Vitellogenin and its receptor play essential roles in the development and reproduction of the brown citrus aphid, Aphis (Toxoptera) citricidus

Plant-derived artificial miRNA effectively reduced the proliferation of aphid (Aphidoidea) through spray-induced gene silencing

BACKGROUND

Aphids (Hemiptera: Aphididae) are notorious sap-sucking insects that rampantly threaten agricultural production worldwide. Current management against aphids in the field heavily relies on chemical pesticides, which makes economical and eco-friendly methods urgently needed. Spray-induced gene silencing (SIGS) offers a powerful and precise approach to pest management. However, the high costs and instability of double-stranded RNA (dsRNA) regulators applied for downstream RNA interference (RNAi) still limit this strategy. It remains uncertain if RNAi regulators applied in SIGS could extend to small RNA (sRNA), especially miRNA.

RESULTS

We chose two sRNA sequences, miR-9b and miR-VgR, whose corresponding targets ABCG4 and VgR are both essential for aphid growth and development. The efficacy of these sequences was initially verified by chemically synthetic single-stranded RNA (syn-ssRNA). Through spray treatment, we observed a significantly decreased survival number and increased abnormality rate of green peach aphids fed on the host under laboratory conditions. Based on our previous study, we generated transgenic plants expressing artificial miR-9b (amiR-9b) and miR-VgR (amiR-VgR). Remarkably, plant-derived amiRNA exerted potent and long-lasting inhibitory efficacy with merely one percent concentration of chemical synthetics. Notably, the simultaneous application of amiR-9b and amiR-VgR exhibited superior inhibitory efficacy.

CONCLUSION

We explored the potential use of sRNA-based biopesticide through SIGS while investigating the dosage requirements. To optimize this strategy, the utilization of plant-derived amiRNA was proposed. The results suggested that attributed to stability and durability, deploying amiRNA in pest management is a potential and promising solution for the field application. © 2024 Society of Chemical Industry.

Precise coordination between nutrient transporters ensures fertility in the malaria mosquito Anopheles gambiae

Females from many mosquito species feed on blood to acquire nutrients for egg development. The oogenetic cycle has been characterized in the arboviral vector Aedes aegypti, where after a bloodmeal, the lipid transporter lipophorin (Lp) shuttles lipids from the midgut and fat body to the ovaries, and a yolk precursor protein, vitellogenin (Vg), is deposited into the oocyte by receptor-mediated endocytosis. Our understanding of how the roles of these two nutrient transporters are mutually coordinated is however limited in this and other mosquito species. Here, we demonstrate that in the malaria mosquito Anopheles gambiae, Lp and Vg are reciprocally regulated in a timely manner to optimize egg development and ensure fertility. Defective lipid transport via Lp knockdown triggers abortive ovarian follicle development, leading to misregulation of Vg and aberrant yolk granules. Conversely, depletion of Vg causes an upregulation of Lp in the fat body in a manner that appears to be at least partially dependent on target of rapamycin (TOR) signaling, resulting in excess lipid accumulation in the developing follicles. Embryos deposited by Vg-depleted mothers are completely inviable, and are arrested early during development, likely due to severely reduced amino acid levels and protein synthesis. Our findings demonstrate that the mutual regulation of these two nutrient transporters is essential to safeguard fertility by ensuring correct nutrient balance in the developing oocyte, and validate Vg and Lp as two potential candidates for mosquito control.

Comparative Analyses of Reproductive Caste Types Reveal Vitellogenin Genes Involved in Queen Fertility in Solenopsis invicta

The red imported fire ant (Solenopsis invicta Buren) is a social pest species with a robust reproductive ability that causes extensive damage. Identification of the genes involved in queen fertility is critical in order to better understand the reproductive biology and screening for the potential molecular targets in S. invicta. Here, we used the mRNA deep sequencing (RNA-seq) approach to identify differentially expressed genes (DEGs) in the transcriptomes of three reproductive caste types of S. invicta, including queen (QA) and winged female (FA) and male (MA) ants. The genes that were specific to and highly expressed in the queens were then screened, and the Vg2 and Vg3 genes were chosen as targets to explore their functions in oogenesis and fertility. A minimum of 6.08 giga bases (Gb) of clean reads was obtained from all samples, with a mapping rate > 89.78%. There were 7524, 7133, and 977 DEGs identified in the MA vs. QA, MA vs. FA, and FA vs. QA comparisons, respectively. qRT-PCR was used to validate 10 randomly selected DEGs, including vitellogenin 2 (Vg2) and 3 (Vg3), and their expression patterns were mostly consistent with the RNA-seq data. The S. invicta Vgs included conserved domains and motifs that are commonly found in most insect Vgs. SiVg2 and SiVg3 were highly expressed in queens and winged females and were most highly expressed in the thorax, followed by the fat body, head, and epidermis. Evaluation based on a loss-of-function-based knockdown analysis showed that the downregulation of either or both of these genes resulted in smaller ovaries, less oogenesis, and less egg production. The results of transcriptional sequencing provide a foundation for clarifying the regulators of queen fertility in S. invicta. The functions of SiVg2 and SiVg3 as regulators of oogenesis highlight their importance in queen fecundity and their potential as targets of reproductive disruption in S. invicta control.

