising methoprene-tolerant and Taiman proteins. Interestingly, Grp78-2 expression in the fat body is about 10-fold higher than that of Grp78-1 Knockdown of either Grp78-1 or Grp78-2 significantly reduced levels of vitellogenin (Vg) protein, accompanied by retarded maturation of oocytes. Depletion of both Grp78-1 and Grp78-2 resulted in ER stress and apoptosis in the fat body and in severely defective Vg synthesis and oocyte maturation. These results indicate a crucial role of Grp78 in JH-dependent vitellogenesis and egg production. The presence and differential regulation of two Grp78 genes in L. migratoria likely help accelerate the production of this chaperone in the fat body to facilitate folding of massively synthesized Vg and other proteins.

Juvenile hormone regulation of female reproduction in the common bed bug, Cimex lectularius

To begin studies on reproduction in common bed bug, Cimex lectularius, we identified three genes coding for vitellogenin (Vg, a protein required for the reproductive success of insects) and studied their hormonal regulation. RNA interference studied showed that expression of Vg3 gene in the adult females is a prerequisite for successful completion of embryogenesis in the eggs laid by them. Juvenile hormone (JH) receptor, Methoprene-tolerant (Met), steroid receptor coactivator (SRC) and GATAa but not ecdysone receptor (EcR) or its partner, ultraspiracle (USP) are required for expression of Vg genes. Feeding and mating working through Vg, Met, SRC, EcR, and GATAa regulate oocyte development. Knockdown of the expression of Met, SRC, EcR, USP, BR-C (Broad-Complex), TOR (target of rapamycin), and GATAa in female adults resulted in a reduction in the number eggs laid by them. Interestingly, Kruppel homolog 1 (Kr-h1) knockdown in the adult females did not reduce their fecundity but affected the development of embryos in the eggs laid by females injected with Kr-h1 double-stranded RNA. These data suggest that JH functioning through Met and SRC regulate both vitellogenesis and oogenesis in C. lectularius. However, JH does not work through Kr-h1 but may work through transcription factors not yet identified.

Juvenile hormone facilitates the antagonism between adult reproduction and diapause through the methoprene-tolerant gene in the female Colaphellus bowringi

In insects, the process whereby juvenile hormone (JH) regulates short-day (SD)-induced reproductive diapause has been previously investigated. However, we still do not understand the mechanism by which JH regulates long-day (LD)-induced reproductive diapause. In this study, we use a cabbage beetle, Colaphellus bowringi, which is a serious pest of cruciferous vegetables in Asia capable of entering reproductive diapause under LD conditions, as a model to test whether JH regulates female reproductive diapause similar to the mechanism of SD-induced diapause. Our results showed that the JH analog (JHA) methoprene significantly induced ovarian development but inhibited lipid accumulation of diapause-destined adults. Meanwhile, the transcripts of the vitellogenin (Vg) genes were upregulated, whereas the expression of the fat synthesis and stress tolerance genes were downregulated. RNA interference of the JH candidate receptor gene methoprene-tolerant (Met) blocked JH-induced ovarian development and Vg transcription, suggesting a positive regulatory function for JH-Met signaling in reproduction. Furthermore, under reproduction-inducing conditions, Met depletion promoted a diapause-like phenotype, including arrested ovarian development and increased lipid storage, and stimulated the expression of diapause-related genes involved in lipid synthesis and stress tolerance, suggesting JH-Met signaling plays an important role in the inhibition of diapause. Accordingly, our data indicate that JH acts through Met to facilitate development of the reproductive system by upregulating Vg expression while inhibiting diapause by suppressing lipid synthesis and stress tolerance in the cabbage beetle. Combined with previous studies in SD-induced reproductive diapause, we conclude that JH may regulate female reproductive diapause using a conserved Met-dependent pathway, regardless of the length of the photoperiod inducing diapause in insects.

Physiology: Female Flies Have the Guts for Reproduction

Females of many species adjust their diet to support the energetic requirements of egg production. New research shows how, in flies, females remodel their gut and modify gut physiology too.

