RNA interference in Lepidoptera: an overview of successful and unsuccessful studies and implications for experimental design

Transgenic double-stranded RNA rice, a potential strategy for controlling striped stem borer (Chilo suppressalis)

BACKGROUND

Although the striped stem borer (SSB, Chilo suppressalis Walker) is a devastating pest of rice that causes significant economic losses, management options are currently limited. Plant-mediated RNA interference (RNAi) is an emerging crop protection technique in which transgenic plants are modified to express insect-specific double-stranded RNAs (dsRNAs) that trigger RNAi silencing in target pests.

RESULT

In this study, an RNAi-based screen of 35 candidate SSB genes identified a small heat shock protein gene (CssHsp) as a potential plant-based RNAi target. To assess its utility in planta, a total of 39 transgenic rice plants were generated, with 11 independent transformants found to contain a single copy of the dsCssHsp expression cassette. In life-time feeding bioassays, three transgenic lines (DS10, DS35, DS36) were found to have significant negative impacts on SSB populations. After feeding for 8 days, mortality in the three transgenic lines exceeded 60%. By pupation, mortality further increased to 90% and few SSB survived to eclosion. Gene expression analyses confirmed that CssHsp transcript levels were significantly reduced after feeding on the transgenic dsCssHsp rice.

CONCLUSION

These results demonstrate the potential for developing a plant-mediated RNAi strategy targeting CssHsp as a more biorational field-based approach for SSB control. © 2021 Society of Chemical Industry.

Functionally characterized arthropod pest and pollinator cytochrome P450s associated with xenobiotic metabolism

The cytochrome P450 family (P450s) of arthropods includes diverse enzymes involved in endogenous essential physiological functions and in the oxidative metabolism of xenobiotics, insecticides and plant allelochemicals. P450s can also establish insecticide selectivity in bees and pollinators. Several arthropod P450s, distributed in different phylogenetic groups, have been associated with xenobiotic metabolism, and some of them have been functionally characterized, using different in vitro and in vivo systems. The purpose of this review is to summarize scientific publications on arthropod P450s from major insect and mite agricultural pests, pollinators and Papilio sp, which have been functionally characterized and shown to metabolize xenobiotics and/or their role (direct or indirect) in pesticide toxicity or resistance has been functionally validated. The phylogenetic relationships among these P450s, the functional systems employed for their characterization and their xenobiotic catalytic properties are presented, in a systematic approach, including critical aspects and limitations. The potential of the primary P450-based metabolic pathway of target and non-target organisms for the development of highly selective insecticides and resistance-breaking formulations may help to improve the efficiency and sustainability of pest control.

Double-Strand RNA (dsRNA) Delivery Methods in Insects: Diaphorina citri

RNAi is a gene-silencing mechanism conserved in the vast majority of eukaryotes. It is widely used to study gene function in animals due to the ease of eliciting gene knockdown. Beyond research applications, RNAi technology based on exogenous dsRNA is a promising candidate for next generation insect pest control. An advantage of using RNAi is that design of dsRNA essentially requires only the sequence of the target gene. The greatest challenge, however, is dsRNA delivery for large-scale insect control. Delivery methods that have widely been used are oral, injection, or via soaking. Unfortunately, each insect presents its own challenges owing to the differences in the presence of dsRNA degrading enzymes, cellular uptake efficiency, expression of core RNAi machinery, the nature of the target gene, the concentration and persistence of the dsRNA, as well as the particular way of feeding of each insect, which together cause variations in the efficiency of RNAi. In this chapter, a protocol for the synthetic production of dsRNA is described along with three methods for delivery that have been successful in one of the more problematic insects, Diaphorina citri.

Chitin metabolism as a potential target for RNAi-based control of the forestry pest Hyblaea puera Cramer (Lepidoptera: Hyblaeidae)

BACKGROUND

Hyblaea puera, commonly known as the teak defoliator, is a serious pest in teak plantations. Despite the availability of control measures, this pest causes losses in yield and quality of timber through voracious feeding. RNA interference (RNAi) is a promising strategy for the control of this pest. Chitin metabolism, which is vital for the growth and development of arthropods, is a potential target for developing RNAi-based insecticides.

RESULTS

To assess the effects of chitin metabolism inhibition, H. puera larvae were treated with a chitin synthesis inhibitor, diflubenzuron (DFB). DFB treatment caused pupal deformities and disrupted eclosion. Partial gene sequences for three key genes of H. puera chitin metabolism were cloned and sequenced: chitin synthase 1 (HpCHS1), chitinase-h (HpChi-h) and ecdysone receptor (HpEcR). Feeding dsRNA cognate for these three target genes to the first instar of H. puera resulted in mortality and reduction in the corresponding transcript levels as assessed through qRT-PCR. This is the first report of RNAi in this forestry pest. The highest mortality was 45.9%, in response to dsHpEcR treatment; HpChi-h transcripts were the most down-regulated in response to dsHpEcR feeding. DsHpEcR RNAi resulted in growth inhibition and molting arrest. The mortalities were 29.7% and 32.4% for dsHpCHS1 and dsHpChi-h feeding, respectively.