Spatio-temporal patterns of ovarian development and VgR gene silencing reduced fecundity in parthenogenetic Artemia

The halophilic zooplankton brine shrimp Artemia has been used as an experimental animal in multidisciplinary studies. However, the reproductive patterns and its regulatory mechanisms in Artemia remain unclear. In this study, the ovarian development process of parthenogenetic Artemia (A. parthenogenetica) was divided into five stages, and oogenesis or egg formation was identified in six phases. The oogenesis mode was assumed to be polytrophic. We also traced the dynamic translocation of candidate germline stem cells (cGSCs) using EdU labelling and elucidated several key cytological events in oogenesis through haematoxylin and eosin staining and fluorescence imaging. Distinguished from the ovary structure of insects and crustaceans, Artemia germarium originated from ovariole buds and are located at the base of the ovarioles. RNA-seq based on five stages of ovarian development identified 2657 upregulated genes related to reproduction by pair-to-pair comparison. Gbb, Dpp, piwi, vasa, nanos, VgA and VgR genes associated with cGSCs recognition and reproductive development were screened and verified using qPCR. Silencing of the VgR gene in A. parthenogenetica (Ap-VgR) at ovarian development Stage II led to a low level of gene expression (less than 10%) within 5 days, which resulted in variations in oogenesis-related gene expression and significantly inhibited vitellogenesis, impeded oocyte maturation, and eventually decreased the number of offspring. In conclusion, we have illustrated the patterns of ovarian development, outlined the key spatio-temporal features of oogenesis and identified the negative impacts of VgR gene knockdown on oogenesis using A. parthenogenetica as an experimental animal. The findings of this study also lay a foundation for the further study of reproductive biology of invertebrates.

Novel compounds ZK-PI-5 and ZK-PI-9 regulate the reproduction of Spodoptera frugiperda (Lepidoptera: Noctuidae), with insecticide potential

Trehalase inhibitors prevent trehalase from breaking down trehalose to provide energy. Chitinase inhibitors inhibit chitinase activity affecting insect growth and development. This is an important tool for the investigation of regulation of trehalose metabolism and chitin metabolism in insect reproduction. There are few studies on trehalase or chitinase inhibitors' regulation of insect reproduction. In this study, ZK-PI-5 and ZK-PI-9 were shown to have a significant inhibitory effect on the trehalase, and ZK-PI-9 significantly inhibited chitinase activity in female pupae. We investigated the reproduction regulation of Spodoptera frugiperda using these new inhibitors and evaluated their potential as new insecticides. Compounds ZK-PI-5 and ZK-PI-9 were injected into the female pupae, and the control group was injected with solvent (2% DMSO). The results showed that the emergence failure rate for pupae treated with inhibitors increased dramatically and aberrant phenotypes such as difficulty in wings spreading occurred. The oviposition period and longevity of female adults in the treated group were significantly shorter than those in the control group, and the ovaries developed more slowly and shrank earlier. The egg hatching rate was significantly reduced by treatment with the inhibitor. These results showed that the two new compounds had a significant impact on the physiological indicators related to reproduction of S. frugiperda, and have pest control potential. This study investigated the effect of trehalase and chitin inhibitors on insect reproduction and should promote the development of green and efficient insecticides.

RNAi-mediated CrebA silencing inhibits reproduction and immunity in Locusta migratoria manilensis

Locusta migratoria manilensis is a major agricultural pest that causes severe direct and indirect damage to several crops. Thus, to provide a theoretical foundation for pest control, the role of CrebA in the reproduction and immune regulation of L. migratoria was investigated. CrebA is a bZIP transcription factor that critically regulates intracellular protein secretion. In this study, CrebA was widely expressed in the brain, fat body, integument, midgut, and reproductive tissues of different maturity stages of adult locusts, especially in the female fat body. RNA interfering (RNAi)-mediated silencing of CrebA inhibited locusts ovarian development, and key reproduction gene expressions, Vgs, VgRs, Chico, and JHAMT were downregulated. After the locusts were injected with Micrococcus luteus or Escherichia coli, M. luteus activated lysozyme expression, while the E. coli activated both phenol oxidase cascade and lysozyme expression. Furthermore, both bacteria stimulated the upregulation of the antimicrobial peptide genes DEF3 and DEF4. However, CrebA silencing is fatal to locusts infection with E. coli, with a mortality rate of up to 96.3%, and resulted in a significant decrease in the expression of DEF3 and DEF4 and changes in the activities of phenol oxidase and lysozyme of locusts infected by bacteria. Collectively, CrebA may be involved in diverse biological processes, including reproduction and immunity. CrebA inhibited locusts reproduction by regulating JH signaling pathway and inhibits the expression of immune genes TLR6, IMD, and AMPs. These results demonstrate CrebA seems to play a crucial role in reproduction and innate immunity.

Identification and functional analysis of isopentenyl pyrophosphate isomerase genes in the whiteflies Bemisia tabaci (Hemiptera: Aleyrodidae)

The juvenile hormone (JH) plays a vital role in the regulation of a number of physiological processes, including development, reproduction, and ovarian maturation. Isopentenyl pyrophosphate isomerase (IPPI) is a key enzyme in the biosynthetic pathway of JH. In this study, we identified an isopentenyl pyrophosphate isomerase protein from Bemisia tabaci and named it BtabIPPI. The open reading frame (ORF) of BtabIPPI is 768 bp and encodes a protein of 255 amino acids that contains a conserved domain of the Nudix family. The temporal and spatial expression profiles showed that BtabIPPI was highly expressed in the female adults.RNA interference (RNAi)-mediated silencing of BtabIPPI reduced JH titers and the relative expression of vitellogenin receptor (VgR) and JH signaling pathway genes, resulting in a dramatic reduction in fecundity and hatchability. These results indicate that the BtabIPPI gene plays an important role in the female fecundity of B. tabaci. This study will broaden our understanding of the function of IPPI in regulating insect reproduction and provide a theoretical basis for targeting IPPI for pest control in the future.

Fitness cost of spinosad resistance related to vitellogenin in Frankliniella occidentalis (Pergande)

BACKGROUND

The western flower thrips Frankliniella occidentalis, a worldwide agricultural pest, has developed resistance to an array of insecticides. Spinosad resistance confers an apparent fitness cost in F. occidentalis. In the present study, we compared the reproductive capacities, ovary development, and the expression of the vitellogenin (Vg) gene in spinosad-susceptible (Ivf03) and -resistant (NIL-R) near isogenetic lines of F. occidentalis in order to clarify the reason for the fitness cost in spinosad resistance.

