Evaluation of RNA Interference for Control of the Grape Mealybug Pseudococcus maritimus (Hemiptera: Pseudococcidae)

Apprehending siRNA Machinery and Gene Silencing in Brinjal Shoot and Fruit Borer, Leucinodes orbonalis

RNA interference (RNAi) technology is widely used in gene functional studies and has been shown to be a promising next generation alternative for insect pest management. To understand the efficiency of RNAi machinery in Leucinodes orbonalis (L. orbonalis) Guenee, a destructive pest of eggplant, core RNAi pathway genes Argonaute-2, Dicer-2, Loquacious, and Sid-1 were mined from the transcriptome and characterized. The transcript abundance of these genes was studied after exposure to exogenous double-stranded RNA (dsRNA). Domain structure analysis revealed that these genes have conserved domains required for the definite protein function in the siRNA pathway. The protein sequences when subjected to phylogenetic analysis showed a close relation with homologs obtained from Ostrinia sp. The insects fed with dsRNA designed for vacuolar sorting protein SNF7 gene showed significant downregulation at 48 h post treatment and about 79% larval mortality. The expression study of genes showed a significant spike in transcript abundance of Dicer-2, Argonatute-2, and downregulation of Loquacious at 24 and 48 h post dsRNA exposure. The results on siRNA machinery genes expression and target gene knockdown implies L. orbonalis has an ample response to exogenous dsRNA.

Efficacy and Fate of RNA Interference Molecules in the Green Pea Aphid, Acyrthosiphon pisum

RNA interference (RNAi) is a promising technology for controlling insect pests of agriculture. This technology is mediated through the application of double-stranded RNAs (dsRNAs), which are processed within the insect cells into small interfering RNAs (siRNAs). These molecules then target and reduce the expression of the insect-specific genes that can kill or reduce the performance of the pest. The application of these RNA biopesticides generally falls under two methods: foliar sprays and expression of RNAi constructs within transgenic plants. Here, we provide evidence supporting feasibility of using transgenic plants to deliver RNAi-based biopesticides against their aphid pests. Our findings suggest that, under the Cucumis melo galactinol synthase 1 promoter, the companion cells of transformed Arabidopsis thaliana plants express dsRNAs but not siRNAs at detectable levels. Further, oral application of either siRNAs or dsRNAs is equally effective in reducing the expression of transcripts of the integral membrane protein aquaporin 1 in Acyrthosiphon pisum pea aphids. We did not find any dsRNAs or siRNAs remaining in the insects or honeydew 48 h post-exposure, suggesting a low risk of contamination of these molecules beyond the target phloem-piercing insect pests.

RNAi efficiency is enhanced through knockdown of double-stranded RNA-degrading enzymes in butterfly Papilio xuthus

The efficiency of RNA interference (RNAi) has always limited the research on the phenotype innovation of Lepidoptera insects. Previous studies have found that double-stranded RNA-degrading enzyme (dsRNase) is an important factor in RNAi efficiency, but there have been no relevant reports in butterflies (Papilionoidea). Papilio xuthus is one of the important models in butterflies with an extensive experimental application value. To explore the effect of dsRNase in the RNAi efficiency on butterflies, six dsRNase genes (PxdsRNase 1-6) were identified in P. xuthus genome, and their dsRNA-degrading activities were subsequently detected by ex vivo assays. The result shows that the dsRNA-degrading ability of gut content (<1 h) was higher than hemolymph content (>12 h). We then investigated the expression patterns of these PxdsRNase genes during different tissues and developmental stages, and related RNAi experiments were carried out. Our results show that different PxdsRNase genes had different expression levels at different developmental stages and tissues. The expression of PxdsRNase2, PxdsRNase3, and PxdsRNase6 were upregulated significantly through dsGFP injection, and PxdsRNase genes can be silenced effectively by injecting their corresponding dsRNA. RNAi-of-RNAi studies with PxEbony, which acts as a reporter gene, observed that silencing PxdsRNase genes can increase RNAi efficiency significantly. These results confirm that silencing dsRNase genes can improve RNAi efficiency in P. xuthus significantly, providing a reference for the functional study of insects such as butterflies with low RNAi efficiency.

RNAi-Mediated Screening of Selected Target Genes Against Culex quinquefasciatus (Diptera: Culicidae)

Culex quinquefasciatus, a member of the Culex pipiens complex, is widespread in Saudi Arabia and other parts of the world. It is a vector for lymphatic filariasis, Rift Valley fever, and West Nile virus. Studies have shown the deleterious effect of RNA interference (RNAi)-mediated knockdown of various lethal genes in model and agricultural pest insects. RNAi was proposed as a tool for mosquito control with a focus on Aedes aegypti and Anopheles gambiae. In this study, we examined the effect of RNAi of selected target genes on both larval mortality and adult emergence of Cx. quinquefasciatus through two delivery methods: soaking and nanoparticles. Ten candidate genes were selected for RNAi based on their known lethal effect in other insects. Disruption of three genes, chitin synthase-1, inhibitor of apoptosis 1, and vacuolar adenosine triphosphatase, resulted in the highest mortality among the selected genes using the two treatment methods. Silencing the other seven genes resulted in a medium to low mortality in both assays. These three genes are also active against a wide range of insects and could be used for RNAi-based mosquito control in the future.

Non-Target Effects of dsRNA Molecules in Hemipteran Insects

Insect pest control by RNA interference (RNAi)-mediated gene expression knockdown can be undermined by many factors, including small sequence differences between double-stranded RNA (dsRNA) and the target gene. It can also be compromised by effects that are independent of the dsRNA sequence on non-target organisms (known as sequence-non-specific effects). This study investigated the species-specificity of RNAi in plant sap-feeding hemipteran pests. We first demonstrated sequence-non-specific suppression of aphid feeding by dsRNA at dietary concentrations ≥0.5 µg µL⁻¹. Then we quantified the expression of NUC (nuclease) genes in insects administered homologous dsRNA (with perfect sequence identity to the target species) or heterologous dsRNA (generated against a related gene of non-identical sequence in a different insect species). For the aphids Acyrthosiphon pisum and Myzus persicae, significantly reduced NUC expression was obtained with the homologous but not heterologous dsRNA at 0.2 µg µL⁻¹, despite high dsNUC sequence identity. Follow-up experiments demonstrated significantly reduced expression of NUC genes in the whitefly Bemisia tabaci and mealybug Planococcus maritimus administered homologous dsNUCs, but not heterologous aphid dsNUCs. Our demonstration of inefficient expression knockdown by heterologous dsRNA in these insects suggests that maximal dsRNA sequence identity is required for RNAi targeting of related pest species, and that heterologous dsRNAs at appropriate concentrations may not be a major risk to non-target sap-feeding hemipterans.