CONCLUSION

Chitin metabolism could be a potential target for RNAi-based control of H. puera, and HpCHS1, HpChi-h and HpEcR could be suitable target genes. However, the RNAi efficacy needs to be improved through formulations that improve stability and uptake, and employing better delivery strategies. © 2021 Society of Chemical Industry.

RNA Interference Suppression of v-ATPase B and Juvenile Hormone Binding Protein Genes Through Topically Applied dsRNA on Tomato Leaves: Developing Biopesticides to Control the South American Pinworm, Tuta absoluta (Lepidoptera: Gelechiidae)

The South American pinworm Tuta absoluta (Meyrick) (Family: Gelechiidae) is one of the most devastating lepidopteran pests in the developing countries of South America, Africa, and Asia. This pest is classified as the most serious threat for tomato production worldwide. In the present study, we analyzed RNAi-mediated control through exogenously applied dsRNA delivery on tomato. The dsRNA treatments were made to target the juvenile hormone binding protein and the v-ATPase B. Both mRNA targets were cloned, validated by sequencing, and used to produce each dsRNA. After treatments the relative transcript expression was analyzed using qRTPCR to assess to efficacy of RNAi. A leaf-dip assay was used to provide late 2nd instar larvae three feeding access periods: 24, 48, and 72 h, to evaluate the effect of gene silencing of each target. Larvae were fed tomato leaves coated with five different RNAi concentrations (10, 20, 30, 40, and 50 micrograms/centimeter-squared), that suppressed two genes (juvenile hormone protein, JHBP, and vacuolar-type adenosine triphosphatase enzyme, v-ATPase). Treatments with dsRNA showed a significant increase in mortality at 24, 48, and 72 h after ingestion (P < 0.01, α = 0.05), along with reduced leaf damage, and increased feeding deterrence. The results suggest that these two RNAi products may provide a suitable treatment for control of this and other lepidopteran pests.

Establishing RNAi for basic research and pest control and identification of the most efficient target genes for pest control: a brief guide

RNA interference (RNAi) has emerged as a powerful tool for knocking-down gene function in diverse taxa including arthropods for both basic biological research and application in pest control. The conservation of the RNAi mechanism in eukaryotes suggested that it should-in principle-be applicable to most arthropods. However, practical hurdles have been limiting the application in many taxa. For instance, species differ considerably with respect to efficiency of dsRNA uptake from the hemolymph or the gut. Here, we review some of the most frequently encountered technical obstacles when establishing RNAi and suggest a robust procedure for establishing this technique in insect species with special reference to pests. Finally, we present an approach to identify the most effective target genes for the potential control of agricultural and public health pests by RNAi.

Innate and adaptive resistance to RNAi: a major challenge and hurdle to the development of double stranded RNA-based pesticides

RNA interference (RNAi) is a post-transcriptional gene silencing process where short interfering RNAs degrade targeted mRNA. Exploration of gene function through reverse genetics is the major achievement of RNAi discovery. Besides, RNAi can be used as a potential strategy for the control of insect pests. This has led to the idea of developing RNAi-based pesticides. Differential RNAi efficiency in the different insect orders is the biggest biological obstacle in developing RNAi-based pesticides. dsRNA stability, the sensitivity of core RNAi machinery, uptake of dsRNA and amplification and spreading of the RNAi signal are the key factors responsible for RNAi efficiency in insects. This review discusses the physiological and adaptive factors responsible for reduced RNAi in insects that pose a major challenge in developing dsRNA- based pesticides.

RNA interference in Pardosa pseudoannulata, an important predatory enemy against several insect pests, through ingestion of dsRNA-expressing Escherichia coli

RNA interference is an important technology for gene functional research in many organisms. The pond wolf spider (Pardosa pseudoannulata) is an important natural enemy of rice field pests. To facilitate large-scale gene functional research in this spider species and others, we developed an RNA interference (RNAi) method via ingestion of bacteria expressing dsRNA. The dsRNA targeting a cytochrome P450 monooxygenase (cyp41g2) was expressed in Escherichia coli HT115 (DE3). And then the bacterial suspension was fed to 14-20 days old spiderlings. The mRNA abundance of the target gene was significantly reduced after 3-day's ingestion of bacteria expressing dsRNA, and between day 5 and 7, RNAi efficiency remained stable. Thus, we selected 5 days as the optimum interference time. Furthermore, the bacteria resuspension containing 20 ng/μl dsRNA was selected as the optimum concentration. To evaluate the applicability of this method, three other genes with different tissue expression pattern were also selected as targets. And the mRNA abundance of all the four target genes was significantly reduced with RNAi efficiency between 66.0% and up to 86.9%. The results demonstrated that the oral delivery of bacteria expressing dsRNA would be an effective RNAi method for the gene functional study in P. pseudoannulata.

