The MEKRE93 (Methoprene tolerant-Krüppel homolog 1-E93) pathway in the regulation of insect metamorphosis, and the homology of the pupal stage

E93 is indispensable for reproduction in ametabolous and hemimetabolous insects

Ecdysone-induced protein 93 (E93), known as the 'adult-specifier' transcription factor in insects, triggers metamorphosis in both hemimetabolous and holometabolous insects. Although E93 is conserved in ametabolous insects, its spatiotemporal expression and physiological function remain poorly understood. In this study, we first discover that, in the ametabolous firebrat Thermobia domestica, the previtellogenic ovary exhibits cyclically high E93 expression, and E93 mRNA is broadly distributed in previtellogenic ovarioles. E93 homozygous mutant females of T. domestica exhibit severe fecundity deficiency due to impaired previtellogenic development of the ovarian follicles, likely because E93 induces the expression of genes involved in ECM (extracellular matrix)-receptor interactions during previtellogenesis. Moreover, we reveal that in the hemimetabolous cockroach Blattella germanica, E93 similarly promotes previtellogenic ovarian development. In addition, E93 is also essential for vitellogenesis that is necessary to guarantee ovarian maturation and promotes the vitellogenesis-previtellogenesis switch in the fat body of adult female cockroaches. Our findings deepen the understanding of the roles of E93 in controlling reproduction in insects, and of E93 expression and functional evolution, which are proposed to have made crucial contributions to the origin of insect metamorphosis.

The mlpt smORF gene is essential for digestive physiology and molting during nymphal stages in the kissing bug Rhodnius prolixus

Chagas disease affects around 8 million people globally, with Latin America bearing approximately 10,000 deaths each year. Combatting the disease relies heavily on vector control methods, necessitating the identification of new targets. Within insect genomes, genes harboring small open reading frames (smORFs - < 100 amino acids) present numerous potential candidates. In our investigation, we elucidate the pivotal role of the archetypal smORF-containing gene, mille-pattes/polished-rice/tarsalless (mlpt/pri/tal), in the post-embryonic development of the kissing bug Rhodnius prolixus. Injection of double-stranded RNA targeting mlpt (dsmlpt) during nymphal stages yields a spectrum of phenotypes hindering post-embryonic growth. Notably, fourth or fifth stage nymphs subjected to dsmlpt do not undergo molting. These dsmlpt nymphs display heightened mRNA levels of JHAMT-like and EPOX-like, enzymes putatively involved in the juvenile hormone (JH) pathway, alongside increased expression of the transcription factor Kr-h1, indicating changes in the hormonal control. Histological examination reveals structural alterations in the hindgut and external cuticle of dsmlpt nymphs compared to control (dsGFP) counterparts. Furthermore, significant changes in the vector's digestive physiology were observed, with elevated hemozoin and glucose levels in the posterior midgut of dsmlpt nymphs. Importantly, dsmlpt nymphs exhibit impaired metacyclogenesis of Trypanosoma cruzi, the causative agent of Chagas disease, underscoring the crucial role of proper gut organization in parasite differentiation. Thus, our findings constitute the first evidence of a smORF-containing gene's regulatory influence on vector physiology, parasitic cycle, and disease transmission.

Juvenile hormone-induced microRNA miR-iab-8 regulates lipid homeostasis and metamorphosis in Drosophila melanogaster

Metamorphosis plays an important role in the evolutionary success of insects. Accumulating evidence indicated that microRNAs (miRNAs) are involved in the regulation of processes associated with insect metamorphosis. However, the miRNAs coordinated with juvenile hormone (JH)-regulated metamorphosis remain poorly reported. In the present study, using high-throughput miRNA sequencing combined with Drosophila genetic approaches, we demonstrated that miR-iab-8, which primarily targets homeotic genes to modulate haltere-wing transformation and sterility was up-regulated by JH and involved in JH-mediated metamorphosis. Overexpression of miR-iab-8 in the fat body resulted in delayed development and failure of larval-pupal transition. Furthermore, metabolomic analysis results revealed that overexpression of miR-iab-8 caused severe energy metabolism defects especially the lipid metabolism, resulting in significantly reduced triacylglycerol (TG) content and glycerophospholipids but enhanced accumulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). In line with this, Nile red staining demonstrated that during the third larval development, the TG content in the miR-iab-8 overexpression larvae was continuously decreased, which is opposite to the control. Additionally, the transcription levels of genes committed to TG synthesis and breakdown were found to be significantly increased and the expression of genes responsible for glycerophospholipids metabolism were also altered. Overall, we proposed that JH induced miR-iab-8 expression to perturb the lipid metabolism homeostasis especially the TG storage in the fat body, which in turn affected larval growth and metamorphosis.

Pyriproxyfen enhances germline stem cell proliferation and reduces reproduction in Drosophila by up-regulating juvenile hormone signaling

BACKGROUND

Pyriproxyfen is an insect growth regulator (IGR) that is effective against various types of insect pests. However, the molecular mechanism underlying pyriproxyfen effects on insect reproduction remains unclear. Thus, in this study, we attempted to uncover the mechanisms underlying the impact of pyriproxyfen on the reproductive system of the model organism Drosophila melanogaster.

RESULTS

A significant decrease in Drosophila reproduction was observed after pyriproxyfen treatment. The juvenile hormone (JH) titer was significantly increased (120.4%) in the ovary samples of pyriproxyfen-treated flies. Likewise, the concentrations of key enzymes and the expression of key genes related to the JH signaling pathway were also increased in the pyriproxyfen-treated group compared with the control group. Furthermore, pyriproxyfen treatment significantly increased (15.6%) the number of germline stem cells (GSCs) and significantly decreased (17%) the number of cystoblasts (CBs). However, no significant differences were observed in the number of somatic cells. We performed RNA interference (RNAi) on five key genes (Met, Tai, gce, ftz-f1, and hairy) related to the JH signaling pathway in germ cells using the germ cell-specific Gal4 driver. Interestingly, RNAi of the selected genes significantly decreased the number of both GSCs and CBs in pyriproxyfen-treated transgenic flies. These results further validate that pyriproxyfen enhances GSC proliferation by up-regulating JH signaling.

CONCLUSION

Our results indicate that pyriproxyfen significantly decreases reproduction by affecting germ cells in female adult ovaries. The effect of pyriproxyfen on germ cell proliferation and differentiation is mediated by an increase in JH signaling. This study has significant implications for optimizing pest control strategies, developing sustainable agriculture practices, and understanding the mechanism of insecticide action. © 2024 Society of Chemical Industry.

Body part-specific development in termite caste differentiation: crosstalk between hormonal actions and developmental toolkit genes

In social insects, interactions among colony members trigger caste differentiation with morphological modifications. During caste differentiation in termites, body parts and caste-specific morphologies are modified during postembryonic development under endocrine controls such as juvenile hormone (JH) and ecdysone. In addition to endocrine factors, developmental toolkit genes such as Hox- and appendage-patterning genes also contribute to the caste-specific body part modifications. These toolkits are thought to provide spatial information for specific morphogenesis. During social evolution, the complex crosstalks between physiological and developmental mechanisms should be established, leading to the sophisticated caste systems. This article reviews recent studies on these mechanisms underlying the termite caste differentiation and addresses implications for the evolution of caste systems in termites.

