Juvenile hormone regulates vitellogenin gene expression through insulin-like peptide signaling pathway in the red flour beetle, Tribolium castaneum

The Integrative Physiology of Hormone Signaling: Insights from Insect Models

Hormones orchestrate virtually all physiological processes in animals and enable them to adjust internal responses to meet diverse physiological demands. Studies in both vertebrates and insects have uncovered many novel hormones and dissected the physiological mechanisms they regulate, demonstrating a remarkable conservation in endocrine signaling across the tree of life. In this review, we focus on recent advances in insect research, which have provided a more integrative view of the conserved interorgan communication networks that control physiology. These new insights have been driven by experimental advantages inherent to insects, which over the past decades have aligned with new technologies and sophisticated genetic tools, to transform insect genetic models into a powerful testbed for posing new questions and exploring longstanding issues in endocrine research. Here, we illustrate how insect studies have addressed classic questions in three main areas, hormonal control of growth and development, neuroendocrine regulation of ion and water balance, and hormonal regulation of behavior and metabolism, and how these discoveries have illuminated our fundamental understanding of endocrine signaling in animals. The application of integrative physiology in insect systems to questions in endocrinology and physiology is expanding and is poised to be a crucible of discovery, revealing fundamental mechanisms of hormonal regulation that underlie animal adaptations to their environments.

PacBio-based de novo transcriptomics of the coconut rhinoceros beetle Oryctes rhinoceros identifies physiologically important full-length genes and sheds insights into the molecular relationship (chitin synthase) between Scarabaeidae (Coleoptera) and Hymenoptera

The sparser molecular data in non-model insects such as Oryctes rhinoceros prompted us to investigate and identify its physiologically important genes using the novel PacBio Iso-Seq Sequel II platform with single-molecule real-time (SMRT) technology. SMRT library was prepared from various tissues and sequenced. In total, 16,916,297 subreads clustered into 17,547 contigs which collapsed to form 8708 full-length sequences out of which 4352 functionally annotated transcripts were identified. Genes involved in innate immunity, growth and development, hormonal regulation, cellular process, peritrophic membrane, melanogenesis, integument, circulation, cuticle formation, glycan metabolism, etc., were identified. The transcripts' orthologues were identified predominantly in Coleoptera and Hymenoptera in which chitin synthase (CHS), toll, haemocytin, serine protease/limulus clotting factor c, vitellogenin and trehalose transporter exhibited significant molecular relationships between these two insect orders. Chitin synthase 8 (CHS-8) found in ant has been identified for the first time in the order Coleoptera. (O. rhinoceros) at the translational level and projected a potential to explore evolution (horizontal gene transfer) of CHS in insects. The findings will bridge the molecular data between the genome and transcriptome of O. rhinoceros, thus helping develop molecular targets for its control and management.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-025-04348-9.

The role of insulin receptor InR in photoperiod-regulated reproductive diapause of Chrysoperla nipponensis

Insects usually diapause, a process regulated by hormonal signals as an adaptive mechanism developed through long-term evolution to survive unfavorable environmental conditions. Chrysoperla nipponensis is classified as a photoperiod-sensitive insect. Treatments with short-day (SD) and long-day (LD) conditions have distinct effects on ovarian development and lipid accumulation in adults, with SD condition inducing diapause. Injecting bovine insulin promoted ovarian development and egg formation in diapause females, while injecting insulin receptor induced diapause-like traits in reproductive females. This study investigate the biological function of insulin signaling in the reproductive diapause of females of C. nipponensis. Under SD treatment the mRNA expression level of InR1 and InR2, as well as the protein expression level of InR1 were significantly reduced. This reduction led to stagnant ovarian development, increased adipose tissue mass, and a significant rise in triglyceride (TG) content. Silencing InR1 under LD conditions resulted in halted ovarian development and enhanced lipid accumulation, with the expression levels of Akt, Kr-h1, and Vg significantly decreased mirroring those observed under SD conditions. Interestingly, silencing InR2 under LD condition did not affect ovarian development. Furthermore, transcriptome analysis identified six genes (Akt, PkN, Skp2, CycB3, BTrC, and AurkA) associated with reproductive regulation and eight genes (FadΔ11, EchA, EcI, Ugts (2A3, 1-9), AR, Gpdh and Cbr) linked to lipid metabolism, all of which are involved in InR1 mediated regulation of C. nipponensis reproduction.

Transcriptome analysis identifies key genes in juvenile hormone and ecdysteroid signaling pathways and their roles in regulating reproductive system development of adult Monochamus saltuarius

Monochamus saltuarius is an important vector of pinewood nematode in Eurasia with a high reproductive capacity. Endocrine hormones play a key role in insect reproduction. Understanding the mechanism of internal regulation can provide targets for pest control. However, this type of research on M. saltuarius remain limited. Our study constructed transcriptome of the internal reproductive systems in male and female M. saltuarius across three development stages. Interference experiments targeting the MSALMet1 and exploring its critical role in reproduction. Transcriptome results revealed that 42 genes related to the juvenile hormone and ecdysteroid pathways were identified. Among them, 12 genes were significantly enriched in reproduction-related pathways, and the expression patterns of 14 genes aligned with the developmental trend of the internal reproductive system, suggesting that they may play a regulatory role in reproductive processes. Furthermore, protein-protein interaction networks elucidated the complex interactions among these genes, shedding light on their diverse functions. Notably, bioinformatics analysis and interference experiments revealed that MSALMet1 having the profound effect on reproductive system development in both sexes. These findings highlight the critical role of endocrine-related genes in regulating reproductive development and provide a theoretical foundation for regulating reproduction at molecular level, potentially contributing to M. saltuarius population control.

Diet influence on male sexual maturation through interplay between insulin signaling and juvenile hormone in insects

In animals, sexual maturation coincides with the development of sexual behaviors and reproductive system. These developmental events are influenced by diet and governed by endocrine signals. Here, for the first time in insects, we explored functional links between nutrition and juvenile hormone (JH) in the male reproductive physiology through the insulin signaling pathway (ISP) acting as a transducer of nutritional signals. We turned to the male moth Agrotis ipsilon for which sexual maturation, including accessory sex glands (ASGs) development concomitantly with antennal lobes (ALs) maturation for female sex pheromone processing and display of sexual behavior, is known to be JH- and diet-dependent. Indeed, a diet rich in sugars with sodium was previously shown to accelerate sexual maturation, which was achieved from the third day of adult life. In this study, we demonstrated that such a diet raised i) the expression of JH signaling actors (Methoprene-tolerant, Taiman, and Krüppel homolog 1) in ALs and ASGs, ii) the biosynthesis and circulating levels of JH, and iii) the expression of both insulin receptor (InR) and insulin-like peptides (ILPs) in corpora allata (CAs) and brain respectively. Insulin injection raised JH biosynthesis following increased HMG-CoA reductase expression in CAs; opposite effects were induced in InR-deficient males. Thus, we highlighted that promoting effects of a diet composed of sugars with sodium on male sexual maturation results from an early induction of ISP causing an increase in JH biosynthesis followed by a potentiation of JH actions on the development of ASGs and ALs in A. ipsilon.