A dsRNA-degrading nuclease (dsRNase2) limits RNAi efficiency in the Asian corn borer (Ostrinia furnacalis)

The efficiency of RNA interference (RNAi) varies substantially among different insect species. Rapid degradation of double-stranded RNA (dsRNA) by dsRNA-degrading nucleases (dsRNases) has been implicated to cause low RNAi efficiency in several insect species. In this study, we identified four dsRNase genes (OfdsRNase1, OfdsRNase2, OfdsRNase3 and OfdsRNase4) from the Asian corn borer (Ostrinia furnacalis) transcriptome database. Bioinformatic analyses showed that each deduced protein sequence contained endonuclease NS domains and signal peptides. Gene expression analysis revealed that OfdsRNase2 was exclusively expressed in the midgut of larvae. RNAi efficiency was investigated in 2-d-old fifth-instar larvae (high expression of dsRNase2) and 2-d-old pupae (low expression of dsRNase2) by feeding or injecting dsRNA targeting a marker gene that encodes the lethal giant larvae protein (OfLgl). Our results showed that OfLgl only partially silenced the expression of OfLgl in pupae, but not in larvae, suggesting that OfdsRNase2 could contribute to lower RNAi efficiency in larval stages. This hypothesis was supported by our RNAi-of-RNAi experiment using a tissue culture technique where the silencing efficiency against the reporter gene, OfHex1, was significantly improved after knockdown of OfdsRNase2. When double luciferase assays were performed to evaluate the role of the four dsRNases in vitro, only OfdsRNase2 expressed in S2 cells significantly affected RNAi efficiency by degrading dsRNA. Taken together, our results suggested that the degradation of dsRNA by OfdsRNase2 in the midgut contributed to low RNAi efficiency in O. furnacalis larvae.

The induced knockdown of GmCAD receptor protein encoding gene in Galleria mellonella decreased the insect susceptibility to a Photorhabdus akhurstii oral toxin

Photorhabdus bacteria secrete a repertoire of protein toxins that can kill the host insect. Among them, toxin complex (Tc) proteins have gained significant attention due to their wider conservation across the different bacterial genera. In our laboratory, a C-terminal domain of TcaB protein was characterized from P. akhurstii bacterium that conferred the potent oral insecticidal effect on Galleria mellonella. However, the role of insect gut receptors in the TcaB intoxication process was yet to be investigated. In the current study, we examined the transcription of candidate midgut receptors in TcaB-infected larvae and subsequently cloned a cadherin-like gene, GmCAD, from G. mellonella. GmCAD was highly transcribed in the fourth-instar larval stage and specifically in the midgut tissues. Our ligand blot and binding ELISA assays indicated that TcaB binds to the truncated peptides from the GmCAD transmembrane-proximal region with greater affinity than that from the transmembrane-distal region. Oral administration of bacterially expressed GmCAD dsRNA in G. mellonella severely attenuated the expression of target mRNA, which in turn alleviated the negative effect of TcaB on insect survival (TcaB-induced mortality in CAD dsRNA pretreated larvae reduced by 72-83% compared to control), implying the association of GmCAD in the TcaB intoxication process. Present findings form a basis of future research related to the insect gut receptor interactions with Photorhabdus toxins.

CRISPR/Cas9 in lepidopteran insects: Progress, application and prospects

Clustered regularly spaced short palindrome repeats (CRISPR) structure family forms the acquired immune system in bacteria and archaea. Recent advances in CRISPR/Cas genome editing as derived from prokaryotes, confirmed the characteristics of robustness, high target specificity and programmability, and also revolutionized the insect sciences field. The successful application of CRISPR in a wide variety of lepidopteran insects, with a high genetic diversity, provided opportunities to explore gene functions, insect modification and pest control. In this review, we present a detailed overview on the recent progress of CRISPR in lepidopteran insects, and described the basic principles of the system and its application. Major interest is on wing development, pigmentation, mating, reproduction, sex determination, metamorphosis, resistance and silkworm breeding innovation. Finally, we outlined the limitations of CRISPR/Cas system and discussed its application prospects in lepidopteran insects.

