Ecdysteroid signalling components in metamorphosis and development of the desert locust, Schistocerca gregaria

Molecular Characterization and Expression of the Ecdysone Receptor and Ultraspiracle Genes in the Wheat Blossom Midge, Sitodiplosis mosellana

20-hydroxyecdysone (20E) is essential for insect development and diapause. Ecdysone receptor (EcR) and ultraspiracle (USP) proteins are crucial regulators of 20E signaling. To explore their potential roles in the development of Sitodiplosis mosellana, a major wheat pest that undergoes obligatory diapause as a larva, one SmEcR and two SmUSPs (SmUSP-A and SmUSP-B) from this species were isolated and characterized. The deduced SmEcR and SmUSP-A/B proteins contained a conserved DNA-binding domain with two zinc finger motifs that bind to specific DNA sequences. Expression of SmEcR and the SmUSPs was developmentally controlled, as was 20E induction. Their transcription levels increased as the larvae entered pre-diapause, followed by downregulation during diapause and upregulation during the shift to post-diapause quiescence, which is highly consistent with ecdysteroid titers in this species. Topical application of 20E to diapausing larvae also elicited a dose-dependent expression of the three genes. Expression of SmEcR and SmUSPs decreased markedly during the pre-pupal stage and was higher in adult females compared to males. These findings suggested that 20E-induced expression of SmEcR and SmUSPs has key roles in diapause initiation and maintenance, post-diapause quiescence, and adult reproduction, while the larval-pupal transformation may be associated with a decrease in their expression levels.

Toxicity of Per- and Polyfluoroalkyl Substances and Their Substitutes to Terrestrial and Aquatic Invertebrates-A Review

Per- and polyfluoroalkyl substances (PFASs) have been widely used in daily life but they cause certain impacts on the environment due to their unique carbon-fluorine chemical bonds that are difficult to degrade in the environment. Toxicological studies on PFASs and their alternatives have mainly focused on vertebrates, while terrestrial and aquatic invertebrates have been studied to a lesser extent. As invertebrates at the bottom of the food chain play a crucial role in the whole ecological chain, it is necessary to investigate the toxicity of PFASs to invertebrates. In this paper, the progress of toxicological studies on PFASs and their alternatives in terrestrial and aquatic invertebrates is reviewed, and the accumulation of PFASs, their toxicity in invertebrates, as well as the neurotoxicity and toxicity to reproduction and development are summarized. This provides a reference to in-depth studies on the comprehensive assessment of the toxicity of PFASs and their alternatives, promotes further research on PFASs in invertebrates, and provides valuable recommendations for the use and regulation of alternatives to PFASs.

Functional analysis of AgJHAMT gene related to developmental period in Aphis gossypii Glover

Aphis gossypii is one of the most economically important agricultural pests that cause serious crop losses worldwide, and the indiscriminate chemical application causes resistance development in A. gossypii, a major obstacle to successful control. In this study, we selected the up-regulated expression gene AgJHAMT, which was enriched into juvenile hormone pathway though transcriptome sequencing analysis of the cotton aphids that fed on transgenic cotton lines expressing dsAgCYP6CY3 (the TG cotton). The AgJHAMT gene was overexpressed in cotton aphids which fed on the TG cotton, and its expression profile during the nymphs was clarified. Then, silencing AgJHAMT could advance the developmental period of cotton aphids by 0.5 days compared with control groups. The T and t values of cotton aphids in the dsJHAMT treatment group (6.88 ± 0.15, 1.65 ± 0.06) were significantly shorter than that of the sprayed H2O control group (7.6 ± 0.14, 1.97 ± 0.09) (P < 0.05), respectively. The fast growth caused by AgJHAMT silencing was rescued by applying the JH analogue, methoprene. Overall, these findings clarified the function of AgJHAMT in the developmental period of A. gossypii. This study contributes to further clarify the molecular mechanisms of delaying the growth and development of cotton aphids by the transgenic cotton lines expressing dsAgCYP6CY3.

The nuclear receptor gene E75 plays a key role in regulating the molting process of the spider mite, Tetranychus urticae

The nuclear receptor gene Ecdysone-induced protein 75 (E75), as the component of ecdysone response genes in the ecdysone signaling pathway, has important regulatory function for insect molting. However, the regulatory function of E75 during the molting process of spider mites is not yet clear. In this study, the expression pattern of E75 in the molting process of the spider mite Tetranychus urticae was analyzed. The results showed that there was a peak at 8 h post-molting, followed by a decline 8 h after entering each respective quiescent stage across various developmental stages. During the deutonymph stage, the expression dynamics of E75, observed at 4-h intervals, indicated that the transcript levels of TuE75 peaked at 24 h, coinciding with the onset of molting in the mites. To investigate the function of TuE75 during the molting process, silencing TuE75 through dsRNA injection into deutonymph mites at the age of 8 h yielded a notable outcome: 78% of the deutonymph mites were unable to progress to the adult stage. Among these phenotypic mites, 37% were incapable of transitioning into the quiescent state and eventually succumbed after a certain period. An additional 41% of the mites successfully entered the quiescent state but encountered difficulties in shedding the old epidermis, leading to eventual mortality. In summary, these results suggested that TuE75 plays a key role in the molting process of T. urticae.

Ecdysteroid Biosynthesis Halloween Gene Spook Plays an Important Role in the Oviposition Process of Spider Mite, Tetranychus urticae

In insects, the ecdysteroid hormone regulates development and reproduction. However, its function in the reproduction process of spider mites is still unclear. In this study, we investigated the effect of the Halloween gene Spook on the oviposition of the reproduction process in a spider mite, Tetranychus urticae. The expression patterns of the ecdysteroid biosynthesis and signaling pathway genes, as analyzed by RT-qPCR, showed that the expression pattern of the Halloween genes was similar to the oviposition pattern of the female mite and the expression patterns of the vitellogenesis-related genes TuVg and TuVgR, suggesting that the Halloween genes are involved in the oviposition of spider mites. To investigate the function of the ecdysteroid hormone on the oviposition of the reproduction process, we carried out an RNAi assay against the Halloween gene Spook by injection in female mites. Effective silencing of TuSpo led to a significant reduction of oviposition. In summary, these results provide an initial study on the effect of Halloween genes on the reproduction in T. urticae and may be a foundation for a new strategy to control spider mites.