MicroRNA miR-7-5p targets MARK2 to control metamorphosis in Galeruca daurica

The MARK2 gene, coding microtubule affinity-regulating kinase or serine/threonine protein kinase, is an important modulator in organism microtubule generation and cell polarity. However, its role in the metamorphosis of insects remains unknown. In this study, we found a conserved miRNA, miR-7-5p, which targets MARK2 to participate in the regulation of the larval-pupal metamorphosis in Galeruca daurica. The dual luciferase reporter assay showed that miR-7-5p interacted with the 3' UTR of MARK2 and repressed its expression. The expression profiling of miR-7-5p and MARK2 displayed an opposite trend during the larval-adult development process. In in-vivo experiments, overexpression of miR-7-5p by injecting miR-7-5p agomir in the final instar larvae down-regulated MARK2 and up-regulated main ecdysone signaling pathway genes including E74, E75, ECR, FTZ-F1 and HR3, which was similar to the results from knockdown of MARK2 by RNAi. In contrast, repression of miR-7-5p by injecting miR-7-5p antagomir obtained opposite effects. Notably, both overexpression and repression of miR-7-5p in the final instar larvae caused abnormal molting and high mortality during the larval-pupal transition, and high mortality during the pupal-adult transition. The 20-hydroxyecdysone (20E) injection experiment showed that 20E up-regulated miR-7-5p whereas down-regulated MARK2. This study reveals that the accurate regulation of miRNAs and their target genes is indispensable for insect metamorphosis.

Untangling the gordian knot: The intertwining interactions between developmental hormone signaling and epigenetic mechanisms in insects

Hormones control developmental and physiological processes, often by regulating the expression of multiple genes simultaneously or sequentially. Crosstalk between hormones and epigenetics is pivotal to dynamically coordinate this process. Hormonal signals can guide the addition and removal of epigenetic marks, steering gene expression. Conversely, DNA methylation, histone modifications and non-coding RNAs can modulate regional chromatin structure and accessibility and regulate the expression of numerous (hormone-related) genes. Here, we provide a review of the interplay between the classical insect hormones, ecdysteroids and juvenile hormones, and epigenetics. We summarize the mode-of-action and roles of these hormones in post-embryonic development, and provide a general overview of epigenetic mechanisms. We then highlight recent advances on the interactions between these hormonal pathways and epigenetics, and their involvement in development. Furthermore, we give an overview of several 'omics techniques employed in the field. Finally, we discuss which questions remain unanswered and possible avenues for future research.

E93 gene in the swimming crab, Portunus trituberculatus: Responsiveness to 20-hydroxyecdysone and methyl farnesoate and role on regulating ecdysteroid synthesis

Ecdysone-induced protein 93 (E93) is a metamorphic determinant involved in crosstalk between 20-hydroxyecdysone (20E) and juvenile hormone (JH) during the insect molting process. The present study identified the E93 gene from the swimming crab, P. trituberculatus, and found it was widely distributed in adult tissues. PtE93 mRNA levels in Y-organ and epidermis fluctuated during the molt cycle, suggesting its involvement in juvenile molting. In vitro and in vivo treatments with 20E led to an induction of PtE93 expression in Y-organ and epidermis, while we found the opposite effect for methyl farnesoate (MF) treatments, a crustacean equivalent of insect JH. We also observed that two genes for ecdysteroid biosynthesis, Spook (Spo) and Shadow (Sad), were suppressed by 20E and induced by MF, showing a negative correlation between PtE93 and ecdysteroid biosynthesis. PtE93 RNA interference (RNAi) induced Spo and Sad expression levels, elevated ecdysteroid content in culture medium, and relieved the 20E inhibitory effect on ecdysteroid synthesis, indicating an inhibitory role of PtE93 on ecdysteroid synthesis. Overall, our results suggest that E93 may be involved in the crosstalk between 20E and MF during crustacean molting, and its presence in Y-organ is closely related to ecdysteroid synthesis.

The CsmiR1579-CsKr-h1 module mediates rice stem borer development and reproduction: An effective target for transgenic insect-resistant rice

The rice stem borer (RSB, Chilo suppressalis) is a significant agricultural pest that mainly depends on chemical control. However, it has grown to varied degrees of pesticide resistance, which poses a severe threat to rice production and emphasizes the need for safer, more efficient alternative pest management strategies. Here, in vitro and in vivo experiments analyses reveal miR-1579 binds to the critical transcription factor Krüppel homologue 1 (Kr-h1) and negatively regulates its expression. Overexpression of miR-1579 in larvae with significantly lower levels of Kr-h1 was associated with a decline in larval growth and survival. Furthermore, in female pupae, miR-1579 overexpression led to abnormalities in ovarian development, suggesting that targeting miR-1579 could be a potential management strategy against C. suppressalis. Therefore, we generated transgenic rice expressing miR-1579 and screened three lines that had a single copy of highly abundant mature miR-1579 transcripts. Expectedly, fed with transgenic miR-1579 rice lines were significantly lower survival rates in larvae and high levels of resistance to damage caused by C. suppressalis infestation. These findings suggest that miRNA-mediated RNAi could provide an effective and species-specific strategy for C. suppressalis control.

Investigating the origin of insect metamorphosis

Experiments exploring the role of juvenile hormone during the life cycle of firebrat insects provide clues about the evolution of metamorphosis.

Nutrition- and hormone-controlled developmental plasticity in Blattodea

Blattodea, which includes cockroaches and termites, possesses high developmental plasticity that is mainly controlled by nutritional conditions and insect hormones. Insulin/insulin-like growth factor signaling (IIS), target of rapamycin complex 1 (TORC1), and adenosine monophosphate-activated protein complex are the three primary nutrition-responsive signals. Juvenile hormone (JH) and 20-hydroxyecdysone (20E) constitute the two most vital insect hormones that might interact with each other through the Met, Kr-h1, E93 (MEKRE93) pathway. Nutritional and hormonal signals interconnect to create a complex regulatory network. Here we summarize recent progress in our understanding of how nutritional and hormonal signals coordinately control the developmental plasticity of metamorphosis, reproduction, and appendage regeneration in cockroaches as well as caste differentiation in termites. We also highlight several perspectives that should be further emphasized in the studies of developmental plasticity in Blattodea. This review provides a general landscape in the field of nutrition- and hormone-controlled developmental plasticity in insects.

