Knockdown of a putative insulin-like peptide gene LdILP2 in Leptinotarsa decemlineata by RNA interference impairs pupation and adult emergence

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

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

Functional characterization of five developmental signaling network genes in the white-backed planthopper: Potential application for pest management

BACKGROUND

The white-backed planthopper (WBPH, Sogatella furcifera) is a major rice pest that exhibits condition dependent wing dimorphisms - a macropterous (long wing) form and a brachypterous (short wing) form. Although, the gene cascade that regulates wing development and dimorphic differentiation has been largely defined, the utility of these genes as targets for pest control has yet to be fully explored.

RESULTS

Five genes typically associated with the developmental signaling network, armadillo (arm), apterous A (apA), scalloped (sd), dachs (d), and yorkie (yki) were identified from the WBPH genome and their roles in wing development assessed following RNA interference (RNAi)-mediated knockdown. At 5 days-post injection, transcript levels for all five targets were substantially decreased compared with the dsGFP control group. Among the treatment groups, those injected with dsSfarm had the most pronounced effects on transcript reduction, mortality (95 ± 3%), and incidence (45 ± 3%) of wing deformities, whereas those injected with dsSfyki had the lowest incidence (6.7 ± 4%). To assess the utility of topical RNAi for Sfarm, we used a spray-based approach that complexed a large-scale, bacteria-based double-stranded RNA (dsRNA) expression pipeline with star polycation (SPc) nanoparticles. Rice seedlings infested with third and fourth instar nymphs were sprayed with SPc-dsRNA formulations and RNAi phenotypic effects were assessed over time. At 2 days post-spray, Sfarm transcript levels decreased by 86 ± 9.5% compared with dsGFP groups, and the subsequent incidences of mortality and wing defects were elevated in the treatment group.

CONCLUSIONS

This study characterized five genes in the WBPH developmental signaling cascade, assessed their impact on survival and wing development via RNAi, and developed a nanoparticle-dsRNA spray approach for potential field control of WBPH. © 2023 Society of Chemical Industry.

Insulin Receptor Substrate-1 (IRS1) Regulates Oogenesis and Vitellogenesis in Propylea japonica by Mediating the FOXO Transcription Factor Expression, Independent of JH and 20E Signaling Pathways

The insulin receptor substrate (IRS), as the core cytoplasmic adapter protein in the insulin/insulin-like signaling (IIS) pathway, is an important mediator of cellular signaling. However, it is still unknown how IRS crosstalk with hormone signaling regulates insect growth, development, and reproduction. In this study, we demonstrated that knockdown of IRS1 significantly inhibited oogenesis, vitellogenesis, and the development of nurse cells and follicular epithelial cells. In addition, qRT-PCR results showed that FOXO transcription factors significantly responded to silencing of the IRS1 gene. However, IRS1 silencing had no significant effect on the expression of juvenile hormone/20-hydroxyecdysone (JH/20E)-signaling genes, JH synthesis, and degradation enzyme-related genes and the JH/20E titers. Our results suggested that the IIS pathway regulated ovarian development and Vg production through FOXO, independent of JH and 20E signaling pathways. This study revealed the reproductive regulation mechanism in Propylea japonica, which provides a theoretical basis for large-scale expansion of P. japonica as an environment-friendly biological control strategy.

Insulin-Like ILP2 Regulates Trehalose Metabolism to Tolerate Hypoxia/Hypercapnia in Tribolium castaneum

RNAi was used to downregulate the expression of insulin-like peptides (ILP2), with air-modulation, and high-concentration CO₂ stress, in the larvae of Tribolium castaneum. We assessed the changes in carbohydrate-related content, trehalase activity, and the expression levels of trehalose pathway genes. And pupation, adult emergence, pupation rate, and mortality were assessed. There was a significant change in the expression of ILPs in T. castaneum, at a certain concentration of CO₂. ILP2 RNAi did not alter the trehalose content significantly, however, the glycogen and glucose content increased significantly. High-concentration CO₂ stress altered the trehalose content and reduced the glycogen and glucose content. The expression levels of TPS and TRE2 were up-regulated by hypoxia/hypercapnia and dsILP2 combination, with the increase of CO₂ concentration, other trehalase genes begin to respond successively. ILP2 knockout raised the mortality and reduced the pupation rate and eclosion rate in CO₂. Understanding the insulin pathway responses to hypoxic stress induced by a high concentration of CO₂ would further elucidate the mechanisms underlying trehalose metabolism in insects.