RESULTS

The NIL-R strain exhibited a 17.9% decrease in fecundity (eggs laid per female) as compared to the Ivf03 strain, and the ovariole was significantly shortened by 2.8% in the NIL-R strain relative to the Ivf03 strain. Compared to the Ivf03 strain, the expression levels of Vg mRNA and protein were downregulated by 33.7% and 32.9% in the NIL-R strain, respectively. Moreover, interference with the Vg gene significantly reduced the expression levels of Vg mRNA and protein, and decreased ovariole length, survival rates and the fecundity of both strains.

CONCLUSION

The results indicate that the downregulated expression of Vg may contribute to the reduction of ovariole length and consequently to a fitness cost in spinosad-resistant F. occidentalis. The results not only increase our understanding of the evolution of insecticide resistance, but also could contribute to the formulation of control strategy of F. occidentalis. © 2022 Society of Chemical Industry.

Evolution and functionalization of vitellogenin genes in the termite Reticulitermes speratus

Eusociality has been commonly observed in distinct animal lineages. The reproductive division of labor is a particular feature, achieved by the coordination between fertile and sterile castes within the same nest. The sociogenomic approach in social hymenopteran insects indicates that vitellogenin (Vg) has undergone neo-functionalization in sterile castes. Here, to know whether Vgs have distinct roles in nonreproductive castes in termites, we investigated the unique characteristics of Vgs in the rhinotermitid termite Reticulitermes speratus. The four Vgs were identified from R. speratus (RsVg1-4), and RsVg3 sequences were newly identified using the RACE method. Molecular phylogenetic analysis supported the monophyly of the four termite Vgs. Moreover, the termites Vg1-3 and Vg4 were positioned in two different clades. The  dN/dS ratios indicated that the branch leading to the common ancestor of termite Vg4 was under weak purifying selection. Expression analyses among castes (reproductives, workers, and soldiers) and females (nymphs, winged alates, and queens) showed that RsVg1-3 was highly expressed in fertile queens. In contrast, RsVg4 was highly expressed in workers and female nonreproductives (nymphs and winged adults). Localization of RsVg4 messenger RNA was confirmed in the fat body of worker heads and abdomens. These results suggest that Vg genes are functionalized after gene duplication during termite eusocial transition and that Vg4 is involved in nonreproductive roles in termites.

Stabilized Double-Stranded RNA Strategy Improves Cotton Resistance to CBW (Anthonomus grandis)

Cotton is the most important crop for fiber production worldwide. However, the cotton boll weevil (CBW) is an insect pest that causes significant economic losses in infested areas. Current control methods are costly, inefficient, and environmentally hazardous. Herein, we generated transgenic cotton lines expressing double-stranded RNA (dsRNA) molecules to trigger RNA interference-mediated gene silencing in CBW. Thus, we targeted three essential genes coding for chitin synthase 2, vitellogenin, and ecdysis-triggering hormone receptor. The stability of expressed dsRNAs was improved by designing a structured RNA based on a viroid genome architecture. We transformed cotton embryos by inserting a promoter-driven expression cassette that overexpressed the dsRNA into flower buds. The transgenic cotton plants were characterized, and positive PCR transformed events were detected with an average heritability of 80%. Expression of dsRNAs was confirmed in floral buds by RT-qPCR, and the T₁ cotton plant generation was challenged with fertilized CBW females. After 30 days, data showed high mortality (around 70%) in oviposited yolks. In adult insects fed on transgenic lines, chitin synthase II and vitellogenin showed reduced expression in larvae and adults, respectively. Developmental delays and abnormalities were also observed in these individuals. Our data remark on the potential of transgenic cotton based on a viroid-structured dsRNA to control CBW.

Effects of Nano-Graphene Oxide on the Growth and Reproductive Dynamics of Spodoptera frugiperda Based on an Age-Stage, Two-Sex Life Table

The development and reproduction of the fall armyworm (FAW), Spodoptera frugiperda, which were reared on artificial diets containing nano-graphene oxide (GO), were determined based on age-stage, two-sex life table analysis. The results showed that GO had adverse effects on FAWs. Compared with the control, the duration of the egg stage and first, second, and sixth instar larval stages increased with increasing GO concentrations; however, the lifespan of male and female adults decreased with increasing GO concentrations. Weights of FAW pupae that were supplied with GO-amended diets increased by 0.17-15.20% compared to the control. Intrinsic growth, limited growth, and net reproductive rates of FAWs feeding on GO supplemented diets were significantly lower than the control, while mean generational periods (0.5 mg/g: 38.47; 1 mg/g: 40.38; 2 mg/g: 38.42) were significantly longer than the control. The expression of genes encoding vitellogenin (Vg) and vitellogenin receptor (VgR) expression was abnormal in female FAW adults feeding on GO-amended diets; the number of eggs laid decreased relative to the control, but Vg expression increased. In conclusion, GO prolonged the developmental period of FAWs, decreased fecundity, and led to a decline in the population size. The study provides a basis for the rational use of GO as a pesticide synergist for FAW control.