The Juvenile-Hormone-Responsive Factor AmKr-h1 Regulates Caste Differentiation in Honey Bees

Honey bees are typical model organisms for the study of caste differentiation, and the juvenile hormone (JH) is a crucial link in the regulatory network of caste differentiation in honey bees. To investigate the mechanism of JH-mediated caste differentiation, we analyzed the effect of the JH response gene AmKr-h1 on this process. We observed that AmKr-h1 expression levels were significantly higher in queen larvae than in worker larvae at the 48 h, 84 h, and 120 h larval stages, and were regulated by JH. Inhibiting AmKr-h1 expression in honey bee larvae using RNAi could lead to the development of larvae toward workers. We also analyzed the transcriptome changes in honey bee larvae after AmKr-h1 RNAi and identified 191 differentially expressed genes (DEGs) and 682 differentially expressed alternative splicing events (DEASEs); of these, many were related to honey bee caste differentiation. Our results indicate that AmKr-h1 regulates caste differentiation in honey bees by acting as a JH-responsive gene.

Regulation and remodeling of microbial symbiosis in insect metamorphosis

Many insects are dependent on microbial mutualists, which are often harbored in specialized symbiotic organs. Upon metamorphosis, insect organs are drastically reorganized. What mechanism regulates the remodeling of the symbiotic organ upon metamorphosis? How does it affect the microbial symbiont therein? Here, we addressed these fundamental issues of symbiosis by experimentally manipulating insect metamorphosis. The stinkbug Plautia stali possesses a midgut symbiotic organ wherein an essential bacterial symbiont resides. By RNAi of master regulator genes for metamorphosis, Kr-h1 over nymphal traits and E93 over adult traits, we generated precocious adults and supernumerary nymphs of P. stali, thereby disentangling the effects of metamorphosis, growth level, developmental stage, and other factors on the symbiotic system. Upon metamorphosis, the symbiotic organ of P. stali was transformed from nymph type to adult type. The supernumerary nymphs and the precocious adults, respectively, developed nymph-type and adult-type symbiotic organs not only morphologically but also transcriptomically, uncovering that metamorphic remodeling of the symbiotic organ is under the control of the MEKRE93 pathway. Transcriptomic, cytological, and biochemical analyses unveiled that the structural and transcriptomic remodeling of the symbiotic organ toward adult emergence underpins its functional extension to food digestion in addition to the original role of symbiont retention for essential nutrient production. Notably, we found that the symbiotic bacteria in the adult-type symbiotic organ up-regulated genes for production of sulfur-containing essential amino acids, methionine and cysteine, that are rich in eggs and sperm, uncovering adult-specific symbiont functioning for host reproduction and highlighting intricate host-symbiont interactions associated with insect metamorphosis.

Juvenile hormone suppresses sensory organ precursor determination to block Drosophila adult abdomen morphogenesis

Juvenile hormone (JH) has a classic "status quo" action at both the pupal and adult molts when administrated exogenously. In Drosophila, treatment with JH at pupariation inhibits the formation of abdominal bristles, which are derived from the histoblasts. However, the mechanism via which JH exerts this effect remains poorly understood. In this study, we analyzed the effect of JH on histoblast proliferation, migration, and differentiation. Our results indicated that whereas the proliferation and migration of histoblasts remained unaffected following treatment with a JH mimic (JHM), their differentiation, particularly the specification of sensor organ precursor (SOP) cells, was inhibited. This effect was attributable to downregulated proneural genes achaete (ac) and Scute (sc) expression levels, which prevented the specification of SOP cells in proneural clusters. Moreover, Kr-h1 was found to mediate this effect of JHM. Histoblast-specific overexpression or knockdown of Kr-h1, respectively mimicked or attenuated the effects exerted by JHM on abdominal bristle formation, SOP determination, and transcriptional regulation of ac and sc. These results indicated that the defective SOP determination was responsible for the inhibition of abdominal bristle formation by JHM, which, in turn, was mainly mediated via the transducing action of Kr-h1.

Transcriptome profiling for developmental stages Protaetia brevitarsis seulensis with focus on wing development and metamorphosis

A white-spotted flower chafer Protaetia brevitarsis seulensis widely distributed in Asian countries is traditionally used in oriental medicine. This study explored gene expression abundance with respect to wing development and metamorphosis in P. b. seulensis based on the large-scale RNA-seq data. The transcriptome assembly consists of 23,551 high-quality transcripts which are approximately 96.7% covered. We found 265 wing development genes, 19 metamorphosis genes, and 1,314 candidates. Of the 1,598 genes, 1,594 are included exclusively in cluster 4 with similar gene co-expression patterns. The network centrality analyses showed that wing development- and metamorphosis-related genes have a high degree of betweenness centrality and are expressed most highly in eggs, moderately in pupa and adults, and lowest in larva. This study provides some meaningful clues for elucidating the genetic modulation mechanism of wing development and metamorphosis in P. b. seulensis.

Burkholderia gut symbiont induces insect host fecundity by modulating Kr-h1 gene expression

Full-length cDNAs of the Broad-Complex (BR-C) from Riptortus pedestris were cloned. Moreover, Kr-h1 and BR-C expression levels in apo-symbiotic and symbiotic host insects were compared to verify whether they are modulated by Burkholderia gut symbionts. Interestingly, Kr-h1 expression level was significantly increased in symbiotic females. To determine how Kr-h1 affects fecundity in insects, the biosynthesis of two reproduction-associated proteins, hexamerin-α and vitellogenin, was investigated in R. pedestris females. Hexamerin-α and vitellogenin expression at the transcriptional and translational levels decreased in Kr-h1-suppressed symbiotic females, subsequently reduced egg production. These results suggest that Burkholderia gut symbiont modulates Kr-h1 expression to enhance ovarian development and egg production of R. pedestris by increasing the biosynthesis of the two proteins.

Noncoding RNA Regulation of Hormonal and Metabolic Systems in the Fruit Fly Drosophila

The importance of RNAs is commonly recognised thanks to protein-coding RNAs, whereas non-coding RNAs (ncRNAs) were conventionally regarded as 'junk'. In the last decade, ncRNAs' significance and roles are becoming noticeable in various biological activities, including those in hormonal and metabolic regulation. Among the ncRNAs: microRNA (miRNA) is a small RNA transcript with ~20 nucleotides in length; long non-coding RNA (lncRNA) is an RNA transcript with >200 nucleotides; and circular RNA (circRNA) is derived from back-splicing of pre-mRNA. These ncRNAs can regulate gene expression levels at epigenetic, transcriptional, and post-transcriptional levels through various mechanisms in insects. A better understanding of these crucial regulators is essential to both basic and applied entomology. In this review, we intend to summarise and discuss the current understanding and knowledge of miRNA, lncRNA, and circRNA in the best-studied insect model, the fruit fly Drosophila.