Knockdown of Helicoverpa armigera protease genes affects its growth and mortality via RNA interference

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), the cotton bollworm, is a destructive pest which is famous for its resistance to a variety of insecticides. RNA interference is a posttranscriptional gene silencing mechanism that has become a popular tool to control insect pests, triggered by double-stranded RNAs (dsRNAs). The effect of ingestion and injection delivery methods of dsRNA related to some protease genes including Trypsin (Ha-TRY39 and Ha-TRY96), Chymotrypsin (Ha-CHY), and Cathepsin L (Ha-CAT) on growth and development of H. armigera was investigated in this study. All protease genes encoded full ORFs and were expressed in all H. armigera larvae stages and tissues. In both injection and feeding bioassays, Ha-RNAi CHY's performance outperformed that of other protease genes. CHY enzyme activity in the midgut of larvae was significantly reduced after treatment with ds-HaCHY. Oral administration of ds-CHY also resulted in significant mortality of H. armigera larvae. However, because of the high RNase activity in the midgut lumen of lepidoptera, a large amount of dsRNA was needed to effectively kill instars of H. armigera. To reduce dsRNA degradation, bacterial expression and dsRNA formulation were used. After oral administration, it was toxic to H. armigera larvae. Before oral administration, bacterial cells were sonicated to increase dsRNA release. The RNA interference efficiency of sonicated bacteria was significantly increased, resulting in higher larval mortality when administered orally. All of these findings point to Ha-CHY as a new candidate for developing an effective dsRNA-based pesticide for H. armigera control.

A simple method for ex vivo honey bee cell culture capable of in vitro gene expression analysis

Cultured cells are a very powerful tool for investigating biological events in vitro; therefore, cell lines have been established not only in model insect species, but also in non-model species. However, there are few reports on the establishment of stable cell lines and development of systems to introduce genes into the cultured cells of the honey bee (Apis mellifera). We describe a simple ex vivo cell culture system for the honey bee. Hemocyte cells obtained from third and fourth instar larvae were cultured in commercial Grace’s insect medium or MGM-450 insect medium for more than two weeks maintaining a normal morphology without deterioration. After an expression plasmid vector bearing the enhanced green fluorescent protein (egfp) gene driven by the immediate early 2 (IE2) viral promoter was transfected into cells, EGFP fluorescence was detected in cells for more than one week from one day after transfection. Furthermore, double-stranded RNA corresponding to a part of the egfp gene was successfully introduced into cells and interfered with egfp gene expression. A convenient and reproducible method for an ex vivo cell culture that is fully practicable for gene expression assays was established for the honey bee.

Molecular Characterizations and Functional Analyses of LmR2D2 in the Locusta migratoria siRNA Pathway

Small interfering RNAs (siRNAs) are non-coding RNAs with a length of 21~23 nucleotides (nt) and present in almost all eukaryotes. The formation of siRNA is a highly conserved post-transcriptional gene-silencing mechanism mediated by key proteins, including Dicer2, Argonaute2 (Ago2) and R2D2. R2D2 has been identified as a double-stranded RNA (dsRNA)-binding protein and reported as an integral component of the siRNA pathway in Drosophila. However, the involvement of R2D2 in the siRNA pathway of Locusta migratoria is still unknown. In the present study, we identified an LmR2D2 gene from the transcriptome of L. migratoria. It consists of a 954-bp open reading frame that encodes a protein of 318 amino acid residues. Further sequence analysis revealed that LmR2D2 possesses two tandem dsRNA-binding domains (dsRBD) at the N-terminus. Analysis of the developmental expression profile of LmR2D2 indicated that its transcript level was stable in third-instar nymphs of L. migratoria, whereas the tissue-dependent expression profile exhibited high levels of expression of LmR2D2 in the testis and ovary. When LmR2D2 was silenced by RNAi, the RNAi efficiency against Lmβ-tubulin as a marker gene was significantly diminished, as indicated by the 37.7% increased Lmβ-tubulin transcript level. Additionally, the prokaryotic expression system was used to obtain the LmR2D2 supernatant protein. By incubating the LmR2D2 protein with biotin-dsRNA, we found that LmR2D2 can bind to dsRNA in vitro, which supports our conclusion that LmR2D2 plays an essential role in the siRNA pathway of L. migratoria.