20-hydroxyecdysone Upregulates Ecdysone Receptor (ECR) Gene to Promote Pupation in the Honeybee, Apis mellifera Ligustica

A heterodimeric complex of two nuclear receptors, the ecdysone receptor (ECR) and ultraspiracle (USP), transduces 20-hydroxyecdysone (20E) signaling to modulate insect growth and development. Here, we aimed to determine the relationship between ECR and 20E during larval metamorphosis and also the specific roles of ECR during larval-adult transition in Apis mellifera. We found that ECR gene expression peaked in the 7-day-old larvae, then decreased gradually from the pupae stage. 20E slowly reduced food consumption and then induced starvation, resulting in small-sized adults. In addition, 20E induced ECR expression to regulate larval development time. Double-stranded RNAs (dsRNAs) were prepared using common dsECR as templates. After dsECR injection, larval transition to the pupal stage was delayed, and 80% of the larvae showed prolonged pupation beyond 18 h. Moreover, the mRNA levels of shd, sro, nvd, and spo, and ecdysteroid titers were significantly decreased in ECR RNAi larvae compared with those in GFP RNAi control larvae. ECR RNAi disrupted 20E signaling during larval metamorphosis. We performed rescuing experiments by injecting 20E in ECR RNAi larvae and found that the mRNA levels of ECR, USP, E75, E93, and Br-c were not restored. 20E induced apoptosis in the fat body during larval pupation, while RNAi knockdown of ECR genes reduced apoptosis. We concluded that 20E induced ECR to modulate 20E signaling to promote honeybee pupation. These results assist our understanding of the complicated molecular mechanisms of insect metamorphosis.

Simultaneous silencing of juvenile hormone metabolism genes through RNAi interrupts metamorphosis in the cotton boll weevil

The cotton boll weevil (CBW) (Anthonomus grandis) is one of the major insect pests of cotton in Brazil. Currently, CBW control is mainly achieved by insecticide application, which is costly and insufficient to ensure effective crop protection. RNA interference (RNAi) has been used in gene function analysis and the development of insect control methods. However, some insect species respond poorly to RNAi, limiting the widespread application of this approach. Therefore, nanoparticles have been explored as an option to increase RNAi efficiency in recalcitrant insects. Herein, we investigated the potential of chitosan-tripolyphosphate (CS-TPP) and polyethylenimine (PEI) nanoparticles as a dsRNA carrier system to improve RNAi efficiency in the CBW. Different formulations of the nanoparticles with dsRNAs targeting genes associated with juvenile hormone metabolism, such as juvenile hormone diol kinase (JHDK), juvenile hormone epoxide hydrolase (JHEH), and methyl farnesoate hydrolase (MFE), were tested. The formulations were delivered to CBW larvae through injection (0.05-2 µg), and the expression of the target genes was evaluated using RT-qPCR. PEI nanoparticles increased targeted gene silencing compared with naked dsRNAs (up to 80%), whereas CS-TPP-dsRNA nanoparticles decreased gene silencing (0%-20%) or led to the same level of gene silencing as the naked dsRNAs (up to 50%). We next evaluated the effects of targeting a single gene or simultaneously targeting two genes via the injection of naked dsRNAs or dsRNAs complexed with PEI (500 ng) on CBW survival and phenotypes. Overall, the gene expression analysis showed that the treatments with PEI targeting either a single gene or multiple genes induced greater gene silencing than naked dsRNA (∼60%). In addition, the injection of dsJHEH/JHDK, either naked or complexed with PEI, significantly affected CBW survival (18% for PEI nanoparticles and 47% for naked dsRNA) and metamorphosis. Phenotypic alterations, such as uncompleted pupation or malformed pupae, suggested that JHEH and JHDK are involved in developmental regulation. Moreover, CBW larvae treated with dsJHEH/JHDK + PEI (1,000 ng/g) exhibited significantly lower survival rate (55%) than those that were fed the same combination of naked dsRNAs (30%). Our findings demonstrated that PEI nanoparticles can be used as an effective tool for evaluating the biological role of target genes in the CBW as they increase the RNAi response.

Reproductive Outbreaks of Sogatella furcifera Mediated by Overexpression of the Nuclear Receptor USP under Pressure from Triflumezopyrim

Long-term pesticide-driven selection pressure is one of the main causes of insect outbreaks. In this study, we found that low doses of triflumezopyrim could increase the fecundity of white-backed planthoppers (Sogatella furcifera). By continuously screening 20 generations with a low dose of triflumezopyrim, a triflumezopyrim-resistant strain (Tri-strain, resistance ratio = 20.9-fold) was obtained. The average oviposition quantity and longevity of the Tri-strain (208.77 eggs and 21.31 days, respectively) were significantly higher than those of the susceptible strain (Sus-strain) (164.62 eggs and 17.85 days, respectively). To better understand the mechanism underlying the effects on reproduction, we detected the expression levels of several reproduction-related transcription factors in both the Tri- and Sus-strains. Ultraspiracle (USP) was significantly overexpressed in the Tri-strain. Knockdown of USP by RNAi severely inhibited the moulting process of S. furcifera and disrupted the development of female adult ovaries. Among the potential downstream target genes of USP, Kr-h1 (0.19-fold), Cht8 (0.56-fold) and GPCR A22 (0.31-fold) showed downregulated expression after USP-RNAi. In contrast, the expression of EcR (2.55-fold), which forms heterodimers with USP, was significantly upregulated. Furthermore, RNAi was performed on Kr-h1 in the Tri-strain, and the results show that larval moulting and the development of female adult ovaries were inhibited, consistent with the USP-RNAi results in S. furcifera. These results suggest that the transcription factors USP and Kr-h1 play important roles in the reproductive development of S. furcifera, and overexpression of USP and Kr-h1 in the Tri-resistant strain may result in reproductive outbreaks of pests.