Autophagy genes AMBRA1 and ATG8 play key roles in midgut remodeling of the yellow fever mosquito, Aedes aegypti

The function of two autophagy genes, an activating molecule BECN1 regulated autophagy (AMBRA1) and autophagy-related gene 8 (ATG8) in the midgut remodeling of Aedes aegypti was investigated. Real-time quantitative polymerase chain reaction (RT-qPCR) analysis of RNA samples collected from the last instar larvae and pupae showed that these two genes are predominantly expressed during the last 12 h and first 24 h of the last larval and pupal stages, respectively. Stable ecdysteroid analog induced and juvenile hormone (JH) analog suppressed these genes. RNA interference (RNAi) studies showed that the ecdysone-induced transcription factor E93 is required for the expression of these genes. JH-induced transcription factor krüppel homolog 1 (Kr-h1) suppressed the expression of these genes. RNAi-mediated silencing of AMBRA1 and ATG8 blocked midgut remodeling. Histological studies of midguts from insects at 48 h after ecdysis to the final larval stage and 12 h after ecdysis to the pupal stage showed that ATG gene knockdown blocked midgut remodeling. AMBRA1 and ATG8 double-stranded (dsRNA)-treated insects retained larval midgut cells and died during the pupal stage. Together, these results demonstrate that ecdysteroid induction of ATG genes initiates autophagy programmed cell death during midgut remodeling. JH inhibits midgut remodeling during metamorphosis by interfering with the expression of ATG genes.

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.

The inhibition of ecdysone signal pathway was the key of pyriproxyfen poisoning for silkworm, Bombyx mori

Pyriproxyfen is a juvenile hormone-like pesticide. Once intake occurs, it leads to a series of poisoning characters consequences in silkworm, Bombyx mori (ID: 7091, Lepidoptera), such as non- cocooning, non-pupation, production of low-active eggs, and extended stages. However, the poisoning mechanism is still unclear. Here, silkworms were fed mulberry leaves soaked with different pyriproxyfen concentrations, and the heads were dissected for transcriptome analysis, while the hemolymph was used for determinations of ecdysone and juvenile hormone titers. As a result, after conjoint analysis of 3 feeding groups and a control group, 555 differentially expressed genes (DEGs) were obtained, which were mainly involved in hormone metabolism, glycometabolism and protein metabolism. Meanwhile, 119 genes were significantly correlated with the pyriproxyfen concentrations, and they were mainly involved in drug metabolism and glycometabolism. The ecdysone titers in several feeding groups were significantly lower than those of the control group, while juvenile hormone was not detected in all groups, including the control and feeding groups. Correspondingly, due to activation of the juvenile hormone signaling pathway by pyriproxyfen, key genes in the ecdysone synthesis pathway were downregulated, and a large number of downstream genes were up- or downregulated. In addition, nearly all genes in the detoxification pathway were upregulated. These results suggested that, affected by the juvenile hormone signaling pathway, ecdysone titers decreased and further affected a series of downstream processes, and this was the key reason for pyriproxyfen poisoning in silkworm, B. mori, which could lay a foundation for the study of pyriproxyfen resistance in silkworm.

A Non-Gradual Development Process of Cicada Eyes at the End of the Fifth-Instar Nymphal Stage to Obtain Visual Ability

Insects' visual system is directly related to ecology and critical for their survival. Some cicadas present obvious differences in color and ultrastructure of compound eyes between nymphal and adult stages, but little is known about when cicadas obtain their visual ability to deal with the novel above-ground habitat. We use transcriptome analyses and reveal that cicada Meimuna mongolica has a trichromatic color vision system and that the eyes undergo a non-gradual development process at the end of the 5th-instar nymphal stage. The white-eye 5th-instar nymphs (i.e., younger 5th-instar nymphs) have no visual ability because critical components of the visual system are deficient. The transformation of eyes toward possessing visual function takes place after a tipping point in the transition phase from the white-eye period to the subsequent red-eye period, which is related to a decrease of Juvenile Hormone. The period shortly after adult emergence is also critical for eye development. Key differentially-expressed genes related to phototransduction and chromophore synthesis play positive roles for cicadas to adapt to above-ground habitat. The accumulation of ommochromes corresponds to the color change of eyes from white to red and dark brown during the end of the 5th-instar nymphal period. Cuticle tanning leads to eye color changing from dark-brown to light-brown during the early adult stage. We hypothesize that the accumulation of ommochromes occurring at the end of 5th-instar nymphal stage and the early adult stage is not only for cicadas to obtain visual ability, but also is a secure strategy to cope with potential photodamage after emergence.

Egfr signaling promotes juvenile hormone biosynthesis in the German cockroach

BACKGROUND

In insects, an interplay between the activities of distinct hormones, such as juvenile hormone (JH) and 20-hydroxyecdysone (20E), regulates the progression through numerous life history hallmarks. As a crucial endocrine factor, JH is mainly synthesized in the corpora allata (CA) to regulate multiple physiological and developmental processes, including molting, metamorphosis, and reproduction. During the last century, significant progress has been achieved in elucidating the JH signal transduction pathway, while less progress has been made in dissecting the regulatory mechanism of JH biosynthesis. Previous work has shown that receptor tyrosine kinase (RTK) signaling regulates hormone biosynthesis in both insects and mammals. Here, we performed a systematic RNA interference (RNAi) screening to identify RTKs involved in regulating JH biosynthesis in the CA of adult Blattella germanica females.

RESULTS

We found that the epidermal growth factor receptor (Egfr) is required for promoting JH biosynthesis in the CA of adult females. The Egf ligands Vein and Spitz activate Egfr, followed by Ras/Raf/ERK signaling, and finally activation of the downstream transcription factor Pointed (Pnt). Importantly, Pnt induces the transcriptional expression of two key enzyme-encoding genes in the JH biosynthesis pathway: juvenile hormone acid methyltransferase (JHAMT) and methyl farnesoate epoxidase (CYP15A1). Dual-luciferase reporter assay shows that Pnt is able to activate a promoter region of Jhamt. In addition, electrophoretic mobility shift assay confirms that Pnt directly binds to the - 941~ - 886 nt region of the Jhamt promoter.

CONCLUSIONS

This study reveals the detailed molecular mechanism of Egfr signaling in promoting JH biosynthesis in the German cockroach, shedding light on the intricate regulation of JH biosynthesis during insect development.

CRISPR-Cas9 Genome Editing Uncovers the Mode of Action of Methoprene in the Yellow Fever Mosquito, Aedes aegypti

Methoprene, a juvenile hormone (JH) analog, is widely used for insect control, but its mode of action is not known. To study methoprene action in the yellow fever mosquito, Aedes aegypti, the E93 (ecdysone-induced transcription factor) was knocked out using the CRISPR-Cas9 system. The E93 mutant pupae retained larval tissues similar to methoprene-treated insects. These insects completed pupal ecdysis and died as pupa. In addition, the expression of transcription factors, broad complex and Krüppel homolog 1 (Kr-h1), increased and that of programmed cell death (PCD) and autophagy genes decreased in E93 mutants. These data suggest that methoprene functions through JH receptor, methoprene-tolerant, and induces the expression of Kr-h1, which suppresses the expression of E93, resulting in a block in PCD and autophagy of larval tissues. Failure in the elimination of larval tissues and the formation of adult structures results in their death. These results answered long-standing questions on the mode of action of methoprene.