Molecular Characterization of Vitellogenin and Its Receptor in Spodoptera frugiperda (J. E. Smith, 1797), and Their Function in Reproduction of Female

The fall armyworm Spodoptera frugiperda is a highly polyphagous invasive pest. The strong reproductive capacity is an important factor in the rapid colonization and expansion of S. frugiperda. Vitellogenin (Vg) and vitellogenin receptor (VgR) play important roles in insect reproduction. As the precursor of vitellin (Vn), Vg provides essential nutrition for embryonic development, and VgR mediates the uptake of Vg by oocytes. In this context, we cloned and characterized these two genes of S. frugiperda (SfVg and SfVgR) and evaluated their expression profiles in different developmental stages and tissues. The RNA interference experiment was used to investigate their function in vitellogenesis. The ORF values of SfVg and SfVgR were 5250 and 5445 bp, encoding 1749 and 1815 amino acid residues, respectively. The qRT-PCR results revealed that both SfVg and SfVgR were highly expressed in female adults; SfVg was specifically expressed in the fat body, whereas SfVgR was highly expressed in the ovary. In addition, the depletion of either SfVg or SfVgR hindered oocyte maturation and ovarian development, leading to a significant decrease in fecundity. The present study reveals the importance of SfVg and SfVgR in the vitellogenesis of S. frugiperda, laying a theoretical foundation for the development of pollution-free pest control strategies with SfVg and SfVgR as new targets.

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

Simple Summary

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

Abstract

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

Suppression of yolk formation, oviposition and egg quality of locust (Locusta migratoria manilensis) infected by Paranosema locustae

Locusta migratoria manilensis is one of the most important agricultural pests in China. The locust has high fecundity and consumes large quantities of food, causing severe damage to diverse crops such as corn, sorghum, and rice. Immunity against pathogens and reproductive success are two important components of individual fitness, and many insects have a trade-off between reproduction and immunity when resources are limited, which may be an important target for pest control. In this study, adult females L. migratoria manilensis were treated with different concentrations (5 × 106 spores/mL or 2 × 107 spores/mL) of the entomopathogenic fungus Paranosema locustae. Effects of input to immunity on reproduction were studied by measuring feeding amount, enzyme activity, vitellogenin (Vg) and vitellogenin receptor (VgR) production, ovary development, and oviposition amount. When infected by P. locustae, feeding rate and phenol oxidase and lysozyme activities increased, mRNA expression of Vg and VgR genes decreased, and yolk deposition was blocked. Weight of ovaries decreased, with significant decreases in egg, length and weight.Thus, locusts used nutritive input required for reproduction to resist invasion by microsporidia. This leads to a decrease in expression of Vg and VgR genes inhibited ovarian development, and greatly decreased total fecundity. P. locustae at 2 × 107 spores/mL had a more obvious inhibitory effect on the ovarian development in migratory locusts. This study provides a detailed trade-off between reproduction and immune input of the female, which provides a reliable basis to find pest targets for biological control from those trade-off processes.

Expression and Role of Vitellogenin Genes in Ovarian Development of Zeugodacus cucurbitae

Vitellogenin (Vg) genes encode the major egg yolk protein precursor in arthropods. In this study, four Vgs were identified in Zeugodacus cucurbitae (Coquillett). Sequence analysis showed that four ZcVgs had the conserved Vg domain. Phylogenetic analysis indicated that four ZcVgs were homologous to the Vgs of Tephritidae insects. The temporal and spatial expression patterns of ZcVgs were analyzed by quantitative real-time polymerase chain reaction (RT-qPCR), and the four ZcVgs showed high expression levels in female adults, especially in the fat body. The expression of ZcVg1 and ZcVg3 was down-regulated by a low dosage (0.5 μg) of 20-hydroxyecdysone (20E), and ZcVg2, ZcVg3, and ZcVg4 were up-regulated by a high dosage (1.0 and 2.0 μg) of 20E. The expression of ZcVg1 and ZcVg2 was up-regulated by 5 μg of juvenile hormone (JH), while all of the ZcVgs were down-regulated by a low and high dosage of JH. Expression of ZcVgs was down-regulated after 24 h of starvation and recovered to normal after nutritional supplementation. After micro-injection of the gene-specific double-stranded RNA, the ZcVgs’ expression was significantly suppressed, and ovarian development was delayed in Z. cucurbitae females. The results indicate that RNA interference of reproduction-related genes is a potential pest control method that works by manipulating female fertility.

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

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

Molecular Identification and Functional Characterization of Vitellogenin Receptor From Harmonia axyridis (Coleoptera: Coccinellidae)

Vitellogenin receptors (VgRs) have vital roles in reproduction by mediating endocytosis of the vitellogenin (Vg) in oviparous insects. Same as most insect species that possess only one VgR, in this study, a single VgR mRNA (HmVgR) was identified in an important natural enemy ladybeetle, Harmonia axyridis (Pallas). The open reading frame of HmVgR was 5,340 bases encoding a protein of 1,779 amino acids. Bioinformatic analyses showed that HmVgR had conserved domain motifs of low-density lipoprotein receptor family. Based on phylogenetic analysis, HmVgR had highly homologous within the Coleoptera. The transcriptional level of HmVgR was initially detected in the newly emerged female adults, gradually increased from day 3 to day 9, peaked on day 13, and then sharply decreased on day 15. Similar to most insects, HmVgR was expressed specifically in ovarian tissue. Moreover, RNA interference (RNAi) clearly decreased the transcription levels of HmVgR, which blocked the deposition of yolk protein in the ovaries, shortened the ovarian length, and led to negative impacts on reproductive-related parameters (i.e., prolonged preoviposition periods, reduced spawning and depressed hatchability). In sum, these results indicated that HmVgR may be critical for yolk protein deposition of oocytes and can play a key role in reproduction of female adults of H. axyridis. Our results provide conclusive proof for the important roles of HmVgR in fecundity, and establish a basis for further research on its interaction with vitellogenin.