Development of RNAi Methods for the Mormon Cricket, Anabrus simplex (Orthoptera: Tettigoniidae)

Mormon crickets are a major rangeland pest in the western United States and are currently managed by targeted applications of non-specific chemical insecticides, which can potentially have negative effects on the environment. In this study, we took the first steps toward developing RNAi methods for Mormon crickets as a potential alternative to traditional broad-spectrum insecticides. To design an effective RNAi-based insecticide, we first generated a de novo transcriptome for the Mormon cricket and developed dsRNAs that could silence the expression of seven housekeeping genes. We then characterized the RNAi efficiencies and time-course of knockdown using these dsRNAs, and assessed their ability to induce mortality. We have demonstrated that it is possible to elicit RNAi responses in the Mormon cricket by injection, but knockdown efficiencies and the time course of RNAi response varied according to target genes and tissue types. We also show that one of the reasons for the poor knockdown efficiencies could be the presence of dsRNA-degrading enzymes in the hemolymph. RNAi silencing is possible in Mormon cricket, but more work needs to be done before it can be effectively used as a population management method.

Knockdown of the Halloween Genes spook, shadow and shade Influences Oocyte Development, Egg Shape, Oviposition and Hatching in the Desert Locust

Ecdysteroids are widely investigated for their role during the molting cascade in insects; however, they are also involved in the development of the female reproductive system. Ecdysteroids are synthesized from cholesterol, which is further converted via a series of enzymatic steps into the main molting hormone, 20-hydoxyecdysone. Most of these biosynthetic conversion steps involve the activity of cytochrome P450 (CYP) hydroxylases, which are encoded by the Halloween genes. Three of these genes, spook (spo), phantom (phm) and shade (shd), were previously characterized in the desert locust, Schistocerca gregaria. Based on recent sequencing data, we have now identified the sequences of disembodied (dib) and shadow (sad), for which we also analyzed spatiotemporal expression profiles using qRT-PCR. Furthermore, we investigated the possible role(s) of five different Halloween genes in the oogenesis process by means of RNA interference mediated knockdown experiments. Our results showed that depleting the expression of SchgrSpo, SchgrSad and SchgrShd had a significant impact on oocyte development, oviposition and hatching of the eggs. Moreover, the shape of the growing oocytes, as well as the deposited eggs, was very drastically altered by the experimental treatments. Consequently, it can be proposed that these three enzymes play an important role in oogenesis.

CYP314A1-dependent 20-hydroxyecdysone biosynthesis is involved in regulating the development of pupal diapause and energy metabolism in the Chinese citrus fruit fly, Bactrocera minax

BACKGROUND

Diapause is an environmentally preprogrammed period of arrested development, and characterized by metabolic depression that can occur during any development stage of insect. The insect steroid hormone 20-hydroxyecdysone (20E), is converted from ecdysone by the cytochrome P450 enzyme shade (CYP314A1), and it exerts a potent effect on the induction and maintenance of diapause in obligatory diapause insects. However, the regulatory mechanism of 20E in obligatory diapause development remains unclear. In this study, the function of 20E in the pupal diapause of Bactrocera minax was investigated.

RESULTS

We determined the expression pattern of Halloween P450 genes from larval to adult B. minax, and found differential expression of CYP314A1 from other P450 genes, with a high level in larvae and a low level in pupae. Dysfunction of CYP314A1 by dsCYP314A1 microinjection in third-instar larvae caused significant larval mortality or abnormal pupae. Compared with dsGFP and DEPC-water, dsCYP314A1-injected larvae had significantly reduced 20E titer and altered energy metabolism, and many individuals failed to pupate. Exogenous 20E microinjected into late third-instar larvae or 20E fed to early third-instar larvae both caused similar energy metabolism changes. The 20E-treated larvae of B. minax had reduced total lipids and increased amounts of trehalose and glycogen. Furthermore, 20E-treated diapause individuals showed rapid pupal development.

CONCLUSION

The 20E biosynthesis was regulated by the expression of CYP314A1, and was involved in the induction and termination phase of obligate diapause by regulating energy metabolism in B. minax. © 2022 Society of Chemical Industry.

Knockdown of ecdysone receptor in male desert locusts affects relative weight of accessory glands and mating behavior

Locusts have been known as pests of agricultural crops for thousands of years. Recently (2018-2021) the world has faced the largest swarms of desert locusts, Schistocerca gregaria, in decades and food security in large parts of Africa and Asia was under extreme pressure. There is an urgent need for the development of highly specific bio-rational pesticides to combat these pests. However, to do so, fundamental research is needed to better understand the molecular mechanisms behind key physiological processes underpinning swarm formation, such as development and reproduction. The scope of this study is to investigate the possible role(s) of the ecdysteroid receptor in the reproductive physiology of male S. gregaria. Ecdysteroids and juvenile hormones are two important classes of insect hormones and are key regulators of post-embryonic development. Ecdysteroids are best known for their role in moulting and exert their function via a heterodimer consisting of the nuclear receptors ecdysone receptor (EcR) and retinoid-X receptor (RXR). To gain insight into the role of SgEcR and/or SgRXR in the male reproductive physiology of S. gregaria we performed RNAi-induced knockdown experiments. A knockdown of SgEcR, but not SgRXR, resulted in an increased relative weight of the male accessory glands (MAG). Furthermore, the knockdown of these genes, either in combination or separately, caused a significant delay in the onset of mating behavior. Nevertheless, the MAG appeared to mature normally and the fertility of mated males was not affected. The high transcript levels of SgEcR in the fat body, especially towards the end of sexual maturation in both males and females, represent a remarkable finding since as of yet the exact role of SgEcR in this tissue in S. gregaria is unknown. Finally, our data suggest that in some cases SgEcR and SgRXR might act independently of each other. This is supported by the fact that the spatiotemporal expression profiles of SgEcR and SgRXR do not always coincide and that knockdown of SgEcR, but not SgRXR, significantly affected the relative weight of the MAG.