Symbiont-Mediated RNA Interference (SMR): Using Symbiotic Bacteria as Vectors for Delivering RNAi to Insects

RNA interference (RNAi) has emerged as a widely used approach for reverse genetic analysis in eukaryotes. In insects, RNAi also has an application in the control of insect pests. Several methods have been developed for delivery of interfering RNA in insects, with varying outcomes for different species. Here we describe how a bacterial symbiont can be exploited for continuous synthesis of interfering double-stranded RNA (dsRNA) in its insect host. This approach, termed symbiont-mediated RNAi (SMR), can overcome problems associated with instability of dietary dsRNA due to action of salivary or foregut nucleases. As insects do not possess RNA-dependent RNA polymerase activity that can amplify and extend RNAi in other organisms, SMR also offers the possibility of long-term systemic RNAi not afforded by single applications of dsRNA to insects by other delivery methods. Here, we describe how SMR can be applied in a globally distributed agricultural pest species, western flower thrips (Frankliniella occidentalis).

Methods for Delivery of dsRNAs for Agricultural Pest Control: The Case of Lepidopteran Pests

RNA interference (RNAi) is a natural mechanism of gene regulation, highly conserved in eukaryotes. Since the elucidation of the gene silencing mechanism, RNAi became an important tool used in insect reverse genetics. The demonstration of effective target-gene silencing by ingestion of double-stranded RNA (dsRNA) produced by transgenic plants indicated the RNAi potential to be used in insect pest management, particularly in agriculture. However, the efficiency of gene silencing by RNAi in insects may vary according to the target taxa, and lepidopteran species have been shown to be quite recalcitrant to RNAi. Developing transgenic plants is a time-consuming and labor-intensive process, so alternative oral delivery systems are required to develop and optimize RNAi settings, such as selecting an efficient target gene, and dsRNA design, length, and stability, among other features. We have developed delivery systems to evaluate dsRNAs to silence genes from two important lepidopteran crop pests of tomato (Solanum lycopersicum) and sugarcane (Saccharum × officinarum): Tuta absoluta (Meyrick), the South American Tomato Pinworm, and Diatraea saccharalis (Fabricius), the Sugarcane Borer, respectively. The protocol described here can be used in similar species and includes (a) direct oral delivery by droplets containing dsRNA; (b) oral delivery by tomato leaflets that absorbed dsRNA solution; (c) delivery by Escherichia coli expressing dsRNA; and (d) delivery by transgenic plants expressing dsRNA.

Characterization of Heat Shock Protein 60 as an Interacting Partner of Superoxide Dismutase 2 in the Silkworm, Bombyx mori, and Its Response to the Molting Hormone, 20-Hydroxyecdysone

Oxidative stress promotes pupation in some holometabolous insects. The levels of superoxide, a reactive oxygen species (ROS), are increased and superoxide dismutase 1 (BmSod1) and superoxide dismutase 2 (BmSod2) are decreased during metamorphic events in silkworm (Bombyx mori). These observations strongly suggest that pupation is initiated by oxidative stress via the down-regulation of BmSod1 and BmSod2. However, the molecular mechanisms underlying ROS production during metamorphic events in silkworm remain unknown. To investigate these molecular mechanisms, the peripheral proteins of BmSod1 and BmSod2 were identified and characterized using dry and wet approaches in this study. Based on the results, silkworm heat shock protein 60 (BmHsp60) was identified as an interacting partner of BmSod2, which belongs to the Fe/MnSOD family. Furthermore, the present study results showed that BmHsp60 mRNA expression levels were increased in response to oxidative stress caused by ultraviolet radiation and that BmHsp60 protein levels (but not mRNA levels) were decreased during metamorphic events, which are regulated by the molting hormone 20-hydroxyecdysone. These findings improve our understanding of the mechanisms by which holometabolous insects control ROS during metamorphosis.

Coleopteran-specific StaufenC functions like Drosophila melanogaster Loquacious-PD in dsRNA processing

In Drosophila melanogaster, PD isoform of the double-stranded RNA binding protein (dsRBP) Loquacious (Loqs-PD) facilitates dsRNA cleavage to siRNA by Dicer-2. StaufenC (StauC) was discovered as a coleopteran-specific dsRBP required for dsRNA processing in coleopteran insects. Here, we show that StauC is essential for the high RNAi efficiency observed in coleopterans. Knockdown of StauC but not the homologs of Loqs-PD and R2D2 evoked a long-lasting insensitivity to RNAi in the coleopteran cell line, Ledp-SL1. The dsRNA insensitivity induced by StauC knockdown could not be overcome merely by an increase in dose or time of exposure to dsRNA or expression of Loquacious or R2D2. Furthermore, StauC but not Loqs and R2D2 are required for processing of dsRNA into siRNA. StauC overexpression also partly restored the impaired RNAi caused by the knockdown of Loqs-PD in D. melanogaster Kc cells. However, StauC was unable to compensate for the loss-of-the function of Dcr-2 or R2D2. Overall, these data suggest that StauC functions like Lops-PD in processing dsRNA to siRNA.