Expression Analyses of Vitellogenin and Target of Rapamycin of Sogatella furcifera (Hemiptera: Delphacidae), and Their Effects on Reproduction

Vitellogenin (Vg) and the target of rapamycin (TOR) are important genes involved in insect reproduction regulation. In this work, the full lengths of the Vg (SfVg) and TOR (SfTOR) genes of the white-backed planthopper Sogatella furcifera were cloned. The expression pattern in females showed that SfVg was highly expressed in fat bodies, and SfTOR was highly expressed in Malpighian tubules. After silencing SfVg or SfTOR, female adults did not deposit eggs. Their ovarian development was delayed, and yolk protein deposition in the oocytes was reduced. However, wild-type females mated with SfTOR-silenced males could lay eggs and produce offsprings normally. The dissections of testes and accessory glands of males with SfTOR knockdown showed that their development was not affected. Therefore, the silencing of the SfVg or SfTOR genes can effectively inhibit female reproduction, but SfTOR knockdown has no significant effect on male reproductive capacity. Furthermore, silencing SfTOR can cause SfVg expression to decrease significantly. All of the above results revealed that SfVg and SfTOR are essential for white-backed planthopper reproduction and may provide a potential target for pest control.

Involvement of clathrin-dependent endocytosis in cellular dsRNA uptake in aphids

RNAi is an essential technology for studying gene function in eukaryotes, and is also considered to be a potential strategy for pest control. However, the mechanism behind the cellular uptake of dsRNA in aphids, a group of important agricultural sucking pests, remains unknown. Here, using the pea aphid Acyrthosiphon pisum as model for aphids, we identified two core genes of clathrin-dependent endocytosis (CDE), Apchc and Apvha16. We confirmed that expression of Apchc, Apvha16 and RNAi core component genes (ApAgo2, ApDcr2 and ApR2d2) were simultaneously induced at 12 h after feeding dsRNA. By using an RNAi-of-RNAi approach, we demonstrated that suppression of Apchc and Apvha16 transcripts by RNAi significantly impaired RNAi efficiency of selected reporter genes (RGs), including ApGNBP1, Apmts and Aphb, suggesting the involvement of CDE in cellular dsRNA uptake in aphids. Further confirmation was also provided using two inhibitors, chlorpromazine (CPZ) and bafilomycin A1 (BafA1). Administration of CPZ and of BafA1 both led to an impaired silencing efficiency of the RGs in the pea aphid. Finally, these RNAi-of-RNAi results were reconfirmed in the peach aphid Myzus persicae. Taking these findings together, we conclude that CDE is involved in cellular dsRNA uptake in aphids.

Sublethal effects of bistrifluron on key biological traits, macronutrients contents and vitellogenin (SeVg) expression in Spodoptera exigua (Hübner)

The beet armyworm, Spodoptera exigua, is a highly polyphagous pest originated from Southeast Asia but has spread globally, attacking economically important crops and fruits. Bistrifluron insecticide is one of the highly active insect growth regulators that has been reported to inhibit development and longevity in other lepidopteran species and could be used in the control of S. exigua. In the present study, the age-stage, two-sex life table technique was applied to assess the sublethal effects of bistrifluron on biological traits and vitellogenin gene (SeVg) expression when 2nd instar larvae fed to sublethal concentrations (LC₁₀, LC₂₀ and LC₄₀) of bistrifluron. Mean generation time from eggs to adults was longer at LC₄₀ (37.79 ± 0.81 d) and LC₂₀ (37.04 ± 0.72) compared to the LC₁₀ (36.89 ± 0.63 d) and control groups (36.07 ± 0.38 d). Fecundity of female at LC₄₀ (279.17 ± 42.8 eggs), LC₂₀ (347 ± 35.4 eggs) and LC₁₀ (411.58 ± 42.38 eggs) were significantly lower than the control treatment (532.47 ± 7.13). Furthermore, the lower intrinsic rates of increase (LC₄₀; r = 0.1207 ± 0.009, LC₂₀; r = 0.1329 ± 0.009 and LC₁₀; r = 0.14398 ± 0.009 compared to the control r = 0.164 ± 0.0076), was observed along with significantly extended mean generation times (LC₄₀; T = 34.825 ± 0.317 days, LC₂₀; T = 33.27 ± 0.368 days and LC₁₀; T = 31.899 ± 0.398 days compared to the control 30.927 ± 0.255 days). Furthermore, the contents of energy reserve macronutrients (carbohydrate, lipid and protein) significantly reduced in dose and time dependent manner in treated insects as compared to control. Furthermore, the expression level of SeVg mRNA significantly decreased by 43.8% in the female adults when one-day-old second instar larvae were treated with sublethal concentrations of bistrifluron in comparison with the control. Documenting these sublethal effects is a vital, and often overlooked factor, in assessing the overall efficacy of insecticides in the management of pest populations.

The RNA helicase DDX3 is required for ovarian development and oocyte maturation in Locusta migratoria

DDX3 represents a well-defined subfamily of DEAD-box RNA helicase and exerts multiple functions in RNA metabolism, cell cycle, tumorigenesis, signal pathway, and fertility. Our previous study has shown that LmDDX3, the ortholog of DDX3 in Locusta migratoria, is ubiquitously expressed, and with a high abundance in testis and ovary. Knockdown of LmDDX3 results in a lethal phenotype in nymph, but it still remains unclear for its role in reproductive process. In this study, we therefore characterized LmDDX3 expression in female adult locust and analyzed its function in oocyte development. LmDDX3 was expressed in all tissues examined with significant more transcripts in ovary and hindgut. In ovary, a strong expression level was detected at the day just after adult eclosion, and a dramatic reduction then occurred during the oocyte development. LmDDX3 RNAi led to a reduced vitellogenin (Vg) expression in fat body via partially at least, the JH signaling pathway, and caused an upregulation of vitellogenin receptor (VgR) in ovary, and thus blocked the ovarian development and oocyte maturation. Sequence and phylogenetic analysis indicated that LmDDX3 was closely related to termite DDX3. Taken together, these data reveal a critical role for LmDDX3 in regulating the transcription of Vg and VgR, two major factors in vitellogenesis that is a key process required for ovary development and oocyte maturation in locust, and contribute thereof a new putative target for locust biological control.