Molting process revealed by the detailed expression profiles of RXR1/RXR2 and mining the associated genes in a spider mite, Panonychus citri

Spider mites have one ecdysone receptor (EcR) and multiple retinoid X receptors (RXRs). However, the function of these RXRs in spider mite development is unknown. Here, we screened the expression dynamics of two PcRXR isoforms at 4 h intervals in the deutonymphal stage of Panonychus citri. The results showed that PcEcR had an expression pattern similar to that of PcRXR2. For PcRXR1, its expression remained at a certain high level, when there was a decrease of both PcEcR and PcRXR2. In situ hybridization showed that PcRXR2 was detected in the central nervous mass, while the ecdysteroid biosynthesis gene PcSpo was mainly expressed at the edge of the central nervous mass. RNAi-based silencing of PcRXR1 or PcRXR2 showed the same phenotype as in mites with that of silencing PcEcR. Furthermore, RNA-seq was used to mine the genes associated with the expression dynamics of PcRXR1 or PcRXR2, which revealed that the heterodimer of EcR-RXR2 in spider mites might be linked with the cell autophagy and tissue remodeling during apolysis, and RXR1 might be linked with new epicuticle and exocuticle secretion during ecdysis. Taken together, these results increase our understanding of the regulation mechanism of ecdysteroid signal pathway in spider mite development.

Prothoracicostatic Activity of the Ecdysis-Regulating Neuropeptide Crustacean Cardioactive Peptide (CCAP) in the Desert Locust

Accurate control of innate behaviors associated with developmental transitions requires functional integration of hormonal and neural signals. Insect molting is regulated by a set of neuropeptides, which trigger periodic pulses in ecdysteroid hormone titers and coordinate shedding of the old cuticle during ecdysis. In the current study, we demonstrate that crustacean cardioactive peptide (CCAP), a structurally conserved neuropeptide described to induce the ecdysis motor program, also exhibits a previously unknown prothoracicostatic activity to regulate ecdysteroid production in the desert locust, Schistocerca gregaria. We identified the locust genes encoding the CCAP precursor and three G protein-coupled receptors that are activated by CCAP with EC₅₀ values in the (sub)nanomolar range. Spatiotemporal expression profiles of the receptors revealed expression in the prothoracic glands, the endocrine organs where ecdysteroidogenesis occurs. RNAi-mediated knockdown of CCAP precursor or receptors resulted in significantly elevated transcript levels of several Halloween genes, which encode ecdysteroid biosynthesis enzymes, and in elevated ecdysteroid levels one day prior to ecdysis. Moreover, prothoracic gland explants exhibited decreased secretion of ecdysteroids in the presence of CCAP. Our results unequivocally identify CCAP as the first prothoracicostatic peptide discovered in a hemimetabolan species and reveal the existence of an intricate interplay between CCAP signaling and ecdysteroidogenesis.

De novo RNA-Seq and Annotation of Sesquiterpenoid and Ecdysteroid Biosynthesis Genes and MicroRNAs in a Spider Mite Eotetranychus kankitus

Eotetranychus kankitus is an important mite pest in citrus, but molecular data on the developmental processes of E. kankitus are lacking. The different development stages mix of E. kankitus was used to sequence for transcriptome and small RNAs to identify genes and predict miRNAs associated with sesquiterpenoid and ecdysteroid biosynthesis and signaling pathways. More than 36 million clean reads were assembled and 67,927 unigenes were generated. Of the unigenes, 19,300 were successfully annotated through annotation databases NR, SwissProt, COG, GO, KEGG, PFAM, and KOG. The transcripts were involved in sesquiterpenoid biosynthesis (11 genes) and ecdysteroid biosynthesis and signaling pathway (13 genes). Another, small RNA library was obtained and 31 conserved miRNAs were identified. Five most abundant miRNAs were Ek-miR-5735, Ek-miR-1, Ek-miR-263a, Ek-miR-184, and Ek-miR-8. The target genes related to sesquiterpenoid and ecdysteroid showed that 10 of the conserved miRNAs could potentially target the sesquiterpenoid and ecdysteroid pathway according to four-prediction software, sRNAT, miRanda, RNAhybrid, and Risearch2. Thus, the results of this study will provide bioinformatics information for further molecular studies of E. kankitus which may facilitate improved pest control strategies.

Sublethal effects of DBE-DBCH diastereomers on physiology, behavior, and gene expression of Daphnia magna

1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (DBE-DBCH) is a brominated flame retardant used in commercial and industrial applications. The use of DBE-DBCH containing products has resulted in an increased release into the environment. However, limited information is available on the long-term effects of DBE-DBCH and its effects in aquatic invertebrates. Thus, the present study was aimed at determining how DBE-DBCH diastereomers (αβ and γδ) affects aquatic invertebrates using Daphnia magna as a model organism. Survival, reproduction, feeding, swimming behavior and toxicogenomic responses to environmental relevant concentrations of DBE-DBCH were analyzed. Chronic exposure to DBE-DBCH resulted in decreased lifespan, and reduced fecundity. Expression of genes involved in reproductive processes, vtg1 and jhe, were also inhibited. DBE-DBCH also induced hypoxia by inhibiting the transcription of genes involved in heme biosynthesis and oxygen transport. Furthermore, DBE-DBCH also inhibited feeding resulting in emptiness of the alimentary canal. Increased expression of the stress response biomarkers was observed following DBE-DBCH exposure. In addition, DBE-DBCH diastereomers also altered the swimming behavior of Daphnia magna. The present study demonstrates that DBE-DBCH cause multiple deleterious effects on Daphnia magna, including effects on reproduction and hormonal systems. These endocrine disrupting effects are in agreement with effects observed on vertebrates. Furthermore, as is the case in vertebrates, DBE-DBCH γδ exerted stronger effects than DBE-DBCH αβ on Daphnia magna. This indicate that DBE-DBCH γδ has properties making it more toxic to all so far studied animals than DBE-DBCH αβ.