Functions of Insulin-like Peptide Genes (CsILP1 and CsILP2) in Female Reproduction of the Predatory Ladybird Coccinella septempunctata (Coleoptera: Coccinellidae)

Insulin-like peptides (ILPs) are important peptide hormones in insects, particularly involved in regulating physiological processes such as growth, development, and reproduction. However, the specific roles of ILPs in the reproduction of natural enemy insects remain unknown. In this study, two ILP genes, CsILP1 and CsILP2, were cloned and their functions were analyzed in female Coccinella septempunctata L. (Coleoptera: Coccinellidae). The open reading frames (ORFs) of CsILP1 and CsILP2 were 384 bp and 357 bp, respectively. The expression of CsILP1 increased on the 6th day after eclosion, reaching its peak on the 12th day, while CsILP2 levels showed a significant increase on the 6th day and then stabilized. In different tissues, CsILP1 was highly expressed in ovaries, while CsILP2 predominated in elytra. Injection of dsRNA targeting CsILP1 and CsILP2 resulted in the down-regulation of insulin pathway genes. The relative expression of ovarian development-related genes Vasa, G2/M, and Vg was reduced by 82.50%, 89.55%. and 96.98% in dsCsILP1-treated females, and by 42.55%, 91.36%, and 55.63% in dsCsILP2-treated females. Furthermore, substantial decreases in 14-day fecundity were observed, with reductions of 89.99% for dsCsILP1 and 83.45% for dsCsILP2. These results confirm the regulatory functions of CsILP1 and CsILP2 in female C. septempunctata reproduction.

The deficiency of acetylcholinesterase gene in Aleuroglyphus ovatus increases its susceptibility to phoxim and natural pyrethrins and inhibits its reproduction

Acetylcholinesterase (AChE), an essential neurotransmitter hydrolase, is targeted by organophosphorus and carbamate pesticides, and its number varies among species. In Aleuroglyphus ovatus, a pest mite that endangers health and economy, Aoace1 and Aoace2 have been identified encoding 590 and 460 amino acids, respectively, with characteristic structures, including catalytic triads, oxyanion holes, acyl pockets, peripheral anion, and catalytic anion sites. Phylogenetic analysis reveals distinct clusters for each gene. Expression patterns indicate that Aoace1 predominates in eggs, while Aoace2 is substantially expressed in adults. Experiments on the response of the Aoace genes to phoxim and natural pyrethrins showed that except for the Aoace2 gene responded to natural pyrethrins, all the experimental groups showed a significant increase at LC30 agent concentration. RNA interference with Aoace1 and Aoace2 significantly reduced AChE activity, and increased mortality with LC30 concentrations of phoxim by 15.8 % and 31.5 %, while increased mortality with LC30 concentrations of natural pyrethrins by 43.4 % and 40.4 %, respectively. Knockdown of ace gene significantly decreased fecundity and vitellogenin gene expression. These findings suggest that Aoace1 and Aoace2 are involved in cholinergic and non-cholinergic functions, with Aoace2 being more influential, offering new insights for A. ovatus control strategies.

miR-276 and miR-182013-5p modulate insect metamorphosis and reproduction via dually regulating juvenile hormone acid methyltransferase

Juvenile hormone (JH) represses insect metamorphosis and stimulates reproduction. JH titers are generally low in juveniles, drop to a nadir during metamorphosis, increase after eclosion and peak in vitellogenic phase. We found that Jhamt, a rate-limiting enzyme in JH biosynthesis, mirrors JH titer patterns in the migratory locust. Knocking down Jhamt reduced JH titers, led to precocious nymphal ecdysis, metamorphosis and impaired vitellogenesis. Jhamt is negatively regulated by miR-276 and positively by miR-182013-5p. miR-276 is abundant in late nymphal but low in adults, while miR-182013-5p shows the opposite pattern. In nymphs, miR-276 binds more to Jhamt, while in adults, miR-182013-5p dominates. Functionally, miR-276 reduced Jhamt and JH levels, shortening nymphal development and inhibiting Vg expression. Conversely, miR-182013-5p increased Jhamt and JH levels, prolonging nymphal development and enhancing Vg expression. Our findings identify miR-276 and miR-182013-5p as dual regulators in JH biosynthesis, acting as "brake" and "accelerator," respectively. This study provides new insights into JH titer fluctuations and miRNA regulation in insect metamorphosis and reproduction.

Regulation Roles of Juvenile Hormone Epoxide Hydrolase Gene 2 in the Female River Prawn Macrobrachium nipponense Reproductive Process

In this study, we investigated the regulatory roles of the juvenile hormone epoxide hydrolase (JHEH) gene in the reproductive process of female Macrobrachium nipponense. Its total cDNA length was 1848 bp, encoding for 460 amino acids. It contained conserved domains typical of epoxide hydrolases, such as the Abhydrolase family domain, the EHN epoxide hydrolase superfamily domain, and the "WWG" and "HGWP" motifs. The qPCR results showed that the expression of Mn-JHEH was the highest in hepatopancreas. Mn-JHEH was expressed at all stages of the embryonic and larval stages. The expression of Mn-JHEH at different developmental periods of the ovary was positively correlated with ovarian maturation. In situ hybridization showed that it was mainly located in the cytoplasmic membrane and nucleus of oocytes. The RNA interference technique was used to study the role of Mn-JHEH in the process of ovarian maturation. The knockdown of Mn-JHEH with dsRNA in the experimental group resulted in a significant decrease in the percentage of ovaries exceeding stage O-III and the gonadal index compared with the control group. On day 14 (the second molt), the molt frequency was significantly higher in the control group than in the experimental group. The results showed that Mn-JHEH played an important role in ovarian maturation and molting.

Akt regulates the fertility of Coridius chinensis by insulin signaling pathway

Akt (also known as protein kinase B) belongs to the multifunctional serine/threonine kinase family and is an important component of the insulin signaling pathway that plays a key role in many biological processes such as cell growth, proliferation, and survival. However, few studies have reported the effect of Akt on reproduction in Hemiptera. In this study, we cloned and characterized the Akt gene from Coridius chinensis (CcAkt). The open reading frame of CcAkt has a length of 1,563 bp and encodes 520 amino acids. It has a conserved pleckstrin homology domain (PH), catalytic domain of serine/threonine protein kinases (S_TKc), and extension of Ser/Thr-type protein kinases (S_TK_X). Phylogenetic analysis showed that CcAkt and HhAkt of Halyomorpha halys had the highest similarity. Analysis of temporal and spatial expression patterns revealed that CcAkt is expressed throughout development and in various tissues of C. chinensis adults. CcAkt was highly expressed in the female adult and the fourth-instar nymph, as well as in the testis and ovary of C. chinensis. Injection of bovine insulin and methoprene induced the CcAkt expression, whereas that of 20-hydroxyecdysone significantly reduced the CcAkt expression. These three hormones, however, induced the expression of vitellogenin (Vg) and vitellogenin receptor (VgR). In unmated females, knockdown of CcAkt resulted in decreased expression of CcVg and CcVgR, stunted the development of the ovarioles, decreased the number of eggs and hatching rate. These findings from RNA interference experiment suggested that CcAkt may be involved in regulating the reproduction of C. chinensis.