Requirement of Myoglianin for metamorphosis in the beetle Henosepilachna vigintioctopunctata

Henosepilachna vigintioctopunctata is a serious defoliating beetle attacking Solanaceae and Cucurbitaceae plants in many Asian countries. In the present paper, we identified a putative myoglianin (myo) gene. Hvmyo was actively transcribed throughout development, from embryo to adult. RNA interference (RNAi)-aided knockdown of Hvmyo delayed larval development by more than 2 days, reduced larval body size, inhibited the growth of antennae, wings and legs and disturbed gut purge. Knockdown of Hvmyo impaired the larval-pupal transition. All the Hvmyo RNAi larvae arrested at the larval stage or formed misshapen pupae or adults. The deformed pupae and adults were partially wrapped with exuviae, bearing separated wings. Moreover, the expression levels of five ecdysteroidogenesis genes (Hvspo, Hvphm, Hvdib, Hvsad and Hvshd), a prothocicotropic hormone (PTTH)/Torso pathway gene (Hvtorso), two 20E receptor genes (HvEcR and HvUSP), and two 20E signalling genes (HvE93 and HvFTZ-F1) were as a result of HvMyo RNAi significantly lowered. Conversely, the expression of a JH biosynthesis gene (Hvjhamt), a JH receptor gene HvMet and a JH signalling gene HvKr-h1 was greatly enhanced. Although ingestion of 20E and Hal rescued the 20E signal, it could not alleviate larval performance and defective phenotypes. Our results suggest that Myo exerts four distinctive roles in ecdysteroidogenesis, JH production, organ growth and larva-pupa-adult transformation in H. vigintioctopunctata.

Knockdown of Ecdysone-Induced Protein 93F Causes Abnormal Pupae and Adults in the Eggplant Lady Beetle

Ecdysone-induced protein 93F (E93) plays triple roles during post-embryonic development in insects whose juvenile instars are more than four. However, it only acts as a specifier of adult structures in Drosophila flies whose larval instars are fixed at three. In this study, we determined the functions of E93 in the eggplant lady beetle (Henosepilachna vigintioctopunctata), which has four larval instars. We uncovered that E93 was abundantly expressed at the prepupal and pupal stages. A precocious inhibition of the juvenile hormone signal by RNA interference (RNAi) of HvKr-h1 or HvHairy, two vital downstream developmental effectors, at the penultimate instar larval stage increased the expression of E93, Conversely, ingestion of JH by the third-instar larvae stimulated the expression of HvKr-h1 but repressed the transcription of either HvE93X1 or HvE93X2. However, disturbance of the JH signal neither drove premature metamorphosis nor caused supernumerary instars. In contrast, depletion of E93 at the third- and fourth-instar larval and prepupal stages severely impaired pupation and caused a larval-pupal mixed phenotype: pupal spines and larval scoli were simultaneously presented on the cuticle. RNAi of E93 at the pupal stage affected adult eclosion. When the beetles had suffered from a dsE93 injection at the fourth-instar larval and pupal stages, a few resultant adults emerged, with separated elytra, abnormally folded hindwings, a small body size and short appendages. Taken together, our results suggest the larval instars are fixed in H. vigintioctopunctata; E93 serves as a repressor of larval characters and a specifier of adult structures during the larval–pupal–adult transition.

Sex- and caste-specific transcriptomes of larval honey bee (Apis mellifera L.) gonads: DMRT A2 and Hsp83 are differentially expressed and regulated by juvenile hormone

The gonads of honey bee, Apis mellifera, queens and drones are each composed of hundreds of serial units, the ovarioles and testioles, while the ovaries of the adult subfertile workers consist of only few ovarioles. We performed a comparative RNA-seq analysis on early fifth-instar (L5F1) larval gonads, which is a critical stage in gonad development of honey bee larvae. A total of 1834 genes were identified as differentially expressed (Padj < 0.01) among the three sex and caste phenotypes. The Gene Ontology analysis showed significant enrichment for metabolism, protein or ion binding, and oxidoreductase activity, and a KEGG analysis revealed metabolic pathways as enriched. In a principal component analysis for the total transcriptomes and hierarchical clustering of the DEGs, we found higher similarity between the queen and worker ovary transcriptomes compared to the drone testis, despite the onset of programmed cell death in the worker ovaries. Four DEGs were selected for RT-qPCR analyses, including their response to juvenile hormone (JH), which is a critical factor in the caste-specific development of the ovaries. Among these, DMRT A2 and Hsp83 were found upregulated by JH and, thus, emerged as potential molecular markers for sex- and caste-specific gonad development in honey bees.

MicroRNA let-7 is required for hormonal regulation of metamorphosis in the silkworm, Bombyx mori

The heterochronic microRNA let-7, which was first identified in Caenorhabditis elegans, controls the timing of developmental programs, and let-7 triggers the onset of the juvenile-adult transition in bilaterians. The expression of let-7 is strongly induced during the last larval stage of C. elegans and is highly expressed in the late last instar larvae/nymphs of the fly Drosophila melanogaster and the cockroach Blattella germanica. In the silkworm Bombyx mori, the expression of let-7 remarkably increases in the corpus cardiacum-corpus allatum complex (CC-CA) at the beginning of the last larval instar and is maintained at high levels during this instar. To determine the biological function of let-7 in B. mori, we generated a let-7 knockout line and a transgenic UAS-let-7 line. The let-7 knockout larvae were developmentally arrested in the prepupal stage and became pupal-adult intermediates after apolysis. When let-7 was ubiquitously overexpressed under the transcriptional control of an Actin3-GAL4 driver, developmental timing and growth of larvae were severely impaired in the penultimate (L4) instar, and these larvae underwent precocious metamorphosis from L4. Furthermore, our results showed that reception and signaling of ecdysteroids and juvenile hormones (JHs) normally occurred in the absence of let-7, whereas the biosynthesis of ecdysone and JHs were affected by disruption and overexpression of let-7. Together, the present study demonstrates that let-7 is required for the coordination of the biosynthesis of ecdysone and JH to ensure the developmental transition during the metamorphosis of B. mori.

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.