Combined Transcriptomic Analysis and RNA Interference Reveal the Effects of Methoxyfenozide on Ecdysone Signaling Pathway of Spodoptera exigua

Spodoptera exigua is a worldwide pest afflicting edible vegetables and has developed varying levels of resistance to insecticides. Methoxyfenozide (MET), an ecdysteroid agonist, is effective against lepidopteran pests such as S. exigua. However, the mechanism of MET to S. exigua remains unclear. In this study, we analyzed the expression patterns of genes related to the ecdysone signaling pathway in transcriptome data treated with sublethal doses of MET and analyzed how expression levels of key genes affect the toxicity of MET on S. exigua. Our results demonstrated that 2639 genes were up-regulated and 2512 genes were down-regulated in S. exigua treated with LC₃₀ of MET. Of these, 15 genes were involved in the ecdysone signaling pathway. qPCR results demonstrated that ecdysone receptor A (EcRA) expression levels significantly increased in S. exigua when treated with different doses of MET, and that the RNAi-mediated silencing of EcRA significantly increased mortality to 55.43% at 72 h when L3 S. exigua larvae were exposed to MET at the LC₃₀ dose. Additionally, knocking down EcRA suppressed the most genes expressed in the ecdysone signaling pathway. The combination of MET and dsEcRA affected the expression of E74 and enhanced the expression of TREA. These results demonstrate that the adverse effects of sublethal MET disturb the ecdysone signaling pathway in S. exigua, and EcRA is closely related to MET toxic effect. This study increases our collective understanding of the mechanisms of MET in insect pests.

RNA-seq analysis of gene expression changes in cuticles during the larval-pupal metamorphosis of Plutella xylostella

The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) is a holometabolous insect that its cuticles must undergo the significant changes during the larval-pupal metamorphosis development. To elucidate these changes at molecular levels, RNA-seq analysis of cuticles from LLS (later fourth instar larval stage), PPS (prepupal stage) and PS (pupal stage) were performed in P. xylostella. In this paper, a total of 17,710 transcripts were obtained in the larval-pupal transition of P. xylostella, and out of which 2293 (881 up-regulated and 1412 down-regulated) and 2989 transcripts (2062 up-regulated and 927 down-regulated) were identified to be differentially expressed between LLS and PPS, as well as PPS and PS, respectively. The further GO and KEGG analysis of differentially expressed genes (DEGs) revealed that the 'structural constituent of cuticle', 'chitin metabolic process', 'chitin binding', 'tyrosine metabolism' and 'insect hormone biosynthesis' pathways were significantly enriched, indicating these pathways might be involved in the process of larval pupation in P. xylostella. Then, we found some genes that encoded cuticular proteins, chitinolytic enzymes, chitin synthesis enzymes, and cuticle tanning proteins changed their expression levels remarkably, indicating these genes might play important roles in the restruction (degradation and biosynthesis) of insect cuticles during the larval metamorphosis. Additionally, the significant changes in the mRNA levels of 20-hydroxyecdysone (20E) and juvenile hormone (JH) related genes suggested their crucial roles in regulating cuticle remodeling during the larval metamorphosis of P. xylostella. In conclusion, the present study provide us the comprehensive gene expression profiles to explore the molecular mechanisms of cuticle metamorphosis in P. xylostella, which laid a molecular basis to study roles of specific pathways and genes in insect development.

Changes in expressions of ecdysteroidogenic enzyme and ecdysteroid signaling genes in relation to Bombyx embryonic development

Although the role of ecdysteroids in regulating egg diapause process in Bombyx mori is well documented, temporal changes in expression levels of genes involved in ecdysteroid biosynthesis and its downstream signaling are less well understood. In the present study, we studied changes in expression levels of genes involved in ecdysteroid biosynthesis and its downstream signaling during embryonic development of B. mori. Results showed that in diapause eggs, the expression of ecdysteroid-phosphate phosphatase (EPPase) gene and Halloween genes (Spook [Spo] and Shade [Shd]) remained at very low levels. However, in eggs whose diapause initiation was prevented by HCl, significant increases in the messenger RNA (mRNA) levels of EPPase, Spo, and Shd were detected during embryonic development. Other Halloween genes (Neverland [Nvd] and Phantom [Phm]) also showed different changes between diapause and HCl-treated eggs. However, genes of Disembodied (Dib) and Shadow (Sad) showed similar changes in both diapause and HCl-treated eggs. We further investigated changes in expression levels of ecdysone receptor genes (EcRA, EcRB1, and USP) and downstream signaling genes (E75A, E75B, E74A, E74B, Br-C, HR3, HR4, KR-H1, and FTZ-F1). Results showed that genes of EcRA and the other nuclear receptors (E75A, E75B, E74A, HR3, HR4, KR-H1, and FTZ-F1) exhibited significant differential patterns between diapause and HCl-treated eggs, with increased levels being detected during later stages of embryonic development in HCl-treated eggs. Differential temporal changes in expressions of genes involved ecdysteroid biosynthesis and its downstream signaling found between diapause and HCl-treated eggs were further confirmed using nondiapause eggs. Our results showed that nondiapause eggs exhibited the same changing patterns as those in HCl-treated eggs, thus clearly indicating potential correlations between expressions of these genes and embryonic development in B. mori. To our knowledge, this is the first comprehensive report to study the transcriptional regulation of ecdysteroidogenic and ecdysteroid signaling genes, thus providing useful information for a clearer understanding of insect egg diapause mechanisms.