The Gustavus Gene Can Regulate the Fecundity of the Green Peach Aphid, Myzus persicae (Sulzer)

Myzus persicae (Sulzer), commonly known as the green peach aphid, is a notorious pest that causes substantial losses to a range of crops and can transmit several plant viruses, including potato virus Y (PVY). Chemical insecticides provide only partial control of this pest and their use is not environmentally sustainable. In recent years, many genes related to growth, development, and reproduction have been used as targets for pest control. These include Gustavus (Gus), a highly conserved gene that has been reported to play an essential part in the genesis of germline cells and, hence, in fecundity in the model insect Drosophila melanogaster. We hypothesized that the Gustavus (Gus) gene was a potential target that could be used to regulate the M. persicae population. In this study, we report the first investigation of an ortholog of Gus in M. persicae, designated MpGus, and describe its role in the fecundity of this insect. First, we identified the MpGus mRNA sequence in the M. persicae transcriptome database, verified its identity with reverse transcription-polymerase chain reaction (RT-PCR), and then evaluated the transcription levels of MpGus in M. persicae nymphs of different instars and tissues with real-time quantitative PCR (RT-qPCR). To investigate its role in regulating the fecundity of M. persicae, we used RNA interference (RNAi) to silence the expression of MpGus in adult insects; this resulted in a significant reduction in the number of embryos (50.6%, P < 0.01) and newborn nymphs (55.7%, P < 0.01) in the treated aphids compared with controls. Interestingly, MpGus was also significantly downregulated in aphids fed on tobacco plants that had been pre-infected with PVY^N, concomitant with a significant reduction (34.1%, P < 0.01) in M. persicae fecundity. Collectively, these data highlight the important role of MpGus in regulating fecundity in M. persicae and indicate that MpGus is a promising RNAi target gene for control of this pest species.

Effects of Sublethal Concentrations of Insecticides on the Fecundity of Sogatella furcifera (Hemiptera: Delphacidae) via the Regulation of Vitellogenin and Its Receptor

White-backed planthopper (Sogatella furcifera, Hemiptera: Delphacidae) is an important migratory pest of rice. It causes severe economic losses by reducing crop production. Vg and VgR are important proteins that help in the successful reproduction of insects and have been studied in many insects. To understand the molecular mechanisms underlying the effects of insecticides on white-backed planthopper reproduction, we studied the expression profiles of SfVg, SfVg-like, and SfVgR in white-backed planthopper exposed to insecticides. SfVg and SfVgR silencing inhibited the ovarian development, number of eggs laid by, and hatching rate of white-backed planthopper. Thiamethoxam LC10 significantly inhibited SfVg-like and SfVgR expression. In contrast, triazophos LC25 significantly promoted SfVg, SfVg-like, and SfVgR expression and increased vitellogenin content in white-backed planthopper. These results demonstrate that insecticides can regulate the reproduction of white-backed planthopper by altering the expression of SfVg and SfVgR, thereby affecting the population density of white-backed planthopper. These findings build a foundation for improving our understanding of the molecular mechanisms underlying the effects of insecticides on the reproduction and resurgence of pests.

RNAi-Based Functional Genomics in Hemiptera

RNA interference (RNAi) is a powerful approach for sequence-specific gene silencing, displaying tremendous potential for functional genomics studies in hemipteran insects. Exploiting RNAi allows the biological roles of critical genes to be defined and aids the development of RNAi-based biopesticides. In this review, we provide context to the rapidly expanding field of RNAi-based functional genomics studies in hemipteran insects. We highlight the most widely used RNAi delivery strategies, including microinjection, oral ingestion and topical application. Additionally, we discuss the key variables affecting RNAi efficacy in hemipteran insects, including insect life-stage, gene selection, the presence of nucleases, and the role of core RNAi machinery. In conclusion, we summarise the application of RNAi in functional genomics studies in Hemiptera, focusing on genes involved in reproduction, behaviour, metabolism, immunity and chemical resistance across 33 species belonging to 14 families.

CRISPR/Cas9-induced vitellogenin knockout lead to incomplete embryonic development in Plutella xylostella

Vitellogenin (Vg) is important for insect egg maturation and embryo development. In the present study, we characterized the molecular structure and expression profile of Vg gene, and analyzed its reproductive functions in diamondback moth, Plutella xylostella (L.), a destructive pest of cruciferous crops, using CRISPR/Cas9 system. The P. xylostella Vg (PxVg) included all conserved domains and motifs that were commonly found in most insect Vgs except for the polyserine tract. PxVg gene was highly expressed in female pupae and adults. PxVg protein was detected in eggs and female adults. PxVg was mainly expressed in the fat body and its protein was detected in most tissues, except in the midgut. CRISPR/Cas9-induced PxVg knockout successfully constructed a homozygous mutant strain with a 5-base pair nucleotide deletion. No PxVg protein was found in the mutant individuals and in their ovaries. There were no significant differences between wild (WT) and mutant (Mut-5) types of P. xylostella in terms of ovariole length and the number of fully developed oocytes in newly emerged females. No significant difference was observed in the number of eggs laid within two days, but there was a lower egg hatchability (84% for WT vs. 47% for Mut-5). This is the first study presenting the functions of Vg in ovary development, egg maturation, oviposition and embryonic development of P. xylostella. Our results suggest that the reproductive functions of Vg may be species-specific in insects. It is possible that Vg may not be the major egg yolk protein precursor in P. xylostella. Other "functional Vgs" closely involved in the yolk formation and oogenesis would need to be further explored in P. xylostella.