CRISPR/Cas9-mediated mutation on an insulin-like peptide encoding gene affects the growth of the ridgetail white prawn Exopalaemon carinicauda

Insulin-like peptides (ILPs) play key roles in animal growth, metabolism and reproduction in vertebrates. In crustaceans, one type of ILPs, insulin-like androgenic gland hormone (IAG) had been reported to be related to the sex differentiations. However, the function of other types of ILPs is rarely reported. Here, we identified another type of ILPs in the ridgetail white prawn Exopalaemon carinicauda (EcILP), which is an ortholog of Drosophila melanogaster ILP7. Sequence characterization and expression analyses showed that EcILP is similar to vertebrate insulin/IGFs and insect ILPs in its heterodimeric structure and expression profile. Using CRISPR/Cas9 genome editing technology, we generated EcILP knockout (KO) prawns. EcILP-KO individuals have a significant higher growth-inhibitory trait and mortality than those in the normal group. In addition, knockdown of EcILP by RNA interference (RNAi) resulted in slower growth rate and higher mortality. These results indicated that EcILP was an important growth regulator in E. carinicauda.

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

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

Modern Techniques in Colorado Potato Beetle (Leptinotarsa decemlineata Say) Control and Resistance Management: History Review and Future Perspectives

Simple Summary

The Colorado potato beetle (CPB) is one of the most important potato pest worldwide. It is native to U.S. but during the 20th century it has dispersed through Europe, Asia and western China. It continues to expand in an east and southeast direction. Damages are caused by larvae and adults. Their feeding on potato plant leaves can cause complete defoliation and lead to a large yield loss. After the long period of using only chemical control measures, the emergence of resistance increased and some new and different methods come to the fore. The main focus of this review is on new approaches to the old CPB control problem. We describe the use of Bacillus thuringiensis and RNA interference (RNAi) as possible solutions for the future in CPB management. RNAi has proven successful in controlling many pests and shows great potential for CPB control. Better understanding of the mechanisms that affect efficiency will enable the development of this technology and boost potential of RNAi to become part of integrated plant protection in the future. We described also the possibility of using single nucleotide polymorphisms (SNPs) as a way to go deeper into our understanding of resistance and how it influences genotypes.

Abstract

Colorado potato beetle, CPB (Leptinotarsa decemlineata Say), is one of the most important pests of the potato globally. Larvae and adults can cause complete defoliation of potato plant leaves and can lead to a large yield loss. The insect has been successfully suppressed by insecticides; however, over time, has developed resistance to insecticides from various chemical groups, and its once successful control has diminished. The number of available active chemical control substances is decreasing with the process of testing, and registering new products on the market are time-consuming and expensive, with the possibility of resistance ever present. All of these concerns have led to the search for new methods to control CPB and efficient tools to assist with the detection of resistant variants and monitoring of resistant populations. Current strategies that may aid in slowing resistance include gene silencing by RNA interference (RNAi). RNAi, besides providing an efficient tool for gene functional studies, represents a safe, efficient, and eco-friendly strategy for CPB control. Genetically modified (GM) crops that produce the toxins of Bacillus thuringiensis (Bt) have many advantages over agro-technical, mechanical, biological, and chemical measures. However, pest resistance that may occur and public acceptance of GM modified food crops are the main problems associated with Bt crops. Recent developments in the speed, cost, and accuracy of next generation sequencing are revolutionizing the discovery of single nucleotide polymorphisms (SNPs) and field of population genomics. There is a need for effective resistance monitoring programs that are capable of the early detection of resistance and successful implementation of integrated resistance management (IRM). The main focus of this review is on new technologies for CPB control (RNAi) and tools (SNPs) for detection of resistant CPB populations.