Dual roles of methoprene-tolerant gene TaMet in male molting and female reproduction of the tomato leafminer, Tuta absoluta (meyrick)

The tomato leafminer (Tuta absoluta) is a highly destructive global quarantine pest. The methoprene-tolerant (Met) protein, a member of the bHLH/PAS family of transcription factors, forms complexes with other family members to transduce the juvenile hormone signal, which regulates insect growth and development. However, the functions of the TaMet gene have rarely been studied in T. absoluta. Herein, we investigated the significance of TaMet in T. absoluta. Spatiotemporal expression analysis revealed that TaMet exhibited comparable expression patterns in males and females, with high expression levels during the early pupal and early adult stages. TaMet was predominantly expressed in the female ovary and male wing. TaMet knockdown impaired ovarian development in female adults, causing irregular arrangement and increased spacing of the egg epithelial cells in the ovary. Silencing TaMet also led to a 67.25% reduction in female spawning and a 67.21% decrease in the offspring hatching rate. Furthermore, the vitellogenin content was significantly diminished, and the expression levels of vitellogenin (Vg) and vitellogenin receptor (VgR) genes were significantly downregulated. In contrast, silencing TaMet in 3-day-old male pupae resulted in an 80% mortality rate and various phenotypic abnormalities, including body melanism, molting defects, and wing deformities. Moreover, the expression levels of wing development and chitin metabolism genes decreased significantly after knocking down TaMet. Our results indicate that TaMet plays a significant dual role in male molting and female reproduction of T. absoluta.

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.

Gene redundancy and gene compensation of insulin-like peptides in the oocyte development of bean beetle

Bean beetle (Callosobruchus maculatus) exhibits clear phenotypic plasticity depending on population density; However, the underlying molecular mechanism remains unknown. Compared to low-density individuals, high-density individuals showed a faster terminal oocyte maturity rate. Four insulin-like peptide (ILP) genes were identified in the bean beetle, which had higher expression levels in the head than in the thorax and abdomen. The population density could regulate the expression levels of CmILP1-3, CmILP2-3, and CmILP1 as well as CmILP3 in the head, thorax, and abdomen, respectively. RNA interference results showed that each CmILP could regulate terminal oocyte maturity rate, indicating that there was functional redundancy among CmILPs. Silencing each CmILP could lead to down-regulation of some other CmILPs, however, CmILP3 was up-regulated in the abdomen after silencing CmILP1 or CmILP2. Compared to single gene silencing, silencing CmILP3 with CmILP1 or CmILP2 at the same time led to more serious retardation in oocyte development, suggesting CmILP3 could be up-regulated to functionally compensate for the down-regulation of CmILP1 and CmILP2. In conclusion, population density-dependent plasticity in terminal oocyte maturity rate of bean beetle was regulated by CmILPs, which exhibited gene redundancy and gene compensation.

Expression and function of the vitellogenin receptor in the hypopharyngeal glands of the honey bee Apis mellifera (Hymenoptera: Apidae) workers

The vitellogenin receptor (VgR) is essential for the uptake and transport of the yolk precursor, vitellogenin (Vg). Vg is synthesized in the fat body, released in the hemolymph, and absorbed in the ovaries, via receptor-mediated endocytosis. Besides its important role in the reproductive pathway, Vg occurs in nonreproductive worker honey bee, suggesting its participation in other pathways. The objective was to verify if the VgR occurs in the hypopharyngeal glands of Apis mellifera workers and how Vg is internalized by these cells. VgR occurrence in the hypopharyngeal glands was evaluated by qPCR analyses of VgR and immunohistochemistry in workers with different tasks. The VgR gene is expressed in the hypopharyngeal glands of workers with higher transcript levels in nurse honey bees. VgR is more expressed in 11-day-old workers from queenright colonies, compared to orphan ones. Nurse workers with developed hypopharyngeal glands present higher VgR transcripts than those with poorly developed glands. The immunohistochemistry results showed the co-localization of Vg, VgR and clathrin (protein that plays a major role in the formation of coated vesicles in endocytosis) in the hypopharyngeal glands, suggesting receptor-mediated endocytosis. The results demonstrate that VgR performs the transport of Vg to the hypopharyngeal glands, supporting the Ovary Ground Plan Hypothesis and contributing to the understanding of the role of this gland in the social context of honey bees.

Molecular and structural analysis of Hdh-MIRP3 and its impact on reproductive regulation in female Pacific abalone, Haliotis discus hannai

Molluscan insulin-related peptides (MIRP) play a crucial role in various biological processes, including reproduction and larval development in mollusk species. To investigate the involvement of MIRP in the ovarian development of Pacific abalone (Haliotis discus hannai), the Hdh-MIRP3 was cloned from cerebral ganglion (CG). Hdh-MIRP3 cDNA was 993 bp long, encoded a 13.22 kDa peptide, comprising 118 amino acids. Fluorescence in situ hybridization confirmed the localization of Hdh-MIRP3 in the CG and ovary. Molecular docking revealed that Hdh-MIRP3 binds to the N-terminal region of Hdh-IRP-R. Tissue expression analysis showed the highest Hdh-MIRP3 expression in the CG, followed by ovarian tissue. Hdh-MIRP3 expression was significantly upregulated in the CG and ovary during the ripe stage of seasonal ovarian development and in effective accumulative temperature conditioned abalone. Furthermore, siRNA silencing of Hdh-MIRP3 significantly downregulated the expression of four reproduction-related genes, including Hdh-GnRH, Hdh-GnRH-R, Hdh-IRP-R, and Hdh-VTG in both the CG and ovary, and Hdh-MIRP3 as well. These results indicate that Hdh-MIRP3 acts as a regulator of ovarian development in Pacific abalone. Additionally, expression analysis indicated that Hdh-MIRP3 plays a role in embryonic and larval development. Overall, the present findings elucidate the role of Hdh-MIRP3 in reproductive development in female Pacific abalone.

MicroR-9c-5p and novel-mir50 co-target Akt to regulate Lasioderma serricorne reproduction

High fecundity is a common characteristic of insect pests which increases the difficulty of population control. Serine/threonine kinase Akt is an indispensable component of the insulin signaling pathway. Silencing of LsAkt severely hinders reproduction in Lasioderma serricorne, a stored product insect pest. However, the post-transcriptional pathway of LsAkt in L. serricorne remains unknown. This study identified 2 binding sites of miR-9c-5p and novel-mir50 in the coding sequences of LsAkt. The expression profiles of 2 microRNAs (miRNAs) and LsAkt displayed an opposite pattern during the adult stages. Luciferase reporter assay showed that novel-mir50 and miR-9c-5p could downregulate the expression of LsAkt. Overexpression of miR-9c-5p and novel-mir50 by injection of mimics inhibited the expression of LsAkt and reduced oviposition, decreased egg hatchability, and blocked ovarian development. It also decreased the expression of genes involved in ovarian development (LsVg and LsVgR) and the nutritional signaling pathway (LsTOR, LsS6K, and Ls4EBP), and reduced the phosphorylation of Akt. Conversely, injection of miR-9c-5p and novel-mir50 inhibitors induced the expressions of LsAkt, LsVg, LsVgR, LsTOR, LsS6K, and Ls4EBP, enhanced Akt phosphorylation level, and accelerated ovarian development. Injection of bovine insulin downregulated the expression of miR-9c-5p and novel-mir50 and upregulated the LsAkt expression. It also rescued the reproductive development defects associated with miR-9c-5p/novel-mir50 overexpression, forming a positive regulatory loop of insulin signaling. These results indicate that miR-9c-5p/novel-mir50 regulates the female reproduction of L. serricorne by targeting Akt in response to insulin signaling. The data also demonstrate the effects of the insulin/miRNA/Akt regulatory axis in insect reproduction.