Photoperiod regulates growth of male accessory glands through juvenile hormone signaling in the linden bug, Pyrrhocoris apterus

Adult reproductive diapause is characterized by lower behavioral activity, ceased reproduction and absence of juvenile hormone (JH). The role of JH receptor Methoprene-tolerant (Met) in female reproduction is well established; however, its function in male reproductive development and behavior is unclear. In the bean bug, Riptortus pedestris, circadian genes are essential for mediating photoperiodically-dependent growth of the male accessory glands (MAGs). The present study explores the role of circadian genes and JH receptor in male diapause in the linden bug, Pyrrhocoris apterus. These data indicate that circadian factors Clock, Cycle and Cry2 are responsible for photoperiod measurement, whereas Met and its partner protein Taiman participate in JH reception. Surprisingly, knockdown of the JH receptor neither lowered locomotor activity nor reduced mating behavior of males. These data suggest existence of a parallel, JH-independent or JH-upstream photoperiodic regulation of reproductive behavior.

Nutritional Signaling Regulates Vitellogenin Synthesis and Egg Development through Juvenile Hormone in Nilaparvata lugens (Stål)

Insect female reproduction which comprises the synthesis of vitellogenein (Vg) in the fat body and its incorporation into developing oocytes, needs a large amount of energy and food resources. Our previous studies found that juvenile hormone (JH) regulates vitellogenesis in the brown planthopper, Nilaparvata lugens. Here, we report on the role of JH in nutrient-regulated Vg synthesis and egg development. We first cloned the genes coding for juvenile hormone acid methyltransferase (JHAMT) which is involved in JH biosynthesis and methoprene-tolerant (Met) for JH action. Amino acids (AAs) induced the expression of jmtN, while showing no effects on the expression of met using an artificial diet culture system. Reduction in JH biosynthesis or its action by RNA interference (RNAi)-mediated silencing of jmtN or met led to a severe inhibition of AAs-induced Vg synthesis and oocyte maturation, together with lower fecundity. Furthermore, exogenous application of JH III partially restored Vg expression levels in jmtN RNAi females. However, JH III application did not rescue Vg synthesis in these met RNAi insects. Our results show that AAs induce Vg synthesis in the fat body and egg development in concert with JH biosynthesis in Nilaparvata lugens (Stål), rather than through JH action.

Multiple factors contribute to anautogenous reproduction by the mosquito Aedes aegypti

Aedes aegypti is an anautogenous mosquito that must blood feed on a vertebrate host to produce and lay a clutch of eggs. The rockpool mosquito, Georgecraigius atropalpus, is related to A. aegypti but is a facultatively autogenous species that produces its first clutch of eggs shortly after emerging without blood feeding. Consumption of a blood meal by A. aegypti triggers the release of ovary ecdysteroidogenic hormone (OEH) and insulin-like peptide 3 (ILP3) from the brain, which stimulate egg formation. OEH and ILP3 also stimulate egg formation in G. atropalpus but are released at eclosion independently of blood feeding. These results collectively suggest that blood meal dependent release of OEH and ILP3 is one factor that prevents A. aegypti from reproducing autogenously. Here, we examined two other factors that potentially inhibit autogeny in A. aegypti: teneral nutrient reserves and the ability of OEH and ILP3 to stimulate egg formation in the absence of blood feeding. Measures of nutrient reserves showed that newly emerged A. aegypti females had similar wet weights but significantly lower protein and glycogen reserves than G. atropalpus females when larvae were reared under identical conditions. OEH stimulated non-blood fed A. aegypti females to produce ecdysteroid hormone and package yolk into oocytes more strongly than ILP3. OEH also reduced host seeking and blood feeding behavior, yet females produced few mature eggs. Overall, our results indicate that multiple factors prevent A. aegypti from reproducing autogenously.