Improving RNAi efficiency for pest control in crop species

The application of RNAi promotes the development of novel approaches toward plant protection in a sustainable way. Genetically modified crops expressing dsRNA have been developed as commercial products with great potential in insect pest management. Alternatively, some nontransformative approaches, including foliar spray, irrigation and trunk injection, are favorable in actual utilization. In this review, we summarize the recent progress and successful cases of RNAi-based pest management strategy, explore essential implications and possibilities to improve RNAi efficiency by delivery of dsRNA through transformative and nontransformative approaches, and highlight the remaining challenges and important issues related to the application of this technology.

Evaluation of a cuticle protein gene as a potential RNAi target in aphids

BACKGROUND

RNA interference (RNAi) has potential as a pest insect control technique. One possible RNAi target is the cuticle protein, which is important in insect molting and development. As an example, here we evaluate the possibility of designing double-stranded RNA (RNA) that is effective for silencing the cuticle protein 19 gene (CP19) in aphids but is harmless to non-target predator insects.

RESULTS

The sequences of CP19s were similar (86.6-94.4%) among the tested aphid species (Aphis citricidus, Acyrthosiphon pisum, and Myzus persicae) but different in the predator Propylaea japonica. Ingestion of species-specific dsRNAs of CP19 by the three aphids produced 39.3-64.2% gene silencing and 45.8-55.8% mortality. Ingestion of non-species-specific dsRNA (dsAcCP19) by Ac. pisum and M. persicae gave gene silencing levels ranging from 40.4% to 50.3% and 43.3-50.8% mortality. The dsApCP19 did not affect PjCP19 expression or developmental duration in P. japonica.

CONCLUSION

The results demonstrate that CP19 is a promising RNAi target for aphid control via one dsRNA design. The targeting of genes that are conserved in insect pests but not present in beneficial insects is a useful RNAi-based pest control strategy. © 2019 Society of Chemical Industry.

Molecular Characterization of Vitellogenin and Its Receptor in Sogatella furcifera, and Their Function in Oocyte Maturation

The yolk protein precursor, vitellogenin (Vg), provides nutrition for embryonic development whereas the vitellogenin receptor (VgR) is responsible for the uptake of yolk protein by maturing oocytes. These two proteins are key reproduction-related proteins in insects. We cloned and characterized Vg and VgR genes in Sogatella furcifera, and investigated their function in oocyte maturation. Cloned SfVg and SfVgR have open reading frames of 6,114 and 5,796 bp, encoding 2,037 and 1,931 amino acid residues, respectively. Structural analysis indicates that SfVg has the three conserved LPD_N, DUF1943, and VWFD domains, SfVgR contains all conservative motifs of the LDLR superfamily. Both genes were highly expressed in adult females; SfVg was most highly expressed in the fat body whereas SfVgR was mainly expressed in the ovary. Knockdown of either gene reduced yolk protein deposition in oocytes and arrested oocyte maturation. However, silencing one of these two genes did not affect the transcript level of the other. These results demonstrate the role of SfVgR in transporting SfVg into oocytes. Both SfVg and SfVgR are essential for oocyte maturation in S. furcifera and both genes could potentially be targeted as means of controlling this pest.

Topical dsRNA delivery induces gene silencing and mortality in the pea aphid

BACKGROUND

With the growing number of available aphid genomes and transcriptomes, an efficient and easy-to-adapt tool for gene function study is urgently required. RNA interference (RNAi), as a post-transcriptional gene silencing mechanism, is important as a research tool for determining gene functions and has potential as a novel insect control strategy. However, these applications have been hampered by the lack of effective dsRNA delivery approaches in aphids.

RESULTS

Here, we developed a convenient and efficient dsRNA delivery method, topical RNAi, in aphids. An investigation of its dose and time-dependent RNAi efficiencies revealed that with as little as 60 ng dsRNA per adult pea aphid (Acyrthosiphon pisum), the indicator gene, Aphunchback, could be significantly silenced within 2 h of exposure. The method was further validated by successfully silencing other different genes, and it was also efficient toward two other aphid species, Aphis citricidus and Myzus persicae. Furthermore, a noticeable mortality was also observed in pea aphids using topical RNAi-mediated gene silencing, within 4 days post-dsRNA application for four out of seven tested genes.

CONCLUSION

Compared with the currently used dsRNA delivery methods in aphids, microinjection and ingestion, topical RNAi is time- and cost-effective, which could greatly influence RNAi-based gene functional studies and potential candidate gene selection for developing RNAi-based aphid control strategies in the future. © 2019 Society of Chemical Industry.

Molecular Features and Expression Patterns of Vitellogenin Receptor in Calliptamus italicus (Orthoptera: Acrididae)

Vitellogenin receptor (VgR) mediates the intake of vitellin via oocytes, thus exerting an important role in vitellogenesis. In this study, reverse transcription-polymerase chain reaction (RT-PCR) and rapid-amplification of cDNA ends techniques were adopted to clone the CiVgR gene, namely the VgR gene of Calliptamus italicus, i.e., Orthopteran. The full length of CiVgR was 5,589 bp, and the open reading frame was estimated to be 5,265 bp, which encoded 1,754 amino acids (aa). Sequence alignment analysis showed that CiVgR belonged to the superfamily of low-density lipoprotein receptor genes, which contained several conserved domains, including ligand-binding domains, epidermal growth factor precursor homology domains, transmembrane domains, and cytoplasmic domains. However, no O-linked sugar domain was identified. Phylogenetic analysis showed that CiVgR had the closest genetic relationship to Blattarias. RT-PCR showed that CiVgR was only specifically expressed in the ovarian tissue of females. quantitative real time polymerase chain reaction showed that the transcription of CiVgR already appeared in the fourth-instar nymph of C. italicus, which gradually increased after adult emergence, peaked at the previtellogenesis stage, and then started to decrease. The expression pattern of CiVgR was closely associated with vitellogenesis. The findings of this study further our understanding of the molecular mechanisms involved in the reproduction of C. italicus, and provide new ideas to control this insect.