Regulation of forkhead box O transcription factor by insulin signaling pathway controls the reproductive diapause of the lady beetle, Coccinella septempunctata

In biological control programs, knowledge about diapause regulation in natural enemy insects provides important insight for improving long-term storage, transportation, and field adoption of these biological control agents. As a natural predator of agricultural pests, the lady beetle Coccinella septempunctata has been commercially mass-cultured and widely employed in pest management. In some insects, insulin signaling, in conjunction with the downstream transcription factor Forkhead box O (FoxO), are master regulators of multiple physiological processes involved in diapause, but it is unclear whether insulin signaling and FoxO affect the diapause of C. septempunctata. In this study, we use a combination of approaches to demonstrate that insulin signaling and FoxO mediate the diapause response in C. septempunctata. In diapausing beetles, application of exogenous insulin and knocking down expression of CsFoxo with RNA interference (RNAi) both rescued beetles from developmental arrest. In non-diapausing beetles, knocking down expression of the insulin receptor (CsInR) with RNA interference (RNAi) arrested ovarian development and decreased juvenile hormone (JH) content to levels comparable to the diapause state. Taken together, these results suggest that a shutdown of insulin signaling prompts the activation of the downstream FoxO gene, leading to the diapause phenotype.

Comparative 4D Label-Free Quantitative Proteomic Analysis of Bombus terrestris Provides Insights into Proteins and Processes Associated with Diapause

Diapause, an adaptative strategy for survival under harsh conditions, is a dynamic multi-stage process. Bombus terrestris, an important agricultural pollinator, is declining in the wild, but artificial breeding is possible by imitating natural conditions. Mated queen bees enter reproductive diapause in winter and recover in spring, but the regulatory mechanisms remain unclear. Herein, we conducted a comparative 4D label-free proteomic analysis of queen bees during artificial breeding at seven timepoints, including pre-diapause, diapause, and post-diapause stages. Through bioinformatics analysis of proteomic and detection of substance content changes, our results found that, during pre-diapause stages, queen bees had active mitochondria with high levels of oxidative phosphorylation, high body weight, and glycogen and TAG content, all of which support energy consumption during subsequent diapause. During diapause stages, body weight and water content were decreased but glycerol increased, contributing to cold resistance. Dopamine content, immune defense, and protein phosphorylation were elevated, while fat metabolism, protein export, cell communication, signal transduction, and hydrolase activity decreased. Following diapause termination, JH titer, water, fatty acid, and pyruvate levels increased, catabolism, synaptic transmission, and insulin signaling were stimulated, ribosome and cell cycle proteins were upregulated, and cell proliferation was accelerated. Meanwhile, TAG and glycogen content decreased, and ovaries gradually developed. These findings illuminate changes occurring in queen bees at different diapause stages during commercial production.

Functional characterisation of Fe (II) and 2OG-dependent dioxygenase TcALKBH4 in the red flour beetle, Tribolium castaneum

Alpha-ketoglutarate-dependent dioxygenase ALKB homologue 4 (ALKBH4) is a member of the Fe (II) and 2-oxoglutarate-dependent ALKB homologue family that plays important roles in epigenetic regulation by alkyl lesions removal in mammals. However, the roles of ALKBH4 in insects are not clear. Here, TcALKBH4 was cloned and functionally characterised in Tribolium castaneum. Temporal expression revealed that TcALKBH4 was highly expressed in early embryos and early pupae. Spatial expression showed that TcALKBH4 was highly expressed in the adult testis, and followed by the ovary. RNA interference targeting TcALKBH4 at different developmental stages in T. castaneum led to apparent phenotypes including the failure of development in larvae, the reduction of food intake and the deficiency of fertility in adult. However, further dot blot analyses showed that TcALKBH4 RNAi does not seem to influence 6 mA levels in vivo. qRT-PCR was used to further explore the underlying molecular mechanisms; the result showed that TcALKBH4 mediates the development of larvae possibly through 20E signalling pathway, and the fertility of female and male adult might be regulated by the expression of vitellogenesis and JH signalling pathway, respectively. Altogether, these findings will provide new insights into the potential function of ALKBH4 in insects.

The serine/threonine kinase Akt gene affects fecundity by reducing Juvenile hormone synthesis in Liposcelis entomophila (Enderlein)

The serine/threonine kinase Akt is an important component of the insulin signalling pathway (ISP) in regulating insect metabolism, growth, and reproduction. The psocid Liposcelis entomophila (Enderlein) is a distasteful stored products pest for its fecundity. However, the molecular mechanism of Akt that controls vitellogenesis and oviposition in L. entomophila remains obscure. In this study, the function of the Akt gene in the female reproduction of L. entomophila (designated as LeAkt) was characterized and investigated. LeAkt contains a 1587 bp open reading frame encoding a 529 amino acid protein that possesses a conserved Pleckstrin Homology domain (PH) and a Ser/Thr-type protein kinase (S_TKc) domain. The mRNA expression of LeAkt was the highest in female adult stages and peaked for 7-day female adults. In female adult tissues, LeAkt was highly expressed in the head and the ovary, indicating that LeAkt was closely correlated with female ovarian development. LeAkt transcription level was significantly suppressed by oral feeding on artificial diets mixed with dsRNA-LeAkt. RNAi-mediated silencing of LeAkt led to a severe inhibition of vitellogenein (Vg) expression and ovarian development, together with lower fecundity and hatchability compared to that of the normal feeding group, suggesting a critical role for LeAkt in L. entomophila reproduction. Further studies revealed that LeAkt silencing significantly decreased the mRNA levels of several signalling and biosynthetic genes in the juvenile hormone (JH) signalling pathway, such as methoprene-tolerant (LeMet), krüppel homolog 1 (LeKr-h1) and JH methyltransferase (LeJHAMT), leading to a severe inhibition of JH biosynthesis in L. entomophila female adults. These results suggested that LeAkt was affecting JH synthesis, thereby influencing Vg synthesis and ultimately L. entomophila reproduction.

A pattern recognition receptor C-type lectin TcCTL14 contributes to immune response and development in the red flour beetle, Tribolium castaneum

Evidence is accumulating that pattern recognition receptor (PRR) C-type lectins (CTL) play essential roles in recognition of pathogens. TcCTL14 (accession no. TC00871) contains the most domains among all CTL of Tribolium castaneum. Yet the biological function of TcCTL14 remains unclear. In this study, TcCTL14 exhibiting typical motif and domain of CTL was cloned from T. castaneum. The expression pattern analysis showed that TcCTL14 was highly expressed in late pupae and central nervous system, and was upregulated after treatment with Escherichia coli and Staphylococcus aureus, respectively. Analysis of binding affinity revealed that recombinant TcCTL14 not only could bind to lipopolysaccharide and peptidoglycan in a dose-dependent fashion, but possibly could bind to and agglutinate different bacteria in a Ca2+ -dependent fashion. Knockdown of TcCTL14 before injection with bacteria led to the downregulation of nuclear factor-κB transcription factors of Toll/IMD and 4 antimicrobial peptides. Knockdown of TcCTL14 also caused suppressed metamorphosis, reduced fecundity, and delayed embryogenesis of T. castaneum. Further observation discovered that knockdown of TcCTL14 inhibited the development of ovaries and embryos. The detection of signaling pathways revealed that TcCTL14 may be involved in metamorphosis and fecundity by impacting 20-hydroxyecdysone and vitellogenin, respectively. Overall, these results indicate that TcCTL14 may contribute to immune response by agglutination or regulating the expression of antimicrobial peptides by the Toll/IMD pathway, and is required for T. castaneum development including metamorphosis, fecundity, and embryogenesis. These findings will improve the functional cognition of PRR CTL in insects and provide the new strategy for pest control.