Perfluorinated alkyl substances impede growth, reproduction, lipid metabolism and lifespan in Daphnia magna

Per- and polyfluorinated alkyl substances (PFASs) are synthetic organofluorine compounds with unique stability accompanied with hydrophobic and lipophobic properties. Perfluorooctane sulfonate (PFOS) and Perfluorooctanoic acid (PFOA) are of high concern due to their wide application in consumer and industrial products, extreme persistence, abundant occurrence in the environment and their toxic effect to humans and animals. However, knowledge on the molecular mechanisms of toxicity and the effects on reproduction output remain scarce. In this study, we analyzed the effects of PFOS and PFOA on Daphnia magna. Acute toxicity, development, reproduction, lipid metabolism (lipid-accumulation) and lifespan was investigated, as well as the expression of genes related to these endpoints. Exposure of PFOS and PFOA at 1, 10 and 25 μM did not cause acute lethality. Hatching was reduced following exposure to both compounds, and lifespan was decreased following exposure to 25 μM PFOS. Body length of Daphnia magna was reduced significantly by 25 μM PFOS following 7 days exposure. Lipid staining revealed that all PFAS exposures increased lipid accumulation. qRT-PCR analysis of genes involved in lipid metabolism suggests that the increase in lipid content could be due to inhibition of genes involved on absorption and catabolism of fatty acids. Exposure to both PFOA and PFOS reduced the fecundity significantly. Downregulation of genes involved in development and reproductive process, including vtg2, vasa, EcRA, EcRB, usp, jhe, HR3, ftz-F1, E74 and E75 were observed. The alterations in developmental and reproductive genes as well as the disturbed lipid metabolism provides mechanistic insight into the possible causes for decreased fecundity and lifespan observed following exposure to both PFOS and PFOA.

Functional analysis of ecdysteroid biosynthetic enzymes of the rice planthopper, Nilaparvata lugens

Ecdysteroids, insect steroid hormones, play key roles in regulating insect development and reproduction. Hemipteran insects require ecdysteroids for egg production; however, ecdysteroid synthesis (ecdysteroidogenesis) details have not been elucidated. We identified all known genes encoding ecdysteroidogenic enzymes in Nilaparvata lugens and clarified their necessity during nymphal and ovarian development. We confirmed that N. lugens utilized 20-hydroxyecdysone as an active hormone. Assays using heterologous expression of enzymes in Drosophila S2 cells showed conserved functions of enzymes Neverland, CYP306A2, CYP314A1 and CYP315A1, but not CYP302A1. RNA interference and rescue analysis using 20-hydroxyecdysone demonstrated that most of the genes were necessary for nymphal development. The identified N. lugens enzymes showed conserved functions and pathways for ecdysteroidogenesis. Knockdown of ecdysteroidogenic enzyme genes in newly molted females caused failure of egg production: less vitellogenic and mature eggs in ovaries, fewer laid eggs and embryonic development deficiency of laid eggs. Considering the high expressions of ecdysteroidogenic enzyme genes in adults and ovaries, ecdysteroidogenesis in ovaries was critical for N. lugens ovarian development. Our study presents initial evidence that hemipteran insects require ecdysteroidogenesis for ovarian development.

Genome-wide identification and analysis of nuclear receptors genes for lethal screening against Bemisia tabaci Q

BACKGROUND

Nuclear receptors (NRs) play an essential role in diverse biological processes, such as insect metamorphosis. Here, transcriptome analysis and functional studies were used to determine whether NRs are involved in metamorphosis of whitefly Bemisia tabaci Q, a serious pest to crops, and to find some potential insecticide targets.

RESULTS

Twenty NRs were identified in the Bemisia tabaci Q genome and categorized into the NR0-NR6 subfamilies. The phylogenetic tree of NRs from Bemisia tabaci Q and other representative species was constructed, which provided evolutionary insight into their genetic distances. The results of spatiotemporal gene expression indicated that the majority of NR gene expression was higher in the head than the abdomen and higher in eggs than adults. Further functional analysis using RNA interference (RNAi) showed that NR genes play an important role in Bemisia tabaci Q pupation and eclosion. With respect to high mortality and effects on growth, this was reflected in the unable to become pupa when the third-stage nymph treated with double-stranded RNA (dsRNA) and the developmental time delay (4-7 days) when pupae were treated with dsRNA for the 12 NR genes during molting compared with the development time in the control.

CONCLUSION

This study provides insight into NR functions during the metamorphosis stages of Bemisia tabaci Q. Several candidate genes could be potential insecticide targets for whitefly pest control due to their important roles in insect development. © 2020 Society of Chemical Industry.

Ecdysone receptor isoforms play distinct roles in larval-pupal-adult transition in Leptinotarsa decemlineata

A heterodimer of two nuclear receptors, ecdysone receptor (EcR) and ultraspiracle, mediates 20-hydroxyecdysone (20E) signaling to modulate many aspects in insect life, such as molting and metamorphosis, reproduction, diapause and innate immunity. In the present paper, we intended to determine the isoform-specific roles of EcR during larval-pupal-adult transition in the Colorado potato beetle. Double-stranded RNAs (dsRNAs) were prepared using the common (dsEcR) or isoform-specific (dsEcRA, dsEcRB1) regions of EcR as templates. Ingestion of either dsEcR or dsEcRA, rather than dsEcRB1, by the penultimate (3rd) and final (4th) instar larvae caused failure of larval-pupal and pupal-adult ecdysis. The RNA interference (RNAi) larvae remained as prepupae, or became deformed pupae and adults. Determination of messenger RNA (mRNA) levels of EcR isoforms found that LdEcRA regulates the expression of LdEcRB1. Moreover, silencing the two EcR transcripts, LdEcRA or LdEcRB1 reduced the mRNA levels of Ldspo and Ldsad, and lowered 20E titer. In contrast, the expression levels of HR3, HR4, E74 and E75 were significantly decreased in the LdEcR or LdEcRA RNAi larvae, but not in LdEcRB1 depleted specimens. Dietary supplement with 20E did not restore the expression of five 20E signaling genes (USP, HR3, HR4, E74 and E75), and only partially alleviated the pupation defects in dsEcR- or dsEcRA-fed beetles. These data suggest that EcR plays isoform-specific roles in the regulation of ecdysteroidogenesis and the transduction of 20E signal in L. decemlineata.

