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

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.

Lipids in Insect Reproduction: Where, How, and Why

Modern insects have inhabited the earth for hundreds of millions of years, and part of their successful adaptation lies in their many reproductive strategies. Insect reproduction is linked to a high metabolic rate that provides viable eggs in a relatively short time. In this context, an accurate interplay between the endocrine system and the nutrients synthetized and metabolized is essential to produce healthy offspring. Lipids guarantee the metabolic energy needed for egg formation and represent the main energy source consumed during embryogenesis. Lipids availability is tightly regulated by a complex network of endocrine signals primarily controlled by the central nervous system (CNS) and associated endocrine glands, the corpora allata (CA) and corpora cardiaca (CC). This endocrine axis provides hormones and neuropeptides that significatively affect tissues closely involved in successful reproduction: the fat body, which is the metabolic center supplying the lipid resources and energy demanded in egg formation, and the ovaries, where the developing oocytes recruit lipids that will be used for optimal embryogenesis. The post-genomic era and the availability of modern experimental approaches have advanced our understanding of many processes involved in lipid homeostasis; therefore, it is crucial to integrate the findings of recent years into the knowledge already acquired in the last decades. The present chapter is devoted to reviewing major recent contributions made in elucidating the impact of the CNS/CA/CC-fat body-ovary axis on lipid metabolism in the context of insect reproduction, highlighting areas of fruitful research.

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.

Juvenile hormone receptor Methoprene tolerant: Functions and applications

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

CRISPR/Cas9-mediated methoprene-tolerant 1 knockout results in precocious metamorphosis of beet armyworm (Spodoptera exigua) only at the late larval stage

Juvenile hormone (JH) controls almost every aspect of an insect, especially metamorphosis. Since RNA interference works on transcripts and is often insufficient in Lepidoptera, how JH affects larval development in these insects is not well studied. Using the CRISPR/Cas9 technique, we knocked out Spodoptera exigua methoprene-tolerant 1 (SeMet1) gene of beet armyworm by modifying two sites in the coding region. However, SeMet1 knockout did not affect egg hatch rate or larval development at L1-L3 stages. In contrast to the consistent five larval instars of the control group, L4 SeMet1 mutants began to show signs of precocious metamorphosis, that is, small patches of pupal cuticle. Most L4 and all L5 SeMet1 mutants died for failing to shed their mosaic cuticles. RNA-seq indicated that most genes encoding pupal cuticle proteins and chitinase genes were altered in SeMet1 mutant L4 larvae. SeKr-h1, a key transcription factor in JH action was significantly down-regulated in L3-L5 larvae, while SeBR-C, a pupal indicator was only upregulated in L4-L5 larvae. These results suggested that S. exigua larvae may initially develop independently of JH, and involve SeMet1 in transducing JH signalling, leading to controlled larval metamorphosis at the late larval stage. We believe our findings will enhance better understanding of JH regulation of larval development.

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.

Effect of Insulin Receptor on Juvenile Hormone Signal and Fecundity in Spodoptera litura (F.)

Simple Summary

The tobacco cutworm, Spodoptera litura (F.), exemplifies strong reproductive capacities and damages many agricultural crops. The insulin signaling pathway is known as a key determinant of female reproduction in insects. However, the detailed molecular mechanisms in these processes are poorly studied. Here, we injected bovine insulin into the newly emerged moth, resulting in gene expression changes in the insulin pathway, while knockdown of SlInR caused an inverse gene expression change involved in the insulin pathway. Further studies indicated that the content of JH-III, Vg, total proteins and triacylgycerol could be suppressed by SlInR dsRNA injection. Furthermore, stunted ovaries and lower fecundity were observed by RNAi. Our studies indicated that SlInR plays a key role in JH-III synthesis and the ovarian development in S. litura.

Abstract

Insulin signaling can regulate various physiological functions, such as energy metabolism and reproduction and so on, in many insects, including mosquito and locust. However, the molecular mechanism of this physiological process remains elusive. The tobacco cutworm, Spodoptera litura, is one of the most important pests of agricultural crops around the world. In this study, phosphoinositide 3-kinase (SlPI3K), protein kinase B (SlAKT), target of rapamycin (SlTOR), ribosomal protein S6 kinase (SlS6K) and transcription factor cAMP-response element binding protein (SlCREB) genes, except transcription factor forkhead box class O (SlFoxO), can be activated by bovine insulin injection. Then, we studied the influence of the insulin receptor gene (SlInR) on the reproduction of S. litura using RNA interference technology. qRT-PCR analysis revealed that SlInR was most abundant in the head. The SlPI3K, SlAKT, SlTOR, SlS6K and SlCREB genes were decreased, except SlFoxO, after the SlInR gene knockdown. Further studies revealed that the expression of vitellogenin mRNA and protein, Methoprene-tolerant gene (SlMet), could be down-regulated by the injection of dsRNA of SlInR significantly. Furthermore, a depletion in the insulin receptor by RNAi significantly decreased the content of juvenile hormone III (JH-III), total proteins and triacylgycerol. These changes indicated that a lack of SlInR could impair ovarian development and decrease fecundity in S. litura. Our studies contribute to a comprehensive insight into reproduction, regulated by insulin and the juvenile hormone signaling pathway through nutrition, and a provide theoretical basis for the reproduction process in pest insects.

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.

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.

Involvement of Methoprene-tolerant and Krüppel homolog 1 in juvenile hormone-signaling regulating the maturation of male accessory glands in the moth Agrotis ipsilon

Male accessory glands (MAGs) produce seminal fluid proteins that are essential for the fertility and also influence the reproductive physiology and behavior of mated females. In many insect species, and especially in the moth Agrotis ipsilon, juvenile hormone (JH) promotes the maturation of the MAGs but the underlying molecular mechanisms in this hormonal regulation are not yet well identified. Here, we examined the role of the JH receptor, Methoprene-tolerant (Met) and the JH-inducible transcription factor, Krüppel homolog 1 (Kr-h1) in transmitting the JH signal that upregulates the growth and synthetic activity of the MAGs in A. ipsilon. We cloned two full length cDNAs encoding Met1 and Met2 which are co-expressed with Kr-h1 in the MAGs where their expression levels increase with age in parallel with the length and protein content of the MAGs. RNAi-mediated knockdown of either Met1, Met2, or Kr-h1 resulted in reduced MAG length and protein amount. Moreover, injection of JH-II into newly emerged adult males induced the transcription of Met1, Met2 and Kr-h1 associated to an increase in the length and protein content of the MAGs. By contrast, JH deficiency decreased Met1, Met2 and Kr-h1 mRNA levels as well as the length and protein reserves of the MAGs of allatectomized old males and these declines were partly compensated by a combined injection of JH-II in operated males. Taken together, our results highlighted an involvement of the JH-Met-Kr-h1 signaling pathway in the development and secretory activity of the MAGs in A. ipsilon.