RNAi-Mediated Functional Analysis Reveals the Regulation of Oocyte Vitellogenesis by Ecdysone Signaling in Two Coleoptera Species

Coleoptera is the largest taxa of animals by far. The robust reproductive capacity is one of the main reasons for such domination. Successful female reproduction partially relies on effective vitellogenesis. However, the hormone regulation of vitellogenesis remains to be explored. In the present paper, in vitro culture of Leptinotarsa decemlineata 1-day-old adult fat bodies in the 20E-contained median did not activate juvenile hormone production and insulin-like peptide pathways, but significantly stimulated the expression of two LdVg genes, in a cycloheximide-dependent pattern. In vivo RNA interference (RNAi) of either ecdysone receptor (LdEcR) or ultraspiracle (Ldusp) by injection of corresponding dsRNA into 1-day-old female adults inhibited oocyte development, dramatically repressed the transcription of LdVg genes in fat bodies and of LdVgR in ovaries; application of JH into the LdEcR or Ldusp RNAi L. decemlineata females did not restore the oocyte development, partially rescued the decreased LdVg mRNA levels but over-compensated LdVgR expression levels. The same RNAi experiments were performed in another Coleoptera species, Henosepilachna vigintioctopunctata. Little yolk substances were seen in the misshapen oocytes in the HvEcR or Hvusp RNAi ovaries, in contrast to larger amounts of yolk granules in the normal oocytes. Correspondingly, the transcript levels of HvVg in the fat bodies and ovaries decreased significantly in the HvEcR and Hvusp RNAi samples. Our results here show that 20E signaling is indispensable in the activation of vitellogenesis in the developing oocytes of the two beetle species.

Identification and functional characterization of insulin-like peptides in a pine beetle

Insulin - including insulin-like peptides (ILPs), relaxins and insulin-like growth factors (IGFs) - is an evolutionarily conserved hormone in all metazoans It is involved in various physiological processes, such as metabolism, growth, reproduction, lifespan and stress resistance. However, there are no reports on the functional role of ILPs in the Chinese white pine beetle, Dendroctonus armandi. In this study, we have cloned and identified two ILP cDNAs in D. armandi. The expression levels of DaILP1 and DaILP2 were significantly changed in different developmental stages. Both ILPs were expressed mostly in the head and fat body. Moreover, starvation induces the reduction of ILP1 mRNA level in adults and larvae, while ILP2 only in larvae of D. armandi, respectively. Additionally, RNA-interference (RNAi) using double stranded RNA to knock down ILP1 and ILP2 reduced the mRNA levels of the target genes, and caused a significant reduction in body weight of D. armandi. Moreover, silencing ILP1 led to an increase of trehalose and glycogen and significantly enhanced starvation resistance in both adults and larvae. The results show that the ILP signaling pathway plays a significant role in growth and carbohydrate metabolism of D. armandi and may provide a potential molecular target for pest control.

MicroRNA miR-2765-3p regulates reproductive diapause by targeting FoxO in Galeruca daurica

The forkhead box O (FoxO), as a conserved transcription factor, plays an indispensable role in regulating insect diapause. However, how FoxO is regulated to control diapause in insects remains unknown. In this study, we discovered functional binding sites for miR-2765-3p in the 3' untranslated region of FoxO in Galeruca daurica. The luciferase reporter assay showed that miR-2765-3p targeted FoxO and suppressed its expression. The expression profiles of miR-2765-3p and FoxO displayed opposite patterns during the female developmental process. Overexpression of miR-2765-3p by the injection of the miR-2765-3p agomir into adult females reduced FoxO expression, leading to the suppression of lipid accumulation, promotion of ovarian development, and inhibition of reproductive diapause. This is similar to the phenotype that results from the depletion of FoxO by injecting dsFoxO into adult females. In addition, the repression of miR-2765-3p by injecting the miR-2765-3p antagomir increased the FoxO transcript level, leading to the stimulation of lipid accumulation, depression of ovarian development, and induction of reproductive diapause. A hormone injection assay showed that the juvenile hormone (JH) agonist (methoprene) upregulated miR-2765-3p and downregulated FoxO. Notably, injecting methoprene rescued ovarian development defects associated with miR-2765-3p inhibition. These findings indicate that the JH/miR-2765-3p/FoxO axis plays a vital role in the regulation of reproductive diapause in G. daurica.

Eukaryotic initiation factor 6 modulates the metamorphosis and reproduction of Tribolium castaneum

Eukaryotic initiation factor 6 (eIF6) is necessary for ribosome biogenesis and translation, but eIF6 has been poorly elucidated in insects. Phylogenetic analysis demonstrated that eIF6 originated from one ancestral gene among animals and exhibited specific duplication in Tribolium, yielding three homologues in Tribolium castaneum, eIF6, eIF6-like 1 (eIF6l1), and eIF6-like 2 (eIF6l2). It was found that eIF6 was highly expressed in the embryonic and early adult stages, eIF6l1 had peak expression at the adult stage, and eIF6l2 showed peak expression in late adults of T. castaneum. Tissue-specific analyses in late-stage larvae demonstrated that eIF6 was abundantly expressed in all tissues, while eIF6l1 and eIF6l2 had the highest expression in the gut and the lowest expression in the head of T. castaneum. Knockdown of eIF6 caused precocious pupation and eclosion, impaired ovary and testis development and completely repressed egg production. The expression levels of vitellogenin 1 (Vg1), Vg2 and Vg receptor (VgR) significantly decreased in ds-eIF6 females 5 days post-adult emergence. Silencing eIF6 activated ecdysteroid biosynthesis and juvenile hormone degradation but reduced the activity of insulin signalling in T. castaneum, which might mediate its roles in metamorphosis, reproduction and gene expression regulation. However, silence of eIF6l1 or eIF6l2 had no effects on metamorphosis and reproduction in T. castaneum. This study provides comprehensive information for eIF6 evolution and function in the insect.

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

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

Insulin peptides and their receptors regulate ovarian development and oviposition behavior in Diaphorina citri

Diaphorina citri is an important vector of Citrus Huanglongbing (HLB) disease. After feeding on young host plant shoots, the population of D. citri can increase significantly. Females also only lay eggs on young shoots. However, there are few studies on the mechanism of this phenomenon. Exogenous nutrient signals can affect the insulin signaling system of D. citri after feeding on young shoots. In this study, the expression of upstream factors DcILP1, DcILP2, and DcIR in the insulin signaling system of D. citri was upregulated after feeding on young shoots. After being silenced by RNA interference technology, the results showed that the number of oviposited eggs of D. citri was significantly decreased and the ovarian development was inhibited with severe vacuolation. In addition, detection using quantitative reverse transcription-polymerase chain reaction showed that the upstream regulatory gene DcRheb of the target of rapamycin (TOR) pathway and the downstream reproduction-related DcVg gene were also significantly downregulated. These results suggest that feeding upon young shoots may upregulate the expression levels of upstream factors DcILP1, DcILP2, and DcIR in the insulin signaling system. The signal will be through upregulating the expression of DcRheb, an upstream gene of the TOR signaling pathway. This in turn influences yolk metabolism, which eventually causes the ovaries of female D. citri to mature and therefore initiate oviposition behavior.