Post-transcriptional regulation of insect metamorphosis and oogenesis

Metamorphic transformation from larvae to adults along with the high fecundity is key to insect success. Insect metamorphosis and reproduction are governed by two critical endocrines, juvenile hormone (JH), and 20-hydroxyecdysone (20E). Recent studies have established a crucial role of microRNA (miRNA) in insect metamorphosis and oogenesis. While miRNAs target genes involved in JH and 20E-signaling pathways, these two hormones reciprocally regulate miRNA expression, forming regulatory loops of miRNA with JH and 20E-signaling cascades. Insect metamorphosis and oogenesis rely on the coordination of hormones, cognate genes, and miRNAs for precise regulation. In addition, the alternative splicing of genes in JH and 20E-signaling pathways has distinct functions in insect metamorphosis and oogenesis. We, therefore, focus in this review on recent advances in post-transcriptional regulation, with the emphasis on the regulatory role of miRNA and alternative splicing, in insect metamorphosis and oogenesis. We will highlight important new findings of miRNA interactions with hormonal signaling and alternative splicing of JH receptor heterodimer gene Taiman.

Transcriptome of pleuropodia from locust embryos supports that these organs produce enzymes enabling the larva to hatch

Background

Pleuropodia are limb-derived glandular organs that transiently appear on the first abdominal segment in embryos of insects from majority of “orders”. They are missing in the genetic model Drosophila and little is known about them. Experiments carried out on orthopteran insects 80 years ago indicated that the pleuropodia secrete a “hatching enzyme” that digests the serosal cuticle to enable the larva to hatch, but evidence by state-of-the-art molecular methods is missing.

Results

We used high-throughput RNA-sequencing to identify the genes expressed in the pleuropodia of the locust Schistocerca gregaria (Orthoptera). First, using transmission electron microscopy we studied the development of the pleuropodia during 11 stages of the locust embryogenesis. We show that the glandular cells differentiate and start secreting just before the definitive dorsal closure of the embryo and the secretion granules outside the cells become more abundant prior to hatching. Next, we generated a comprehensive embryonic reference transcriptome for the locust and used it to study genome wide gene expression across ten morphologicaly defined stages of the pleuropodia. We show that when the pleuropodia have morphological markers of functional organs and produce secretion, they are primarily enriched in transcripts associated with transport functions. They express genes encoding enzymes capable of digesting cuticular protein and chitin. These include the potent cuticulo-lytic Chitinase 5, whose transcript rises just before hatching. Unexpected finding was the enrichment in transcripts for immunity-related enzymes. This indicates that the pleuropodia are equipped with epithelial immunity similarly as barrier epithelia in postembryonic stages.

Conclusions

These data provide transcriptomic support for the historic hypothesis that pleuropodia produce cuticle-degrading enzymes and function in hatching. They may also have other functions, such as facilitation of embryonic immune defense. By the genes that they express the pleuropodia are specialized embryonic organs and apparently an important though neglected part of insect physiology.

Functional characterization of ultraspiracle in Leptinotarsa decemlineata using RNA interference assay

A heterodimer of ultraspiracle (USP) and ecdysone receptor (EcR) mediates 20-hydroxyecdysone (20E) signalling cascade to regulate insect moulting and metamorphosis. However, at least two questions remain to be addressed in terms of the molecular importance of USP in insect species. First, is USP involved in both regulation of ecdysteroidogenesis and mediation of 20E signalling in non-drosophilid insects, as in Drosophila melanogaster? Second, does USP play any role in larval metamorphosis except as the partner of heterodimeric receptor to activate the downstream 20E signalling genes? In this paper, we found that RNA interference (RNAi) of LdUSP in the final (fourth) instar larvae reduced the messenger RNA levels of four ecdysteroidogenesis genes (Ldspo, Ldphm, Lddib and Ldsad) and 20E titre, and repressed the expression of five 20E signal genes (EcRA, HR3, HR4, E74 and E75) in Leptinotarsa decemlineata. The LdUSP RNAi larvae remained as prepupae, with developing antennae, legs and discs of forewings and hindwings. Dietary supplement with 20E restored the expression of the five 20E signal genes, but only partially alleviated the decreased pupation rate in LdUSP RNAi beetles. Knockdown of LdUSP at the penultimate (third) instar larvae did not affect third-fourth instar moulting. However, silencing LdUSP caused similar but less severe impairments on pupation. Accordingly, we propose that USP is undoubtedly necessary for ecdysteroidogenesis, for mediation of 20E signalling and for initiation of metamorphosis in L. decemlineata.

Expression dynamics of key ecdysteroid and juvenile hormone biosynthesis genes imply a coordinated regulation pattern in the molting process of a spider mite, Tetranychus urticae

In insects, the ecdysteroid 20-hydroxyecdysone coordinates with juvenile hormone (JH) to regulate the process of molting, development and metamorphosis; however, this interaction is still unclear in the mites. In this study, we investigated the gene related to ecdysteroid and JH biosynthesis pathways, including four ecdysteroid and 11 JH biosynthesis genes. We examined their expression patterns during molting of different developmental stages of the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), an important agricultural pest that feeds on more than 1100 plant species. The expression of ecdysteroid biosynthesis Halloween genes exhibited a positive zigzag-like pattern, with a peak after 8 h of molting and a drop 8 h after entering each quiescent stage. In contrast, JH biosynthesis genes expression displayed a negative zigzag-like pattern, with a peak at 8 h after entering each quiescent stage and a drop after 8 h of each molting. These opposite patterns imply that ecdysteroid and JH expression is coordinated during the developmental transition. Our data provide an initial perspective on the co-expression of ecdysteroid and JH biosynthesis genes to regulate this important developmental process in the two-spotted spider mite.

An update on ecdysone signaling during insect oogenesis

An overview is presented of the different functions of ecdysone signaling during insect oogenesis. An extensive genetic toolkit allowed analysis with unprecedented temporal and spatial detail in Drosophila where functions were revealed in stem cell proliferation and niche maintenance, germline cyst differentiation and follicle formation, integration of nutrient and lipid signaling, follicle maturation and ovulation. Besides putative autocrine/paracrine signaling, hormonal networks were identified that integrate ecdysone with other endocrine signaling pathways. In other insects, progress in oogenesis has lagged behind although recently RNAi emerged as a new tool to analyze gene function in ovaries in hemimetabolous insects and Tribolium.