Double-Stranded RNA-Degrading Enzymes Reduce the Efficiency of RNA Interference in Plutella xylostella

DsRNA-degrading enzymes (dsRNases) have been recognized as important factors in reducing RNA interference (RNAi) efficiency in different insect species. However, dsRNases in Plutella xylostella are still unknown. We identified the full-length cDNAs of PxdsRNase1, PxdsRNase2, PxdsRNase3, and PxdsRNase4. Gene expression profile showed that PxdsRNase1 was mainly expressed in the hemolymph; and that PxdsRNase2 and PxdsRNase3 were mainly expressed in the intestinal tract. The expression of PxCht (Chitinase of P. xylostella) in P. xylostella larvae injected with the mixture of dsPxCht (dsRNA of PxCht) and dsPxdsRNase1 (dsRNA of PxdsRNase1), dsPxdsRNase2 (dsRNA of PxdsRNase2), or dsPxdsRNase3 (dsRNA of PxdsRNase3) was significantly higher than that in the larvae injected with the mixture of dsGFP (dsRNA of green fluorescent protein gene, GFP) and dsPxCht; the transcription level of PxCht in the larvae feeding on the mixture of dsPxCht and dsPxdsRNase1, dsPxdsRNase2, or dsPxdsRNase3 was significantly higher than that in the larvae feeding on the mixture of dsPxCht and dsGFP. The recombinant protein of PxdsRNase1 degraded dsRNA rapidly, PxdsRNase3 cleaved dsRNA without complete degradation, and PxdsRNase2 could not degrade dsRNA in vitro. These results suggested that PxdsRNases1, PxdsRNases2, and PxdsRNases3 were involved in the dsRNA degradation to reduce RNAi efficiency with different mechanisms.

Effect of RNAi targeting CYP6CY3 on the growth, development and insecticide susceptibility of Aphis gossypii by using nanocarrier-based transdermal dsRNA delivery system

RNA interference (RNAi) has been proved to be a viable method for agricultural pest control. Due to the limited uptake of dsRNA in hemiptera insects, this study used nanocarrier SPc (star polycation) transdermal delivery systems to deliver two truncated fragments (P1/P2) dsRNA of the CYP6CY3 for silencing this target gene in Aphis gossypii. After the cotton aphid was sprayed with the SPc + dsP1/P2 mixture, the expression level of target gene in SPc + dsP1 treatment group was not different from that in dsP1 group at 24 h, 48 h, and significantly lower than that in dsP1 group at 60 h, 72 h, respectively; and the expression level of target gene in SPc + dsP2 treatment group was not different from that in dsP2 group at 24 h, and significantly lower than that in dsP2 group from 48 h, 60 h, 72 h, respectively. In addition, the expression level was continuously silenced after spraying the SPc + dsP1/P2 mixture and significant reduced by 79.7% and 84.3% at 48 h compared with the H₂O control group, the mortality rate reached 48.09% and 43.18% at 84 h, respectively. And the cumulative reproduction number of cotton aphids also decreased, but the cumulative death number of newborn nymphs had an increase trend, compared with the control groups. Bioassays after RNAi showed that the silencing of CYP6CY3 increased the susceptibility of the 4^th instar aphid to imidacloprid, and increased mortality by 67.21% and 58.69% at 96 h, respectively. The life table parameters of the offspring from the 4^th instar cotton aphids from the SPc + dsP1/P2 treatment groups showed that the offspring had a longer pre-reproductive period and post-reproductive period. The intrinsic growth rate was 0.231 ± 0.005, 0.210 ± 0.013 and the finite growth rate was 1.260 ± 0.007 and 1.234 ± 0.016 in the SPc + dsP1/P2 treatment group, these two parameters of the two groups were lower than that of the corresponding control，the population doubling time of the two groups was prolonged and the developmental duration was delayed. These results indicate that CYP6CY3 plays a key role in the growth, development, reproduction and detoxification ability in cotton aphids, and may be as a potential RNAi target for controlling aphids, laying the foundation for the development of new environmentally-friendly RNA pesticides in this field.