Effects of Pyriproxyfen on Female Reproduction in the Common Cutworm, Spodoptera litura (F.) (Lepidoptera: Noctuidae)

The common cutworm, Spodoptera litura, is a rapidly reproducing pest of numerous agricultural ecosystems worldwide. The use of pesticides remains the primary means for controlling S. litura, despite their negative ecological impact and potential threat to human health. The use of exogenous hormone analogs may represent an alternative to insecticides. Juvenile hormones (JHs) play an important role in the reproductive systems of female insects, but the effects of pyriproxyfen, a JH analog, on reproduction in S. litura were poorly understood. In this paper, we topically treated the newly emerged females with 20, 60, or 100 μg of pyriproxyfen to determine its effects on reproduction. Then, we examined the expression of vitellogenin (Vg) and three hormone receptors, USP, HR3, and EcR, using quantitative reverse transcription and real-time polymerase chain reaction (qRT-PCR), and found that pyriproxyfen up-regulated the expression of Vg, USP, and HR3, whereas the expression of EcR was unaffected. An analysis of fecundity showed that the peak oviposition day, lifespan, and oviposition period were progressively shortened as the pyriproxyfen dosage increased. We also found that pyriproxyfen decreased egg laying amount, whereas the number of mature eggs that remained in the ovarioles of dead females increased as the pyriproxyfen dosage increased. We examined oocytes using transmission electron microscopy and found that treatment with 100 μg of pyriproxyfen increased the metabolism by increasing the amount of rough endoplasmic reticulum and mitochondria in the primary oocytes. Our results suggest that the topical application of pyriproxyfen on newly emerged females can efficiently reduce reproduction in S. litura and may represent an alternative to the use of insecticides for controlling the agricultural pest.

Molecular basis of juvenile hormone signaling

Despite important roles played by juvenile hormone (JH) in insects, the mechanisms underlying its action were until recently unknown. A breakthrough has been the demonstration that the bHLH-PAS protein Met is an intracellular receptor for JH. Binding of JH to Met triggers dimerization of Met with its partner protein Tai, and the resulting complex induces transcription of target genes. In addition, JH can potentiate this response by phosphorylating Met and Tai via cell membrane, second-messenger signaling. An important gene induced by the JH-Met-Tai complex is Kr-h1, which inhibits metamorphosis. Kr-h1 represses an 'adult specifier' gene E93. The action of this JH-activated pathway in maintaining the juvenile status is dispensable during early postembryonic development when larvae/nymphs lack competence to metamorphose.

RXR/USP and EcR are critical for the regulation of reproduction and the control of JH biosynthesis in Diploptera punctata

During development and reproduction the response to ecdysteroids is mediated by a heterodimeric receptor complex comprising the retinoid X receptor/ultraspiracle (RXR/USP) and the ecdysone receptor (EcR). Here, the role of these receptors in the endocrine control of reproduction is examined in the cockroach Diploptera punctata. We report the sequence of four DpRXR and three DpEcR splice variants, including the first description of a Drosophila EcRB2-like isoform in a hemimetabolous insect. DpRXR and DpEcR are broadly expressed in the tissues of adult females, with relatively high transcript levels in the corpora allata (CA), nervous tissue and ovary. Developmental profiling revealed an inverse correlation between DpRXR and DpEcR expression and the activity of the CA. RNAi-mediated depletion of DpRXR and DpEcR did not affect oocyte growth, but inhibited oviposition and impaired chorion formation. Retained oocytes exhibited a degenerating follicular epithelium and were slowly resorbed. Treated animals showed significantly higher rates of JH biosynthesis and a decrease in ecdysteroid titers at the end of vitellogenesis. Reduction of DpRXR and DpEcR expression resulted in an upregulation of genes involved in JH production and a downregulation of allatostatin receptor mRNA in the CA. Treatment with dsRNA also affected the expression of genes downstream of JH in target tissues including vitellogenin and Krüppel-homolog 1 as well as Broad-Complex, an early ecdysone response gene. Overall, results suggest that DpRXR and DpEcR are not required early in the reproductive cycle when events are JH-dependent, but do mediate critical ecdysteroid feedback to the CA late in the gonadotropic cycle.