The Role of Peptide Hormones in Insect Lipid Metabolism

Lipids are the primary storage molecules and an essential source of energy in insects during reproduction, prolonged periods of flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. The fat body is primarily composed of adipocytes, which accumulate triacylglycerols in intracellular lipid droplets. Genomics and proteomics, together with functional analyses, such as RNA interference and CRISPR/Cas9-targeted genome editing, identified various genes involved in lipid metabolism and elucidated their functions. However, the endocrine control of insect lipid metabolism, in particular the roles of peptide hormones in lipogenesis and lipolysis are relatively less-known topics. In the current review, the neuropeptides that directly or indirectly affect insect lipid metabolism are introduced. The primary lipolytic and lipogenic peptide hormones are adipokinetic hormone and the brain insulin-like peptides (ILP2, ILP3, ILP5). Other neuropeptides, such as insulin-growth factor ILP6, neuropeptide F, allatostatin-A, corazonin, leucokinin, tachykinins and limostatin, might stimulate lipolysis, while diapause hormone-pheromone biosynthesis activating neuropeptide, short neuropeptide F, CCHamide-2, and the cytokines Unpaired 1 and Unpaired 2 might induce lipogenesis. Most of these peptides interact with one another, but mostly with insulin signaling, and therefore affect lipid metabolism indirectly. Peptide hormones are also involved in lipid metabolism during reproduction, flight, diapause, starvation, infections and immunity; these are also highlighted. The review concludes with a discussion of the potential of lipid metabolism-related peptide hormones in pest management.

The expression of cockroach insulin-like peptides is differentially regulated by physiological conditions and affected by compensatory regulation

In insects, the insulin receptor (InR) pathway is involved in regulating key physiological processes, including juvenile hormone (JH) synthesis, vitellogenin production, and oocyte growth. This raises the question about which ligand (or ligands) binds to InR to trigger the above effects. We have cloned seven insulin-like peptides (BgILP1 to 7) from female Blattella germanica cockroaches and found that the brain expresses BgILP1 to 6, the fat body BgILP7, and the ovary BgILP2. Starvation induces the reduction of BgILP3, 5, and 6 mRNA levels in the brain, and the various BgILPs are differentially expressed during the gonadotrophic cycle. In addition, by knocking down the BgILPs we were able to identify compensatory regulation at transcriptional level between the different BgILPs, although none of the BgILP knockdown assays, including the knockdown of the seven BgILPs, produced the same phenotypes that we achieved by depleting InR. Taken together, the results indicate that B. germanica ILPs are differentially expressed in tissues and in response to physiological conditions, and that they are affected by compensatory regulation.

Silencing chitin deacetylase 2 impairs larval-pupal and pupal-adult molts in Leptinotarsa decemlineata

Insect chitin deacetylases (CDAs) are carbohydrate esterases that catalyze N-deacetylation of chitin to generate chitosan, a process essential for chitin organization and compactness during the formation of extracellular chitinous structure. Here we identified two CDA2 splice variants (LdCDA2a and LdCDA2b) in Leptinotarsa decemlineata. Both splices were abundantly expressed in larval foregut, rectum, and epidermis; their levels peaked immediately before ecdysis within each instar. In vivo results revealed that the two isoforms transcriptionally responded, positively and negatively respectively, to 20-hydroxyecdysone and juvenile hormone signaling pathways. RNA interference (RNAi)-aided knockdown of the two LdCDA2 variants (hereafter LdCDA2) or LdCDA2b, rather than LdCDA2a, resulted in three negative effects. First, foliage consumption was significantly reduced, larval developing period was lengthened, and larval growth was retarded. Second, chitin contents were reduced, whereas glucose, trehalose, and glycogen contents were increased in the LdCDA2 and LdCDA2b RNAi larvae. Third, approximately 20% of LdCDA2 and LdCDA2b RNAi larvae were trapped within the exuviae and finally died. About 60% of the abnormal pupae died as pharate adults. Around 20% of the RNAi pupae emerged as deformed adults, with small size and wrinkled wings. These adults eventually died within 1 week after molting. Our results reveal that knockdown of CDA2 affects chitin accumulation. Consequently, LdCDA2 may be a potential target for control of L. decemlineata larvae.