Identification and functional analysis of dsRNases in spotted-wing drosophila, Drosophila suzukii

RNAi efficiency in insects is different from species to species; some species in Coleoptera are relatively more amenable to RNA interference (RNAi) than other species. One of the major factors is the presence of dsRNA-degrading enzymes, called dsRNases, in saliva, gut, or hemolymph in insects, which degrade the double-stranded RNA (dsRNA) introduced, resulting in the low efficacy of RNAi. In this study, we report a dsRNA-degrading activity in the gut homogenates from the spotted-wing drosophila, Drosophila suzukii, by ex vivo assay. Then, we identified two Drosophila suzukii dsRNase genes, named DrosudsRNase1 and DrosudsRNase2. In silico analysis shows that the gene structures are similar to dsRNases found in other insects. When dsRNases expressed in Sf9 cells were compared for their dsRNA degrading activities, dsRNase1 was more vital than dsRNase2. Both dsRNases were expressed highly and exclusively in the gut compared to the rest of body. Also, they were highly expressed during larval and adult stages but not in embryonic and pupal stages, suggesting the dsRNases protect foreign RNA molecules received during the feeding periods. DsRNase1 was expressed at a higher level in adults, whereas dsRNase2 showed more expression in early larvae. Our study on the tissue and development-specific patterns of dsRNases provides an improved understanding of the RNAi application for the management of D. suzukii.

Transcript level is a key factor affecting RNAi efficiency

Efficiency is the basis for the application of RNA interference (RNAi) technology. Actually, RNAi efficiency varies greatly among insect species, tissues and genes. Previous efforts have revealed the mechanisms for variation among insect species and tissues. Here, we investigated the reason for variable efficiency among the target genes in the same insect. First, we tested the genes sampled randomly from Tribolium castaneum, Locusta migratoria and Drosophila S2 cells for both their expression levels and sensitivity to RNAi. The results indicated that the genes with higher expression levels were more sensitive to RNAi. Statistical analysis showed that the correlation coefficients between transcript levels and knockdown efficiencies were 0.8036 (n = 90), 0.7255 (n = 18) and 0.9505 (n = 13), respectively in T. castaneum, L. migratoria and Drosophila S2 cells. Subsequently, ten genes with varied expression level in different tissues (midgut and carcass without midgut) of T. castaneum were tested. The results indicated that the higher knockdown efficiency was always obtained in the tissue where the target gene expressed higher. In addition, three genes were tested in different developmental stages, larvae and pupae of T. castaneum. The results found that when the expression level increased after insect pupation, these genes became more sensitive to RNAi. Thus, all the proofs support unanimously that transcript level is a key factor affecting RNAi sensitivity. This finding allows for a better understanding of the RNAi efficiency variation and lead to effective or efficient use of RNAi technology.

Improvement of host-induced gene silencing efficiency via polycistronic-tRNA-amiR expression for multiple target genes and characterization of RNAi mechanism in Mythimna separata

Host-induced gene silencing (HIGS) emerged as a new strategy for pest control. However, RNAi efficiency is reported to be low in Lepidoptera, which are composed of many important crop pests. To address this, we generated transgenic plants to develop HIGS effects in a maize pest, Mythimna separata (Lepidoptera, Noctuidae), by targeting chitinase encoding genes. More importantly, we developed an artificial microRNA (amiR) based PTA (polycistronic-tRNA-amiR) system for silencing multiple target genes. Compared with hpRNA (hairpin RNA), transgenic expression of a PTA cassette including an amiR for the gut-specific dsRNA nuclease gene MsREase, resulted in improved knockdown efficiency and caused more pronounced developmental abnormalities in recipient insects. When target gene siRNAs were analysed after HIGS and direct dsRNA/siRNA feeding, common features such as sense polarity and siRNA hotspot regions were observed, however, they differed in siRNA transitivity and major 20-24nt siRNA species. Core RNAi genes were identified in M. separata, and biochemical activities of MsAGO2, MsSID1 and MsDcr2 were confirmed by EMSA (electrophoretic mobility shift assay) and dsRNA cleavage assays, respectively. Taken together, we provide compelling evidence for the existence of the RNAi mechanism in M. separata by analysis of both siRNA signatures and RNAi machinery components, and the PTA system could potentially be useful for future RNAi control of lepidopteran pests.

New Frontiers in Pest Control: Chitosan Nanoparticles-Shielded dsRNA as an Effective Topical RNAi Spray for Gram Podborer Biocontrol