RNA Interference in Insects: Protecting Beneficials and Controlling Pests

Insects constitute the largest and most diverse group of animals on Earth with an equally diverse virome. The main antiviral immune system of these animals is the post-transcriptional gene-silencing mechanism known as RNA(i) interference. Furthermore, this process can be artificially triggered via delivery of gene-specific double-stranded RNA molecules, leading to specific endogenous gene silencing. This is called RNAi technology and has important applications in several fields. In this paper, we review RNAi mechanisms in insects as well as the potential of RNAi technology to contribute to species-specific insecticidal strategies. Regarding this aspect, we cover the range of strategies considered and investigated so far, as well as their limitations and the most promising approaches to overcome them. Additionally, we discuss patterns of viral infection, specifically persistent and acute insect viral infections. In the latter case, we focus on infections affecting economically relevant species. Within this scope, we review the use of insect-specific viruses as bio-insecticides. Last, we discuss RNAi-based strategies to protect beneficial insects from harmful viral infections and their potential practical application. As a whole, this manuscript stresses the impact of insect viruses and RNAi technology in human life, highlighting clear lines of investigation within an exciting and promising field of research.

The ecdysone receptor complex is essential for the reproductive success in the female desert locust, Schistocerca gregaria

Ecdysteroid hormones influence the development and reproduction of arthropods by binding a heterodimeric complex of nuclear receptors, the ecdysone receptor (EcR) and the retinoid-X-receptor/ultraspiracle (RXR/USP). Here, we report on the in vivo role(s) of the ecdysone receptor complex, SchgrEcR/SchgrRXR, in the female reproductive physiology of a major phytophagous pest insect, i.e. the desert locust, Schistocerca gregaria. Tissue and temporal distribution profiles were analysed during the first gonadotrophic cycle of adult female locusts. RNA interference was used as a reverse genetics tool to investigate the in vivo role of the ecdysone receptor complex in ovarian maturation, oogenesis, fertility and fecundity. We discovered that silencing the ecdysone receptor complex in S. gregaria resulted in impaired ovulation and oviposition, indicative for a crucial role of this complex in chorion formation. We also found evidence for a feedback of SchgrEcR/SchgrRXR on juvenile hormone biosynthesis by the corpora allata. Furthermore, we observed a tissue-dependent effect of the SchgrEcR/SchgrRXR knockdown on the transcript levels of the insulin receptor and neuroparsin 3 and 4. The insulin receptor transcript levels were upregulated in the brain, but not the fat body and gonads. Neuroparsins 3 and 4 transcript levels were down regulated in the brain and fat body, but not in the gonads.

Structure and function of the alternatively spliced isoforms of the ecdysone receptor gene in the Chinese mitten crab, Eriocheir sinensis

Alternative splicing is an essential molecular mechanism that increase the protein diversity of a species to regulate important biological processes. Ecdysone receptor (EcR), an essential nuclear receptor, is essential in the molting, growth, development, reproduction, and regeneration of crustaceans. In this study, the whole sequence of EcR gene from Eriocheir sinensis was obtained. The sequence was 45,481 bp in length with 9 exons. Moreover, four alternatively spliced EcR isoforms (Es-EcR-1, Es-EcR-2, Es-EcR-3 and Es-EcR-4) were identified. The four isoforms harbored a common A/B domain and a DNA-binding region but different D domains and ligand-binding regions. Three alternative splicing patterns (alternative 5′ splice site, exon skipping, and intron retention) were identified in the four isoforms. Functional studies indicated that the four isoforms have specific functions. Es-EcR-3 may play essential roles in regulating periodic molting. Es-EcR-2 may participate in the regulation of ovarian development. Our results indicated that Es-EcR has broad regulatory functions in molting and development and established the molecular basis for the investigation of ecdysteroid signaling related pathways in E. sinensis.

The ecdysis triggering hormone system is essential for successful moulting of a major hemimetabolous pest insect, Schistocerca gregaria

Insects are enclosed in a rigid exoskeleton, providing protection from desiccation and mechanical injury. To allow growth, this armour needs to be replaced regularly in a process called moulting. Moulting entails the production of a new exoskeleton and shedding of the old one and is induced by a pulse in ecdysteroids, which activates a peptide-mediated signalling cascade. In Holometabola, ecdysis triggering hormone (ETH) is the key factor in this cascade. Very little functional information is available in Hemimetabola, which display a different kind of development characterized by gradual changes. This paper reports on the identification of the ETH precursor and the pharmacological and functional characterisation of the ETH receptor in a hemimetabolous pest species, the desert locust, Schistocerca gregaria. Activation of SchgrETHR by SchgrETH results in an increase of both Ca²⁺ and cyclic AMP, suggesting that SchgrETHR displays dual coupling properties in an in vitro cell-based assay. Using qRT-PCR, an in-depth profiling study of SchgrETH and SchgrETHR transcripts was performed. Silencing of SchgrETH and SchgrETHR resulted in lethality at the expected time of ecdysis, thereby showing their crucial role in moulting.

Oral delivery of dsRNA by microbes: Beyond pest control

RNA interference (RNAi) by oral delivery of dsRNA in insects has great potential as a tool for integrated pest management (IPM), especially with respect to addressing the need to reduce off-target effect and slow down resistance development to chemical insecticides. Employing the natural association existing between insect and yeast, we developed a novel method to enable the knock down of vital genes in the pest insect Drosophila suzukii through oral delivery of species-specific dsRNA using genetically modified Saccharomyces cerevisae. D. suzukii that were fed with our “yeast biopesticide” showed a significant decrease in fitness. In this perspective article, we postulate that this approach could be adapted to a large number of species, given the great diversity of symbiotic interactions involving microorganisms and host species. Furthermore, we speculate that beyond its application as biopesticide, dsRNA delivery by genetically modified microbes can also serve to facilitate reverse genetic applications, specifically in non-model organisms.