Hormonal signaling cascades required for phototaxis switch in wandering Leptinotarsa decemlineata larvae

Many animals exploit several niches sequentially during their life cycles, a fitness referred to as ontogenetic niche shift (ONS). To successfully accomplish ONS, transition between development stages is often coupled with changes in one or more primitive, instinctive behaviors. Yet, the underlining molecular mechanisms remain elusive. We show here that Leptinotarsa decemlineata larvae finish their ONS at the wandering stage by leaving the plant and pupating in soil. At middle wandering phase, larvae also switch their phototactic behavior, from photophilic at foraging period to photophobic. We find that enhancement of juvenile hormone (JH) signal delays the phototactic switch, and vise verse. Moreover, RNA interference (RNAi)-aided knockdown of LdPTTH (prothoracicotropic hormone gene) or LdTorso (PTTH receptor gene) impairs avoidance response to light, a phenotype nonrescuable by 20-hydroxyecdysone. Consequently, the RNAi beetles pupate at the soil surface or in shallow layer of soil, with most of them failing to construct pupation chambers. Furthermore, a combination of depletion of LdPTTH/LdTorso and disturbance of JH signal causes no additive effects on light avoidance response and pupation site selection. Finally, we establish that TrpA1 (transient receptor potential (TRP) cation channel) is necessary for light avoidance behavior, acting downstream of PTTH. We conclude that JH/PTTH cascade concomitantly regulates metamorphosis and the phototaxis switch, to drive ONS of the wandering beetles from plant into soil to start the immobile pupal stage.

Many animals occupy distinct niches and utilize diverse resources at different development stages in order to meet stage-dependent requirements and overcome stage-specific limitations. This fitness is referred to as ontogenetic niche shift (ONS). During the preparation for ONS, animals often change one or more primitive, instinctive behaviors. Holometabolous insects, with four discrete developmental periods usually in different niches, are a suitable animal group to explore the molecular modes of these behavioral switches. Here we find that Leptinotarsa decemlineata larvae, an insect defoliator of potatoes, switch their phototactic behavior, from photophilic at feeding period to photophobic during the larval-pupal transition (wandering stage). This phototactic switch facilitates the wandering larvae to accomplish the ONS from potato plants to their pupation sites below ground. We show that JH/PTTH cascade controls the phototaxis switch, through a step in photo transduction between the photoreceptor molecule and the transient receptor potential cation channel.

RNA interference against the putative insulin receptor substrate gene chico affects metamorphosis in Leptinotarsa decemlineata

It is noted that insect insulin/insulin-like growth factor/target of rapamycin signaling is critical for the regulation of metamorphosis in holometabolous insects. However, the molecular mechanism remains undetermined. Our previous findings reveal that RNA interference (RNAi)-mediated knockdown of an insulin gene (LdILP2) in Leptinotarsa decemlineata disturbs both 20-hydroxyecdysone (20E) and juvenile hormone (JH) signaling, and impairs pupation. In the present paper, we further observed that the expression of the insulin receptor substrate gene chico (Ldchico) and the phosphoinositide-3-kinase gene pi3k (Ldpi3k92E) was repressed in LdILP2 depleted larvae. Moreover, RNAi of Ldchico or Ldpi3k92E decreased food consumption, affected absorption and metabolism of amino acids and sugars, and reduced expression of several 20E (LdEcR, LdHR3 and LdE75) and JH (LdJHAMT, LdKr-h1 and LdHairy) signaling genes. As a result, larval development was postponed and larval growth was inhibited. Intriguingly, knockdown of Ldchico, rather than Ldpi3k92E, impaired larval-pupal and pupal-adult ecdysis, and specifically repressed transcription of another 20E signaling gene LdUSP. Ingestion of 20E rescued the expression of LdEcR, LdHR3 and LdE75, whereas 20E feeding restored neither the decreased LdUSP mRNA level, nor the reduced pupation and adult emergence rates in Ldchico RNAi larvae. Therefore, Chico is critical for the regulation of larval-pupal-adult transition by a PI3K-independent pathway, perhaps through activation of USP in L. decemlineata.