FoxO directly regulates the expression of odorant receptor genes to govern olfactory plasticity upon starvation in Bactrocera dorsalis

Under nutrient-poor conditions, animals must save energy by adjusting their behavior and physiology in order to survive. Although the impact of feeding state on olfactory sensory neuron activity has been well studied, the regulatory mechanisms underlying the transcriptional changes in odorant receptors (Ors) induced by feeding signals are seldom mentioned. Here, we showed that starvation could attenuate antennal responses of Bactrocera dorsalis toward multiple odorants, which could be reverted by sugar re-feeding, but not by a protein-rich diet. Using methyl eugenol (ME) as a paradigm, our study provided molecular evidence that Forkhead Box protein O (FoxO) can be expressed in antennal tissues to govern starvation-induced olfactory modifications by binding to the upstream regulatory regions of ME-responsive Ors and regulating their expressions. Since the consensus FoxO binding motif was also identified in other 17 Ors whose expression levels were also significantly altered upon FoxO knockdown and starvation, our data suggest that FoxO-dependent binding is likely a universal regulatory mechanism for Or genes during starvation and re-feeding cycles. Taken together, the FoxO-Ors axis elucidated in this study provides an improved understanding of how the insulin signaling pathway senses the feeding state and certain macronutrient composition to shape olfactory plasticity, allowing flies to dynamically alter chemosensory sensitivities toward different odors. Our study also highlights sugar as a satiety signal, which could increase ME-mediated trap efficiency in the field.

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.

Functional Analysis of a Juvenile Hormone Inducible Transcription Factor, Krüppel homolog 1, in the Bean Bug, Riptortus pedestris

Juvenile hormone (JH) has two major functions in insects, i.e., suppression of metamorphosis in the larval or nymphal stage and promotion of reproduction in the adult stage. Krüppel homolog 1 (Kr-h1), a C2H2 zinc-finger type transcription factor, is reported to act downstream of the JH receptor complex. In the present study, the function of Kr-h1 was examined in adults and nymphs of Riptortus pedestris by RNA interference (RNAi). After injection of adults with dsRNA of Kr-h1, the expression level of Kr-h1 was significantly decreased in the abdomen. Kr-h1 dsRNA-injection resulted in a lower proportion of individuals with developed ovaries, but the difference was not statistically significant. The transcript levels of cyanoprotein-α and vitellogenin-1, which are JH-inducible genes encoding yolk proteins, were not affected in the abdomen by Kr-h1 knockdown. Kr-h1 dsRNA-injection was effective for suppression of Kr-h1 expression in nymphs. Some Kr-h1 dsRNA-injected fifth (final) instar nymphs had morphological defects in the wing bud. Moreover, they had several adult morphological features, including ocelli in the head, connexivum in the abdomen, coloring of the dorsal abdomen, and genitals. The nymphs possessing adult features did not emerge as adults during 1 month. These results demonstrated that Kr-h1 is necessary for maintaining nymphal characters in R. pedestris. The function of Kr-h1 in ovarian development remains unclear in R. pedestris.

Choline acetyltransferase and vesicular acetylcholine transporter are required for metamorphosis, reproduction, and insecticide susceptibility in Tribolium castaneum

Choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT) are essential enzymes for synthesizing and transporting acetylcholine (ACh). But their functions in metamorphosis, reproduction, and the insecticide susceptibility were poorly understood in the insects. To address these issues, we identified the orthologues of chat and vacht in Tribolium castaneum. Spatiotemporal expression profiling showed Chat has the highest expression at the early adult stage, while vacht shows peak expression at the early larval stage. Both of them were highly expressed at the head of late adult. RNA interference (RNAi) of chat and vacht both led to a decrease in ACh content at the late larval stage. It is observed that chat knockdown severely affected larval development and pupal eclosion, but vacht RNAi only disrupted pupal eclosion. Further, parental RNAi of chat or vacht led to 35 % or 30 % reduction in fecundity, respectively, and knockdown of them completely inhibited egg hatchability. Further analysis has confirmed that both the reduction in fecundity and hatchability caused through the maternal specificity in T. castaneum. Moreover, the transcript levels of chat and vacht were elevated after carbofuran or dichlorvos treatment. Reduction of chat or vacht decreased the resistance to carbofuran and dichlorvos. This study provides the evidence for chat and vacht not only involved in development and reproduction of insects but also could as the potential targets of insecticides.

RNA interference knockdown of insulin receptor inhibits ovarian development in Chilo suppressalis

BACKGROUND

The nutritional signaling pathway regulates an insect's size, development, and lifespan, as well as playing a vital role in reproduction. The insulin/insulin-like growth factor signaling (IIS) pathway plays a key role in the nutrition signaling pathway. As an integral component of the IIS pathway, insulin receptor (InR), a receptor tyrosine kinase, plays a role in the insulin pathway by controlling reproduction in many insect species. However, the precise molecular function of InR in non-model insect reproduction is poorly understood.

METHODS

In our study, Chilo suppressalis, a well-known rice pest, was used as a molecular system to determine the role of InR in insect reproduction. Sequencing the InR gene of C. suppressalis, comparing the amino acid sequence-specific structure, and constructing a phylogenetic tree revealed that this gene has four main domains: ligand binding L domain, Furin-like region, fibronectin type III domains, and Tyrosine kinase catalytic domain, which were all highly conserved in insects.

RESULTS

By characterizing the spatiotemporal expression profile of InR in different developmental stages and tissues, we found that InR gene expression was highest on the 3-day old in female pupae, 6th instar larvae, and fat body on the 6-day old in female pupae. InR gene expression may promote the molting and pupation of larvae and play a role in reproduction in the fat body. Furthermore, the RNA interference knockdown of InR dramatically reduced yolk deposition and blocked oocyte maturation. After suppression of InR, the expression of several other genes fluctuated to varying degrees.

CONCLUSION

In conclusion, InR is vital to reproduction and is expected to become a new target for pest management.

Nutrition regulates the expression of storage proteins in Bombyx mori via insulin-like/FoxO signaling pathway

Insect serum proteins, also termed storage proteins (SPs), are hexamer proteins that form amino acid reservoirs important for the development of pupae and embryos in most insects. In this study, we investigated the SP genes expression and regulation pathways in silkworms (Bombyx mori). We observed that B. mori SPs (BmSPs) in the fat body of larvae were strongly decreased by starvation, suggesting they respond to nutrition deprivation. Further, we examined the relationship between BmSP expression and the insulin-like signaling pathway (ILS) to study the regulation of BmSPs expression. The results showed that insulin up-regulated the expression of BmSPs, but an inhibitor of the ILS pathway protein PI3K downregulated the expression of BmSPs in B. mori larvae. Similar results were observed in cultured fat body in vitro and BmE cells. We then over-expressed FoxO, an ILS transcriptional factor, in BmE cells and B. mori larvae to further verify the regulatory role of ILS on expression of BmSPs and found BmFoxO negatively regulates the expression of BmSPs in both BmE cells and larvae. Moreover, BmFoxO was dephosphorylated and translocated from the cytoplasm to the nucleus under starvation treatment. Finally, an element on -2627-2644 bp upstream of the transcription start site of BmSP1 was identified as the binding site of BmFoxO by electrophoretic mobility shift assay and verified by chromatin immunoprecipitation. In summary, our results indicate that nutrient uptake triggers the expression of BmSPs via the ILS/FoxO signaling pathway. This study provides a reference for further study on the expression and regulation of insect SP genes.

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

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

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

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.