Knockdown of the juvenile hormone receptor gene inhibits soldier-specific morphogenesis in the damp-wood termite Zootermopsis nevadensis (Isoptera: Archotermopsidae)

The Methoprene-tolerant (Met) protein has been established as a juvenile hormone (JH) receptor. Knockdown of the Met gene caused precocious metamorphosis and suppression of ovarian development. However, the function of Met in caste development of social insects is unclear. In termites, JH acts as a central factor for caste development, especially for soldier differentiation, which involves two molts from workers via a presoldier stage. Increased JH titer in workers is needed for the presoldier molt, and the high JH titer is maintained throughout the presoldier period. Although presoldiers have the fundamental morphological features of soldiers, the nature of the cuticle is completely different from that of soldiers. We expected that JH signals via Met are involved in soldier-specific morphogenesis of the head and mandibles during soldier differentiation, especially in the presoldier period, in natural conditions. To test this hypothesis, we focused on soldier differentiation in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Met homolog (ZnMet) expression in heads increased just after the presoldier molt. This high expression was reduced by ZnMet double stranded (dsRNA) injection before the presoldier molt. Although this treatment did not cause any morphological changes in presoldiers, it caused strong effects on soldiers, their mandibles being significantly shorter and head capsules smaller than those of control soldiers. Injection of ZnMet dsRNA throughout the presoldier stage did not affect the formation of soldier morphology, including cuticle formation. These results suggested that the rapid increase in ZnMet expression and subsequent activation of JH signaling just after the presoldier molt are needed for the formation of soldier-specific weapons. Therefore, besides its established role in insect metamorphosis, the JH receptor signaling also underlies soldier development in termites.

High juvenile hormone titre and abdominal activation of JH signalling may induce reproduction of termite neotenics

Termite castes are a key example of polyphenism, in which reproductive division of labour is clearly seen in colonies. The reproductive castes in termites include primary and neotenic reproductives; primary reproductives found a new colony whereas neotenics succeed them in the reproductive role when the primary reproductives die or become senescent. Neotenics usually differentiate from nymphs or workers by developing functional gonads while retaining juvenile characteristics; however, the developmental mechanism during neotenic differentiation remains poorly understood. Juvenile hormone (JH) mediates a number of aspects of developmental regulation in caste differentiation in termites. In the present study we quantified JH titres in neotenic reproductives of Reticulitermes speratus, and compared these with other developmental stages. In addition, expression changes in JH signalling gene homologues (Methoprene-tolerant [Met], Krüppel-homolog1, Broad-Complex) in the head, thorax and abdomen were investigated during neotenic differentiation. Finally, we examined the function of Met in reproduction of neotenics by RNA interference (RNAi). Our results showed that the JH titres of neotenics were significantly higher than those of nymphs and workers. JH signalling genes were highly expressed in neotenic abdomens, compared with those in workers and nymphs. Met RNAi resulted in the inhibition of vitellogenin gene expression in newly moulted neotenics. These results suggest that the fertility of neotenics might be controlled by a large increase of JH titres and body-part-specific activation of JH signalling pathways.

Genomic and functional characterization of a methoprene-tolerant gene in the kissing-bug Rhodnius prolixus

Metamorphosis, which depends upon a fine balance between two groups of lipid-soluble hormones such as juvenile hormones (JHs) and ecdysteroids, is an important feature in insect evolution. While it is clear that the onset of metamorphosis depends on the decrease of JH levels, the way in which these hormones exert their activities is not fully understood in Triatominae species. The discovery of a Drosophila melanogaster mutant resistant to the treatment with the JH analog methoprene, led finally to the description of the methoprene-tolerant gene in Tribolium castaneum (TcMet) as a putative JH receptor. Here we present the genomic and functional characterization of an ortholog of the methoprene-tolerant gene in the hemimetabolous insect Rhodnius prolixus (RpMet). The analysis of the R. prolixus gene showed that the exonic structure is different from that described for holometabolous species, although all the critical protein motifs are well conserved. Expression analysis showed the presence of RpMet mRNA in all the tested tissues: ovary, testis, rectum, Malpighian tubules and salivary glands. When juvenile individuals were treated with RpMet specific double strand RNA (dsRNA), we observed abnormal molting events that resulted in individuals with morphological alterations (adultoids). Similarly, treatment of newly emerged fed females with dsRNA resulted in an abnormal development of the ovaries, with eggs revealing anomalies in size and accumulation of yolk, as well as a decrease in the amount of heme-binding protein. Altogether, our results validate that RpMet is involved in the transduction of JH signaling, controlling metamorphosis and reproduction in R. prolixus.