Prediction of a peptidome for the western tarnished plant bug Lygus hesperus

Many strategies for controlling insect pests require an understanding of their hormonal signaling agents, peptides being the largest and most diverse single class of these molecules. Lygus hesperus is a pest species of particular concern, as it is responsible for significant damage to a wide variety of commercially important plant crops. At present, little is known about the peptide hormones of L. hesperus. Here, transcriptomic data were used to predict a peptidome for L. hesperus. Fifty-three L. hesperus transcripts encoding peptide precursors were identified, with a subset amplified by PCR for sequence verification. The proteins deduced from these transcripts allowed for the prediction of a 119-sequence peptidome for L. hesperus. The predicted peptides include isoforms of allatostatin A, allatostatin B (AST-B), allatostatin C, allatotropin, bursicon, CCHamide, corazonin, crustacean cardioactive peptide, crustacean hyperglycemic hormone/ion transport peptide, diuretic hormone 31, GSEFLamide, insulin-like peptide, myosuppressin, neuroparsin, neuropeptide F, orcokinin, orcomyotropin, pyrokinin, short neuropeptide F, SIFamide, sulfakinin and tachykinin-related peptide. Of note were several isoforms of AST-B that possess -WX₇Wamide carboxyl-termini rather than the stereotypical -WX₆Wamide (e.g., KWQDMQNPGWamide), an allatotropin ending in -SARGFamide rather than -TARGFamide (GLKNGPLNSARGFamide), a GSEFLamide ending in -GTEFLamide (TVGTEFLamide), several orcokinins with PMDEIDR- rather than NFDEIDR- amino-termini (e.g., PMDEIDRAGFTHFV), and an eight rather than 12 amino acid long isoform of SIFamide (PPFNGSIFamide). Collectively, the L. hesperus peptidome predicted here provides a resource for initiating physiological investigations of peptidergic signaling in this species, including studies directed at the biological control of this agricultural pest.

Asian Citrus Psyllid RNAi Pathway - RNAi evidence

Diaphorina citri, known as the Asian citrus psyllid, is an important pest of citrus because it transmits a phloem-limited bacteria strongly implicated in huanglongbing (citrus greening disease). Emerging biotechnologies, such as RNA interference, could provide a new sustainable and environmentally friendly strategy for the management of this pest. In this study, genome and functional analysis were performed to verify whether the RNAi core genes are present in the Asian psyllid genome and if the RNAi machinery could be exploited to develop a management strategy for this pest. Analyses of RNAi-related genes in the Asian citrus psyllid genome showed an absence of sequences encoding R2D2, a dsRNA-binding protein that functions as a cofactor of Dicer-2 in Drosophila. Nevertheless, bioassays using an in Planta System showed that the Asian citrus psyllid was very sensitive to ingested dsRNA, demonstrating a strong RNAi response. A small dose of dsRNA administered through a citrus flush was enough to trigger the RNAi mechanism, causing significant suppression of the targeted transcript, and increased psyllid mortality. This study provides evidence of a functional RNAi machinery, which could be further exploited to develop RNAi based management strategies for the control of the Asian citrus psyllid.

Diversity of insulin-like peptide signaling system proteins in Calanus finmarchicus (Crustacea; Copepoda) - Possible contributors to seasonal pre-adult diapause

Calanus finmarchicus, an abundant calanoid copepod in the North Atlantic Ocean, is both a major grazer on phytoplankton and an important forage species for invertebrate and vertebrate predators. One component of the life history of C. finmarchicus is the overwintering dormancy of sub-adults, a feature key for the annual recruitment of this species in early spring. While little is known about the control of dormancy in C. finmarchicus, one hypothesis is that it is an insect-like diapause, where the endocrine system is a key regulator. One group of hormones implicated in the control of insect diapause is the insulin-like peptides (ILPs). Here, C. finmarchicus transcriptomic data were used to predict ILP signaling pathway proteins. Four ILP precursors were identified, each possessing a distinct A- and B-chain peptide; these peptides are predicted to form bioactive heterodimers via inter-chain disulfide bridging. Two ILP receptors, which likely represent splice variants of a common gene, were identified. Three insulin-degrading enzymes were also discovered, as were proteins encoding the transcription factor FOXO, a downstream target of ILP that has been implicated in the regulation of insect diapause, and insulin receptor substrate, a protein putatively linking the ILP receptor and FOXO. RNA-Seq data suggest that some C. finmarchicus insulin pathway transcripts are differentially expressed across development. As in insects, the ILP signaling system may be involved in controlling C. finmarchicus' organism-environment interactions (e.g., regulation of seasonal sub-adult diapause), a hypothesis that can now be investigated using these data.