Characterization of MicroRNAs Associated with Reproduction in the Brown Planthopper, Nilaparvata lugens

Insects have a robust capacity to produce offspring for propagation, and the reproductive events of female insects have been achieved at the molecular and physiological levels via regulatory gene pathways. However, the roles of MicroRNAs (miRNAs) in the reproductive development of the brown planthopper (BPH), Nilaparvata lugens, remain largely unexplored. To understand the roles of miRNAs in reproductive development, miRNAs were identified by Solexa sequencing in short-winged (SW) female adults of BPH. Small RNA libraries derived from three developmental phases (1 day, 3 days, and 5 days after emergence) were constructed and sequenced. We identified 905 miRNAs, including 263 known and 642 novel miRNAs. Among them, a total of 43 miRNAs were differentially expressed in the three developmental phases, and 14,568 putative targets for 43 differentially expressed miRNAs (DEMs) were predicted by TargetScan and miRanda. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the predicted miRNA targets illustrated the putative roles for these DEMs in reproduction. The progress events were annotated, including oogenesis, lipid biosynthetic process, and related pathways such as apoptosis, ABC transporters, and amino acid metabolism. Four highly abundant DEMs (miR-9a-5p, miR-34-5p, miR-275-3p, and miR-317-3p) were further screened, and miR-34-5p was confirmed to be involved in the regulation of reproduction. Overexpression of miR-34-5p via injecting its mimics reduced fecundity and decreased Vg expression. Moreover, target genes prediction for miR-34-5p showed they might be involved in 20E signaling cascades, apoptosis, and gonadal development, including hormone receptor 4 (HR4), caspase-1 (Cp-1), and spermatogenesis-associated protein 20 (SPATA20). These findings provide a valuable resource for future studies on the role of miRNAs in BPH reproductive development.

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.

Characterization of Insulin-like Peptide (ILP) and Its Potential Role in Ovarian Development of the Cuttlefish Sepiella japonica

The insulin-like peptide (ILP) family is well known for regulating reproduction in invertebrates, while its role in mollusks remains largely unknown. In this study, we first isolated and characterized the ILP gene in the cuttlefish Sepiella japonica. The full-length SjILP cDNA obtained was 926 bp and encoded a precursor protein of 161 amino acids. The precursor protein consisted of a signal peptide, a B chain, a C-peptide, and an A chain. It possessed the typical features of ILP proteins, including two cleavage sites (KR) and eight conserved cysteines. To define the function of SjILP, the expression of SjILP in different tissues and ovarian development stages were analyzed using qRT-PCR. SjILP was mainly expressed in the ovary, and its gene expression correlated with ovarian development. Furthermore, silencing SjILP using RNA interference (RNAi) dramatically decreased the expression levels of four ovarian-development-related genes (vitellogenin1, vitellogenin2, cathepsin L1-like, and follistatin). These data suggest the critical role of SjILP in the regulation of ovarian development in S. japonica.

Kr-h1, a Cornerstone Gene in Insect Life History

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

Molecular Characterization and RNA Interference Analysis of SLC26A10 From Nilaparvata lugens (Stål)

SLC26A10 is a member of the SLC26 gene family, but its role in insects is still unclear. We cloned the SLC26A10 gene of Nilaparvata lugens (NlSLC26A10) and found NlSLC26A10 contained 11 transmembrane regions and a STAS domain. Expression pattern analysis showed NlSLC26A10 expression was more upregulated in adults than in nymphs, highest in the ovary. After injection of double-stranded RNA (dsRNA) of NlSLC26A10, the mRNA level of NlSLC26A10 significantly decreased and, consequently, the ovarian development of adult females was hindered; the amount and the hatchability of eggs and yeast-like symbionts in mature oocytes decreased. Further study showed that NlSLC26A10 might result in decreased juvenile hormone level and vitellogenin expression. These results indicate that NlSLC26A10 plays an essential role in the reproduction of N. lugens.

Endocrine Regulation of Lifespan in Insect Diapause

Diapause is a physiological adaptation to conditions that are unfavorable for growth or reproduction. During diapause, animals become long-lived, stress-resistant, developmentally static, and non-reproductive, in the case of diapausing adults. Diapause has been observed at all developmental stages in both vertebrates and invertebrates. In adults, diapause traits weaken into adaptations such as hibernation, estivation, dormancy, or torpor, which represent evolutionarily diverse versions of the traditional diapause traits. These traits are regulated through modifications of the endocrine program guiding development. In insects, this typically includes changes in molting hormones, as well as metabolic signals that limit growth while skewing the organism's energetic demands toward conservation. While much work has been done to characterize these modifications, the interactions between hormones and their downstream consequences are incompletely understood. The current state of diapause endocrinology is reviewed here to highlight the relevance of diapause beyond its use as a model to study seasonality and development. Specifically, insect diapause is an emerging model to study mechanisms that determine lifespan. The induction of diapause represents a dramatic change in the normal progression of age. Hormones such as juvenile hormone, 20-hydroxyecdysone, and prothoracicotropic hormone are well-known to modulate this plasticity. The induction of diapause-and by extension, the cessation of normal aging-is coordinated by interactions between these pathways. However, research directly connecting diapause endocrinology to the biology of aging is lacking. This review explores connections between diapause and aging through the perspective of endocrine signaling. The current state of research in both fields suggests appreciable overlap that will greatly contribute to our understanding of diapause and lifespan determination.

A heat shock protein protects against oxidative stress induced by lambda-cyhalothrin in the green peach aphid Myzus persicae

Lambda-cyhalothrin (LCT) is a pyrethroid insecticide widely used to control insect pests. Insect exposure to LCT may cause abnormal accumulation of reactive oxygen species (ROS) and result in oxidative damage. Heat shock proteins (HSPs) may help protect against oxidative stress. However, little is known about the role of HSPs in response to LCT in the green peach aphid, Myzus persicae. This insect is an important agricultural pest causing severe yield losses in crops. In this study, we characterized a cDNA sequence (MpHsp70) encoding a member of the HSP70 family in M. persicae. MpHsp70 encoded a 623 amino acid protein putatively localized in the cytosol. The highest expression level of MpHsp70 occurred in fourth-instar nymphs. Treatment of M. persicae with LCT resulted in oxidative stress and significantly increased H₂O₂ and malondialdehyde levels. This led to an elevated transcription level of MpHsp70. Injection of H₂O₂ into M. persicae also upregulated the MpHsp70 expression level, suggesting that MpHsp70 is responsive to ROS, particularly H₂O₂, induced by LCT. Recombinant MpHSP70 protein was expressed in Escherichia coli. E. coli cells overexpressing MpHSP70 exhibited significant tolerance to H₂O₂ and the ROS generators, cumene hydroperoxide and paraquat. This indicated that MpHSP70 protects against oxidative stress. Furthermore, knockdown of MpHsp70 by RNA interference resulted in increased susceptibility in apterous adults of M. persicae to LCT. These findings indicate that MpHsp70 plays an important role in defense against LCT-induced oxidative stress and insecticide susceptibility in M. persicae.

Regulation of insulin-like peptide expression in adult Blattella germanica females

The insulin-IGF-signalling (IIS) pathway regulates key processes in metazoans. The pathway is activated through the binding of the ligands, which in insects are usually referred to as insulin-like peptides (ILPs), to a class of receptor tyrosine kinases, the insect insulin receptor. To study the pathway regulation, it is therefore essential to understand how ILPs are produced and released. In this study we analysed the factors that regulate the expression of the seven ILPs (BgILPs) expressed in adult females of the German cockroach, Blattella germanica. The results showed that the starvation-induced expression reduction of brain BgILP3, 5 and 6 and fat body BgILP7 is not due to reduced juvenile hormone (JH) or decreased TOR pathway activity. In addition, depletion of FoxO in starved females did not correct the low levels of these BgILPs, but even reduced further BgILP5 expression, indicating the need to maintain certain basal levels of BgILP5 even during starvation. Furthermore, JH promoted increased BgILP5 and decreased BgILP3 expression in the brain, an effect that required Methoprene-tolerant (Met), the JH receptor, but not Krüppel homolog 1 (Kr-h1), the main JH transducer. On the other hand, JH inhibited the expression of BgILP7 in the fat body, although in this case, the action required both Met and Kr-h1. In addition, JH reduction treatments produced a decrease in the expression of the insulin receptor in the fat body, which suggests an increase in IIS. The results show a peculiar regulation of ILP expression in adult B. germanica females, which is clearly different than that seen in other species. This is understandable given that gene duplications in recent clades have resulted in different sets of ILP genes, involving substantial changes in gene regulatory networks.