Smad on X is vital for larval-pupal transition in a herbivorous ladybird beetle

Insect development is regulated by a combination of juvenile hormone (JH) and 20-hydroxyecdysone (20E). Production of both JH and 20E is regulated by transforming growth factor-β (TGFβ) signaling. TGFβ can be classified into two branches, the Activin and Bone Morphogenetic Protein (BMP) pathways. In Drosophila melanogaster, BMP signaling is critical for JH synthesis, whereas Activin signal is required to generate the large pulse of 20E necessary for entering metamorphosis. However, to which extent the roles of these signals are conserved remains unknown. Here we studied the role of an Activin component Smad on X (Smox) in post-embryonic development in a defoliating ladybird Henosepilachna vigintioctopunctata. RNA interference (RNAi)-aided knockdown of Hvsmox inhibited larval growth, and impaired larval development. All Hvmyo RNAi larvae arrested at the fourth-instar larval stage. Moreover, knockdown of Hvsmox delayed gut and Malpighian tubules remodeling. Furthermore, the expression of a JH biosynthesis gene (Hvjhamt), a JH receptor gene HvMet and a JH response gene HvKr-h1 was greatly enhanced. Conversely, the expression levels of an ecdysteroidogenesis gene (Hvspo), a 20E receptor gene (HvEcR) and six 20E response genes (HvBrC, HvE74, HvE75, HvE93, HvHR3 and HvHR4) were significantly lowered. Knockdown of HvMet partially restored the negative phenotypes in the Hvsmox RNAi beetles. Our results suggest that Smox exerts regulative roles in JH production, ecdysteroidogenesis and organ remodeling, thus contributing to modulate the larva-pupa-adult transformation in H. vigintioctopunctata.

Silencing of transcription factor E93 inhibits adult morphogenesis and disrupts cuticle, wing and ovary development in Locusta migratoria

Ecdysone-induced protein 93F (E93) plays important roles during the metamorphosis process in insects. In this study, a cDNA of the LmE93 gene was identified from the transcriptome of Locusta migratoria, which consists of the 3378-nucleotide open-reading frame (ORF) and encodes 1125 amino acids with helix-turn-helix (HTH) motifs. Reverse transcription quantitative polymerase chain reaction analysis revealed that LmE93 was highest expressed in ovary. The LmE93 expression level was markedly low from the 3rd to 4th instar nymphs, and greatly increased in 1-day-old 5th instar nymphs with a peak on middle nymphal days, then declined in the late nymphal days. Moreover, injected dsLmE93 into 4th and 5th instar nymphs greatly reduced LmE93 transcripts, respectively, and prevented the process of metamorphosis, causing supernumerary nymphal stages. Hematoxylin-eosin staining of the integument showed that the apolysis occurred in advance in 4th instar nymphs, and old cuticle degradation was decreased in dsLmE93-injected locusts of 5th instar nymphs. Smaller and no fully developed wings with reduced columns between the anterior and posterior regions were found in N6 and N7 supernumerary nymphs. In addition, the development of the ovary in dsLmE93-injected locusts was severely blocked, the yolk was almost not formed and there was no development of ovarioles. The results indicated that LmE93 play key roles in the metamorphosis, cuticle, wing and ovarian development of locusts.

MicroRNA let-7-5p targets the juvenile hormone primary response gene Krüppel homolog 1 and regulates reproductive diapause in Galeruca daurica

MicroRNAs (miRNAs) regulate various biological processes in insects. However, their roles in the regulation of insect diapause remain unknown. In this study, we address the biological function of a conserved miRNA, let-7-5p in the regulation of a juvenile hormone primary response gene, Krüppel homolog 1 (Kr-h1), which modulates reproductive diapause in Galeruca daurica. The dual luciferase reporter assay showed that let-7-5p depressed the expression of Kr-h1. The expression profiles of let-7-5p and Kr-h1 displayed opposite patterns in the adult developmental stage. Injection of let-7-5p agomir in pre-diapause adult females inhibited the expression of Kr-h1, which consequently led to delay ovarian development, increase lipid accumulation, expand fat body, and induce reproductive diapause just as depleting Kr-h1 did. Conversely, injection of let-7-5p antagomir resulted in opposite effects by reducing fat storage and stimulating reproduction. Moreover, JH receptor agonist methoprene reduced the expression of let-7-5p, and rescued the ovarian development defects associated with let-7-5p overexpression. These results indicate that let-7-5p plays an important role in the regulation of reproductive diapause and development of G. daurica adults through its target gene Kr-h1.

Temporal analysis of microRNAs associated with wing development in the English grain aphid, Sitobion avenae (F.) (Homoptera: Aphidiae)

Molecular mechanisms underlying wing evolution and development have been a point of scientific inquiry for decades. Phloem-feeding aphids are one of the most devastating global insect pests, where dispersal of winged morphs lead to annual movements, migrations, and range expansions. Aphids show a polyphenic wing dimorphism trait, and offer a model to study the role of environment in determining morphological plasticity of a single genotype. Despite recent progresses in the genetic understanding of wing polyphenism, the influence of environmental cues remains unclear. To investigate the involvement of miRNAs in wing development, we sequenced small RNA libraries of the English grain aphid, Sitobion avenae (F.) across six different developmental stages. As a result, we identified 113 conserved and 193 S. avenae-specific miRNAs. Gene Ontology and KEGG pathway analyses of putative target mRNAs for the six differentially expressed miRNAs are enriched for wing development processes. Dietary uptake of miR-263a, miR-316, and miR-184a agomirs and antagomirs led to significantly higher mortality (>70%) and a lower proportion of winged morphs (<5%). On the other hand, wing malformation was observed in miR-2 and miR-306 agomirs and miR-2 and miR-14 antagomirs, respectively, suggesting their involvement in S. avenae wing morphogenesis. These combined results not only shed light on the regulatory role of miRNAs in wing dimorphism, but also provide potential novel targets for the long-term sustainable management of S. avenae, a devastating global grain pest.

Molecular mechanisms underlying metamorphosis in the most-ancestral winged insect

As caterpillars metamorphose to butterflies, insects change their appearance dramatically through metamorphosis. Some insects have an immobile pupal stage for morphological remodeling (homometaboly). Other insects, such as cockroaches, have no pupal stage, and the juveniles and adults are morphologically similar (hemimetaboly). Notably, among the most-ancestral hemimetabolous insects, dragonflies drastically alter their appearance from aquatic nymphs to aerial adults. In dragonflies, we showed that transcription factors Kr-h1 and E93 are essential for regulating metamorphosis as in other insects, while broad, the master gene for pupation in holometabolous insects, regulates a number of both nymph-specific genes and adult-specific genes, providing insight into what evolutionary trajectory the key transcription factor broad has experienced before ending up with governing pupation and holometaboly.

Insects comprise over half of the described species, and the acquisition of metamorphosis must have contributed to their diversity and prosperity. The order Odonata (dragonflies and damselflies) is among the most-ancestral insects with drastic morphological changes upon metamorphosis, in which understanding of the molecular mechanisms will provide insight into the evolution of incomplete and complete metamorphosis in insects. In order to identify metamorphosis-related genes in Odonata, we performed comprehensive RNA-sequencing of the blue-tailed damselfly Ischnura senegalensis at different developmental stages. Comparative RNA-sequencing analyses between nymphs and adults identified eight nymph-specific and seven adult-specific transcripts. RNA interference (RNAi) of these candidate genes demonstrated that three transcription factors, Krüppel homolog 1 (Kr-h1), broad, and E93 play important roles in metamorphosis of both I. senegalensis and a phylogenetically distant dragonfly, Pseudothemis zonata. E93 is essential for adult morphogenesis, and RNAi of Kr-h1 induced precocious metamorphosis in epidermis via up-regulation of E93. Precocious metamorphosis was also induced by RNAi of the juvenile hormone receptor Methoprene-tolerant (Met), confirming that the regulation of metamorphosis by the MEKRE93 (Met-Kr-h1-E93) pathway is conserved across diverse insects including the basal insect lineage Odonata. Notably, RNAi of broad produced unique grayish pigmentation on the nymphal abdominal epidermis. Survey of downstream genes for Kr-h1, broad, and E93 uncovered that unlike other insects, broad regulates a substantial number of nymph-specific and adult-specific genes independently of Kr-h1 and E93. These findings highlight the importance of functional changes and rewiring of the transcription factors Kr-h1, broad, and E93 in the evolution of insect metamorphosis.