Knockdown of Methoprene-Tolerant Arrests Ovarian Development in the Sogatella furcifera (Hemiptera: Delphacidae)

Juvenile hormone (JH) is responsible for repressing larval metamorphosis and inducing vitellogenesis and egg production in insects. Methoprene-tolerant (Met) is known to be an intracellular receptor and transducer of JH. We examined the role of Met in ovarian development in the rice pest Sogatella furcifera (Horváth). We first cloned and sequenced S. furcifera Met (SfMet). The SfMet protein belongs to the basic helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) family with a bHLH domain and two PAS domains (PAS-A and PAS-B). SfMet was expressed in all developmental stages and tissues but was most highly expressed in the ovaries of adult females. Furthermore, RNA interference (RNAi) mediated silencing of SfMet substantially reduced the expression of SfVg, decreased yolk protein deposition and blocked oocyte maturation and ovarian development. These results demonstrate that SfMet plays a key role in female reproduction in S. furcifera and suggest that targeting this gene could be an effective way of controlling this pest.

Identification and Characterization of the Vitellogenin Receptor Gene and Its Role in Reproduction in the Alligatorweed Flea Beetle, Agasicles hygrophila

Agasicles hygrophila is an effective biological control agent for the invasive weed Alternanthera philoxeroides, and because of this it has been introduced to many parts of the world where A. philoxeroides is a problem. Despite this, there are no reports at present about the reproduction of this important insect. Vitellogenin receptors (VgRs) belong to the superfamily of low-density lipoprotein receptors (LDLRs). One of the roles of VgRs is to regulate the absorption of yolk protein in insects. In this study, the full length vitellogenin receptor gene (AhVgR) from A. hygrophila was sequenced and found to encode a predicted protein of 1,642 amino acids. Sequence analysis of AhVgR revealed that it contains conserved structural motifs common to LDLR family members, and a phylogenetic analysis placed AhVgR as a separate group among the order Coleoptera. AhVgR was found to be specifically expressed in ovarian tissues, and it is first transcribed in the newly-emerged females. The expression patterns are consistent with VgR genes in other insects. RNA interference (RNAi)-mediated suppression of AhVgR gene expression in adult A. hygrophila females inhibited yolk protein deposition in the ovaries, shortened the ovariole, drastically reduced egg production, and ultimately led to a decrease in fecundity. In summary, our work shows that AhVgR is critical for transporting Vg into the oocytes and plays an important role in A. hygrophila reproduction.

Identification and Expression Patterns of Three Vitellogenin Genes and Their Roles in Reproduction of the Alligatorweed Flea Beetle Agasicles hygrophila (Coleoptera: Chrysomelidae)

The alligatorweed flea beetle Agasicles hygrophila is an insect used for biological control of the aquatic weed Alternanthera philoxeroides (alligatorweed). Because these insects are oviparous, synthesis, and transportation of yolk proteins is integral to reproduction. Vitellin, the chief protein constituent in egg yolk, is mainly synthesized in the fat body and its synthesis is regulated by the transcript levels of Vitellogenin (Vg). In our study, we first cloned and characterized three Vg genes from A. hygrophila and quantified the expression levels of these Vgs in different tissues and developmental stages by RT-qPCR. Analysis of the full-length cDNA sequences of the three A. hygrophila Vg genes revealed that the open reading frames of AhVg1, AhVg2, and AhVg3 were 5175, 5346, and 5385 bp, encoding 1724, 1781, and 1794 amino acids, respectively. RT-qPCR analysis revealed that these three AhVgs have similar expression patterns; expression in the fat body was significantly higher than that in other tissues, and the highest expression was observed in the adult developmental stage. RNA interference was used to explore the functions of the AhVgs. A. hygrophila female adults injected with dsRNA targeting the AhVg genes showed decreased AhVg gene expression. Down regulation of all three AhVgs significantly affected ovary development, reduced egg laying capacity, and reduced the egg hatch rate compared with the control groups. Our findings provide the basis for further study of the functions of Vg genes in other insect species.

NADPH-Cytochrome P450 Reductase Mediates the Resistance of Aphis (Toxoptera) citricidus (Kirkaldy) to Abamectin

NADPH-cytochrome P450 reductase (CPR) plays an essential role in the cytochrome P450 enzyme system, which aids in the metabolism of endogenous and exogenous compounds including the detoxification of insecticides. In this study, the CPR transcript in Aphis (Toxoptera) citricidus (Kirkaldy) was cloned, and the deduced amino acid sequence contained an N-terminal membrane anchor, three conserved binding domains (flavin mononucleotide, flavin adeninedinucleotide, and nicotinamide adenine dinucleotide phosphate), a flavin adeninedinucleotide-binding motif, and catalytic residues. Based on phylogenetic analysis, AcCPR was grouped in the hemipteran branch. AcCPR was ubiquitously expressed at all developmental stages and was most abundant in the adults and least abundant in third instar nymphs. Compared with other tested tissues of adults, the expression level of AcCPR was significantly high in the gut. Feeding double-stranded RNA of AcCPR reduced the AcCPR mRNA level and the activity of AcCPR in aphids, and the treated insects exhibited higher susceptibility to abamectin than the control group. Furthermore, the heterologous overexpression of AcCPR in Sf9 cells resulted in a greater viability than control cells when treated with abamectin. All results demonstrated that AcCPR may contribute to the resistance of A.citricidus to abamectin, and CPR may be a potential target for novel insecticide design or a new factor in the development of insecticide resistance.