Neuropeptide Bursicon Influences Reproductive Physiology in Tribolium Castaneum

Bursicon is a neuropeptide belonging to the cystine knot family and is composed of burs and partner of burs (pburs) subunits. It can form heterodimers or homodimers to execute different biological functions. Bursicon heterodimers regulate cuticle sclerotization and wing maturation, whereas bursicon homodimers mediate innate immunity and midgut stem cell proliferation. A recent study has shown that bursicon potentially induces the expression of vitellogenin (Vg) in the black tiger shrimp Penaeus monodon; however, the underlying mechanism remains unknown. In this study, we investigated the role of bursicon in the reproductive physiology of the red flour beetle, Tribolium castaneum. The knockdown of burs, pburs, or its receptor T. castaneum rickets (Tcrk) in 2-day pupae significantly downregulated the expression levels of Vg1, Vg2, and Vg receptor (VgR) genes in females 3- and 5-day post-adult emergence, leading to abnormal oocytes with limited Vg content. The silencing of burs repressed the number of eggs laid and completely inhibited egg hatch, whereas the silencing of pburs dramatically decreased the number of eggs laid, hatch rate, and offspring larval size, and this RNA interference (RNAi) effects persisted to the next generation. Furthermore, the knockdown of burs or pburs downregulated the expression of the insulin/insulin-like signaling/target of rapamycin (TOR) signaling genes encoding insulin receptor (InR), protein kinase B (Akt), TOR, and ribosomal protein S6 kinase (S6K). Most importantly, the injection of recombinant pburs (r-pburs) protein was able to upregulate the expression of Vg, VgR, InR, Akt, TOR, S6K, JH synthesis (JHAMT), Methoprene-tolerant (Met), and Taiman (Tai) in normal females and rescue the expression of Vg and VgR in pburs RNAi females but failed to rescue Vg and VgR in Tcrk knockdown females. We infer that bursicon homodimers influence Vg expression via the receptor Tcrk, possibly by mediating the expression of the juvenile hormone (JH) and IIS/TOR pathway genes, thereby regulating reproduction in T. castaneum.

Disruption of the Serine/Threonine Kinase Akt Gene Affects Ovarian Development and Fecundity in the Cigarette Beetle, Lasioderma serricorne

Serine/threonine kinase Akt, an important component of the insulin signaling pathway, plays an essential role in many physiological processes. In this study, we identified and characterized an Akt gene (designated LsAkt) from the cigarette beetle, Lasioderma serricorne. LsAkt contains a 1614 bp open reading frame encoding a 537 amino acid protein that possesses a conserved pleckstrin homology domain and a serine/threonine kinase domain. The expression of LsAkt was high in pupal stages and peaked in day-4 female pupae. In adult tissues, LsAkt was highly expressed in the thorax, ovary, and midgut. The expression of LsAkt was induced by methoprene or bovine insulin in vivo, but significantly decreased by 20-hydroxyecdysone. RNA interference-mediated knockdown of LsAkt resulted in severely blocked ovarian development and reduced fecundity and hatchability. The vitellogenin (Vg) content and juvenile hormone (JH) titers of LsAkt-depletion beetles were decreased, and expressions of Vg and four JH signaling and biosynthetic genes were significantly decreased. Silencing of LsAkt reduced the amounts of glucose, glycogen, and trehalose in female adults and affected the expressions of seven key carbohydrate metabolic genes. Taken together, it is inferred that Akt implicates in L. serricorne reproduction by modification of Vg synthesis, juvenile hormone production and carbohydrate metabolism.

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.

Vitellogenin regulates estrogen-related receptor expression by crosstalk with the JH and IIS-TOR signaling pathway in Polyrhachis vicina Roger (Hymenoptera, Formicidae)

The Estrogen-related receptor (ERR) can regulate the growth and development, metabolism, reproduction, and other physiological activities of insects, but its specific mechanism of action is still unclear. The aim of this study was to explore the relationship between expression of ERR and Vitellogenins (Vg) and the juvenile hormone (JH) and insulin/insulin-like growth factor/target of rapamycin (IIS/TOR) signaling pathways in Polyrhachis vicina Roger. P. vicina was used as the experimental model to clone the PvVg gene, perform double-stranded RNA synthesis and delivery and observe the effects of pharmacological treatments. The full-length PvVg cDNA product is 5586 bp. Higher PvVg mRNA expression was seen in the pupa and adults, and varying levels were seen in the different body parts of three different castes. RNA interference of PvVg expression led to disturbed development, an abnormal phenotype, and high mortality. PvVg RNAi also led to a reduction in mRNA levels of PvERR, ultraspiracle (PvUSP), forkhead box protein O (PvFOXO) and PvTOR genes in fourth instar larval, but a significant increase was seen in pupa and females. No significant change was seen in workers and males. After PvVg knockdown, application of exogenous JHIII reduced the expression of these genes in pupa and females, increased expression in workers, and decreased PvUSP mRNA expression in males. Both protein and mRNA expression levels of PvFOXO were affected by PvVg RNAi. PvERR RNAi increased PvVg expression in pupa and females and Kruppel-homolog 1 (PvKr-h1) and PvFOXO expression in males. The results of this study suggest that there is an interaction between PvERR and PvVg, and that crosstalk with the JH and IIS/TOR signaling pathways can affect development and reproduction. This effect is caste and developmental stage specific. We also speculate that the FOXO/USP complex participates in JH regulation of PvVg in P. vicina.

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.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

Cell-specific expression and individual function of prohormone convertase PC1/3 in Tribolium larval growth highlights major evolutionary changes between beetle and fly neuroendocrine systems

Background

The insect neuroendocrine system acts in the regulation of physiology, development and growth. Molecular evolution of this system hence has the potential to allow for major biological differences between insect groups. Two prohormone convertases, PC1/3 and PC2, are found in animals and both function in the processing of neuropeptide precursors in the vertebrate neurosecretory pathway. Whereas PC2-function is conserved between the fly Drosophila and vertebrates, ancestral PC1/3 was lost in the fly lineage and has not been functionally studied in any protostome.

Results

In order to understand its original functions and the changes accompanying the gene loss in the fly, we investigated PC1/3 and PC2 expression and function in the beetle Tribolium castaneum. We found that PC2 is broadly expressed in the nervous system, whereas surprisingly, PC1/3 expression is restricted to specific cell groups in the posterior brain and suboesophageal ganglion. Both proteases have parallel but non-redundant functions in adult beetles’ viability and fertility. Female infertility following RNAi is caused by a failure to deposit sufficient yolk to the developing oocytes. Larval RNAi against PC2 produced moulting defects where the larvae were not able to shed their old cuticle. This ecdysis phenotype was also observed in a small subset of PC1/3 knockdown larvae and was strongest in a double knockdown. Unexpectedly, most PC1/3-RNAi larvae showed strongly reduced growth, but went through larval moults despite minimal to zero weight gain.

Conclusions

The cell type-specific expression of PC1/3 and its essential requirement for larval growth highlight the important role of this gene within the insect neuroendocrine system. Genomic conservation in most insect groups suggests that it has a comparable individual function in other insects as well, which has been replaced by alternative mechanisms in flies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-021-00179-w.