Juvenile Hormone Studies in Drosophila melanogaster

In the field of insect endocrinology, juvenile hormone (JH) is one of the most wondrous entomological terms. As a unique sesquiterpenoid hormone produced and released by the endocrine gland, corpus allatum (CA), JH is a critical regulator in multiple developmental and physiological processes, such as metamorphosis, reproduction, and behavior. Benefited from the precise genetic interventions and simplicity, the fruit fly, Drosophila melanogaster, is an indispensable model in JH studies. This review is aimed to present the regulatory factors on JH biosynthesis and an overview of the regulatory roles of JH in Drosophila. The future directions of JH studies are also discussed, and a few hot spots are highlighted.

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.

Juvenile hormone membrane signaling phosphorylates USP and thus potentiates 20-hydroxyecdysone action in Drosophila

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) coordinately regulate development and metamorphosis in insects. Two JH intracellular receptors, methoprene-tolerant (Met) and germ-cell expressed (Gce), have been identified in the fruit fly Drosophila melanogaster. To investigate JH membrane signaling pathway without the interference from JH intracellular signaling, we characterized phosphoproteome profiles of the Met gce double mutant in the absence or presence of JH in both chronic and acute phases. Functioning through a potential receptor tyrosine kinase and phospholipase C pathway, JH membrane signaling activated protein kinase C (PKC) which phosphorylated ultraspiracle (USP) at Ser35, the PKC phosphorylation site required for the maximal action of 20E through its nuclear receptor complex EcR-USP. The usp^S35A mutant, in which Ser was replaced with Ala at position 35 by genome editing, showed decreased expression of Halloween genes that are responsible for ecdysone biosynthesis and thus attenuated 20E signaling that delayed developmental timing. The usp^S35A mutant also showed lower Yorkie activity that reduced body size. Altogether, JH membrane signaling phosphorylates USP at Ser35 and thus potentiates 20E action that regulates the normal fly development. This study helps better understand the complex JH signaling network.

Reproductive plasticity and oogenesis in the queen honey bee (Apis mellifera)

In the honey bee (Apis mellifera), queen and worker castes originate from identical genetic templates but develop into different phenotypes. Queens lay up to 2000 eggs daily whereas workers are sterile in the queen's presence. Periodically queens stop laying: during swarming, when resources are scarce in winter, and when they are confined to a cage by beekeepers. We used confocal microscopy and gene expression assays to investigate the control of oogenesis in the ovaries of honey bee queens that were caged inside and outside the colony. We find evidence that queens use a different combination of 'checkpoints' to regulate oogenesis compared to honey bee workers and other insect species. However, both queen and worker castes likely use the same programmed cell death pathways to terminate oocyte development at their caste-specific checkpoints. Our results also suggest that a key factor driving the termination of oogenesis in queens is nutritional stress. Thus, queens may regulate oogenesis via the same regulatory pathways that were utilised by ancestral solitary species but likely have adjusted physiological checkpoints to suit their highly-derived life history.

Drosophila E93 promotes adult development and suppresses larval responses to ecdysone during metamorphosis

Pulses of the steroid hormone ecdysone act through transcriptional cascades to direct the major developmental transitions during the Drosophila life cycle. These include the prepupal ecdysone pulse, which occurs 10 ​hours after pupariation and triggers the onset of adult morphogenesis and larval tissue destruction. E93 encodes a transcription factor that is specifically induced by the prepupal pulse of ecdysone, supporting a model proposed by earlier work that it specifies the onset of adult development. Although a number of studies have addressed these functions for E93, little is known about its roles in the salivary gland where the E93 locus was originally identified. Here we show that E93 is required for development through late pupal stages, with mutants displaying defects in adult differentiation and no detectable effect on the destruction of larval salivary glands. RNA-seq analysis demonstrates that E93 regulates genes involved in development and morphogenesis in the salivary glands, but has little effect on cell death gene expression. We also show that E93 is required to direct the proper timing of ecdysone-regulated gene expression in salivary glands, and that it suppresses earlier transcriptional programs that occur during larval and prepupal stages. These studies support the model that the stage-specific induction of E93 in late prepupae provides a critical signal that defines the end of larval development and the onset of adult differentiation.

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

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

Evaluation of Reference Genes for Transcriptional Profiling in Two Cockroach Models

The German cockroach, Blattella germanica, and the American cockroach, Periplaneta americana are the most common and synanthropic household pests of interest to public health. While they have increasingly served as model systems in hemimetabolous insects for studying many biological issues, there is still a lack of stable reference gene evaluation for reliable quantitative real-time PCR (qPCR) outputs and functional genomics. Here, we evaluated the expression variation of common insect reference genes, including the historically used actin, across various tissues and developmental stages, and also under experimental treatment conditions in these two species by using three individual algorithms (geNorm, BestKeeper, and NormFinder) and a comprehensive program (RefFinder). RPL32 in B. germanica and EF1α in P. americana showed the overall lowest variation among all examined samples. Based on the stability rankings by RefFinder, the optimal but varied reference genes under specific conditions were selected for qPCR normalization. In addition, the combination of RPL32 and EF1α was recommended for all the tested tissues and stages in B. germanica, whereas the combination of multiple reference genes was unfavorable in P. americana. This study provides a condition-specific resource of reference gene selection for accurate gene expression profiling and facilitating functional genomics in these two important cockroaches.

Epoxidation of juvenile hormone was a key innovation improving insect reproductive fitness

Methyl farnesoate (MF) plays hormonal regulatory roles in crustaceans. An epoxidated form of MF, known as juvenile hormone (JH), controls metamorphosis and stimulates reproduction in insects. To address the evolutionary significance of MF epoxidation, we generated mosquitoes completely lacking either of the two enzymes that catalyze the last steps of MF/JH biosynthesis and epoxidation, respectively: the JH acid methyltransferase (JHAMT) and the P450 epoxidase CYP15 (EPOX). jhamt ^-/- larvae lacking both MF and JH died at the onset of metamorphosis. Strikingly, epox ^-/- mutants, which synthesized MF but no JH, completed the entire life cycle. While epox ^-/- adults were fertile, the reproductive performance of both sexes was dramatically reduced. Our results suggest that although MF can substitute for the absence of JH in mosquitoes, it is with a significant fitness cost. We propose that MF can fulfill most roles of JH, but its epoxidation to JH was a key innovation providing insects with a reproductive advantage.

Krüppel-homolog 1 exerts anti-metamorphic and vitellogenic functions in insects via phosphorylation-mediated recruitment of specific cofactors

Background

The zinc-finger transcription factor Krüppel-homolog 1 (Kr-h1) exerts a dual regulatory role during insect development by preventing precocious larval/nymphal metamorphosis and in stimulating aspects of adult reproduction such as vitellogenesis. However, how Kr-h1 functions both as a transcriptional repressor in juvenile metamorphosis and an activator in adult reproduction remains elusive. Here, we use the insect Locusta migratoria to dissect the molecular mechanism by which Kr-h1 functions as activator and repressor at these distinct developmental stages.

Results

We report that the kinase PKCα triggers Kr-h1 phosphorylation at the amino acid residue Ser¹⁵⁴, a step essential for its dual functions. During juvenile stage, phosphorylated Kr-h1 recruits a corepressor, C-terminal binding protein (CtBP). The complex of phosphorylated Kr-h1 and CtBP represses the transcription of Ecdysone induced protein 93F (E93) and consequently prevents the juvenile-to-adult transition. In adult insects, phosphorylated Kr-h1 recruits a coactivator, CREB-binding protein (CBP), and promotes vitellogenesis by inducing the expression of Ribosomal protein L36. Furthermore, Kr-h1 phosphorylation with the concomitant inhibition of E93 transcription is evolutionarily conserved across insect orders.

Conclusion

Our results suggest that Kr-h1 phosphorylation is indispensable for the recruitment of transcriptional cofactors, and for its anti-metamorphic and vitellogenic actions in insects. Our data shed new light on the understanding of Kr-h1 regulation and function in JH-regulated insect metamorphosis and reproduction.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01157-3.

Constraints and Opportunities for the Evolution of Metamorphic Organisms in a Changing Climate

We argue that developmental hormones facilitate the evolution of novel phenotypic innovations and timing of life history events by genetic accommodation. Within an individual’s life cycle, metamorphic hormones respond readily to environmental conditions and alter adult phenotypes. Across generations, the many effects of hormones can bias and at times constrain the evolution of traits during metamorphosis; yet, hormonal systems can overcome constraints through shifts in timing of, and acquisition of tissue specific responses to, endocrine regulation. Because of these actions of hormones, metamorphic hormones can shape the evolution of metamorphic organisms. We present a model called a developmental goblet, which provides a visual representation of how metamorphic organisms might evolve. In addition, because developmental hormones often respond to environmental changes, we discuss how endocrine regulation of postembryonic development may impact how organisms evolve in response to climate change. Thus, we propose that developmental hormones may provide a mechanistic link between climate change and organismal adaptation.

Regulation of metamorphosis in neopteran insects is conserved in the paleopteran Cloeon dipterum (Ephemeroptera)

In the Paleozoic era, more than 400 Ma, a number of insect groups continued molting after forming functional wings. Today, however, flying insects stop molting after metamorphosis when they become fully winged. The only exception is the mayflies (Paleoptera, Ephemeroptera), which molt in the subimago, a flying stage between the nymph and the adult. However, the identity and homology of the subimago still is underexplored. Debate remains regarding whether this stage represents a modified nymph, an adult, or a pupa like that of butterflies. Another relevant question is why mayflies have the subimago stage despite the risk of molting fragile membranous wings. These questions have intrigued numerous authors, but nonetheless, clear answers have not yet been found. By combining morphological studies, hormonal treatments, and molecular analysis in the mayfly Cloeon dipterum, we found answers to these old questions. We observed that treatment with a juvenile hormone analog in the last nymphal instar stimulated the expression of the Kr-h1 gene and reduced that of E93, which suppress and trigger metamorphosis, respectively. The regulation of metamorphosis thus follows the MEKRE93 pathway, as in neopteran insects. Moreover, the treatment prevented the formation of the subimago. These findings suggest that the subimago must be considered an instar of the adult mayfly. We also observed that the forelegs dramatically grow between the last nymphal instar, the subimago, and the adult. This necessary growth spread over the last two stages could explain, at least in part, the adaptive sense of the subimago.

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

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

Characterization of E93 in neometabolous thrips Frankliniella occidentalis and Haplothrips brevitubus

Insect metamorphosis into an adult occurs after the juvenile hormone (JH) titer decreases at the end of the juvenile stage. This generally coincides with decreased transcript levels of JH-response transcription factors Krüppel homolog 1 (Kr-h1) and broad (br), and increased transcript levels of the adult specifier E93. Thrips (Thysanoptera) develop through inactive and non-feeding stages referred to as “propupa” and “pupa”, and this type of distinctive metamorphosis is called neometaboly. To understand the mechanisms of hormonal regulation in thrips metamorphosis, we previously analyzed the transcript levels of Kr-h1 and br in two thrips species, Frankliniella occidentalis (Thripidae) and Haplothrips brevitubus (Phlaeothripidae). In both species, the transcript levels of Kr-h1 and br decreased in the “propupal” and “pupal” stages, and their transcription was upregulated by exogenous JH mimic treatment. Here we analyzed the developmental profiles of E93 in these two thrips species. Quantitative RT-PCR revealed that E93 expression started to increase at the end of the larval stage in F. occidentalis and in the “propupal” stage of H. brevitubus, as Kr-h1 and br mRNA levels decreased. Treatment with an exogenous JH mimic at the onset of metamorphosis prevented pupal-adult transition and caused repression of E93. These results indicated that E93 is involved in adult differentiation after JH titer decreases at the end of the larval stage of thrips. By comparing the expression profiles of Kr-h1, br, and E93 among insect species, we propose that the “propupal” and “pupal” stages of thrips have some similarities with the holometabolous prepupal and pupal stages, respectively.

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.

Vitellogenin expression in the ovaries of adult honeybee workers provides insights into the evolution of reproductive and social traits

Social insects are notable for having two female castes that exhibit extreme differences in their reproductive capacity. The molecular basis of these differences is largely unknown. Vitellogenin (Vg) is a powerful antioxidant and insulin-signalling regulator used in oocyte development. Here we investigate how Royal Jelly (the major food of honeybee queens) and queen mandibular pheromone (a major regulator of worker fertility), affect the longevity and reproductive status of honey bee workers, the expression of Vg, its receptor VgR and associated regulatory proteins. We find that Vg is expressed in the ovaries of workers and that workers fed a queen diet of Royal Jelly have increased Vg expression in the ovaries. Surprisingly, we find that expression of Vg is not associated with ovary activation in workers, suggesting that this gene has potentially acquired non-reproductive functions. Therefore, Vg expression in the ovaries of honeybee workers provides further support for the Ovarian Ground Plan Hypothesis, which argues that genes implicated in the regulation of reproduction have been co-opted to regulate behavioural differences between queens and workers.