Chickpea pod borer, Helicoverpa armigera, displays resistance to chemical insecticides and transgenics. The potential nontransformative RNAi approach of specific gene silencing by mRNA breakdown through exogenous double-stranded (dsRNA) delivery to Helicoverpa faces problems of degradation by nucleases and insect gut pH. We demonstrate that chitosan nanoparticles (CNPs) effectively mediate specific dsRNA delivery against Helicoverpa armigera juvenile hormone methyltransferase (JHAMT) and acetylcholine esterase (ACHE) target genes. Ionotropically synthesized cationic CNPs (100 nm size, +32 mV charge) loaded dsRNA efficiently and protected it effectively from degradation by nucleases and insect gut pH. Tagging CNPs with Calcofluor fluorescence illustrated its efficient uptake in columnar insect gut cells. The potential of CNPs-mediated dsRNA delivery was elucidated with effective silencing of green fluorescent protein transformed Sf9 cells. Furthermore, CNPs-dsRNA complexes were stable for 5 d on leaf surfaces, and their ingestion with leaf effectively silenced H. armigera JHAMT and ACHE genes to suppress related enzyme activities and caused 100% insect mortality. Further, in planta bioassay with CNPs-dsRNA spray confirmed the RNAi induced insect mortality. Moreover, CNPs-dsRNA fed nontarget insects Spodoptera litura and Drosophila melanogaster were unaffected, and no toxicity was observed for CNPs in cell line studies. Remarkably, only two low dose (0.028 g/ha) topical CNPs-ache-dsRNA sprays on chickpea displayed reduced pod damage with high yields on par with chemical control in the field, which was followed by CNPs-jhamt-dsRNA nanoformulation. These studies can pave the way for the development of topical application of CNPs-dsRNA spray as a safe, specific, innovative insecticide for sustainable crop protection.

Evaluating toxicity of Varroa mite (Varroa destructor)-active dsRNA to monarch butterfly (Danaus plexippus) larvae

Varroa mites (Varroa destructor) are parasitic mites that, combined with other factors, are contributing to high levels of honey bee (Apis mellifera) colony losses. A Varroa-active dsRNA was recently developed to control Varroa mites within honey bee brood cells. This dsRNA has 372 base pairs that are homologous to a sequence region within the Varroa mite calmodulin gene (cam). The Varroa-active dsRNA also shares a 21-base pair match with monarch butterfly (Danaus plexippus) calmodulin mRNA, raising the possibility of non-target effects if there is environmental exposure. We chronically exposed the entire monarch larval stage to common (Asclepias syriaca) and tropical (Asclepias curassavica) milkweed leaves treated with concentrations of Varroa-active dsRNA that are one- and ten-fold higher than those used to treat honey bee hives. This corresponded to concentrations of 0.025-0.041 and 0.211-0.282 mg/g leaf, respectively. Potassium arsenate and a previously designed monarch-active dsRNA with a 100% base pair match to the monarch v-ATPase A mRNA (leaf concentration was 0.020-0.034 mg/g) were used as positive controls. The Varroa mite and monarch-active dsRNA's did not cause significant differences in larval mortality, larval or pupal development, pupal weights, or adult eclosion rates when compared to negative controls. Irrespective of control or dsRNA treatment, larvae that consumed approximately 7500 to 10,500-mg milkweed leaf within 10 to 12 days had the highest pupal weights. The lack of mortality and sublethal effects following dietary exposure to dsRNA with 21-base pair and 100% base pair match to mRNAs that correspond to regulatory genes suggest monarch mRNA may be refractory to silencing by dsRNA or monarch dsRNase may degrade dsRNA to a concentration that is insufficient to silence mRNA signaling.

Strategies for enhancing the efficiency of RNA interference in insects

Low RNA interference (RNAi) efficiency in many insect pests has significantly prevented its widespread application for insect pest management. This article provides a comprehensive review of recent research in developing various strategies for enhancing RNAi efficiency. Our review focuses on the strategies in target gene selection and double-stranded RNA (dsRNA) delivery technologies. For target gene selection, genome-wide or large-scale screening strategies have been used to identify most susceptible target genes for RNAi. Other strategies include the design of dsRNA constructs and manipulate the structure of dsRNA to maximize the RNA efficiency for a target gene. For dsRNA delivery strategies, much recent research has focused on the applications of complexed or encapsulated dsRNA using various reagents, polymers, or peptides to enhance dsRNA stability and cellular uptake. Other dsRNA delivery strategies include genetic engineering of microbes (e.g. fungi, bacteria, and viruses) and plants to produce insect-specific dsRNA. The ingestion of the dsRNA-producing organisms or tissues will have lethal or detrimental effects on the target insect pests. This article also identifies obstacles to further developing RNAi for insect pest management and suggests future avenues of research that will maximize the potential for using RNAi for insect pest management. © 2021 Society of Chemical Industry.

Accelerated delivery of dsRNA in lepidopteran midgut cells by a Galanthus nivalis lectin (GNA)-dsRNA-binding domain fusion protein

Lepidopteran insects are highly refractory to oral RNA interference (RNAi). Degradation, impaired cellular uptake and intracellular transport of double-stranded RNA (dsRNA) are considered the major factors responsible for the reduced RNAi efficiency in these insects. In this study, the potential of lectins to improve dsRNA delivery and RNAi efficacy was evaluated. First, a fusion protein consisting of the Galanthus nivalis agglutinin (GNA) and a dsRNA binding domain was developed, further referred to as GNA:dsRBD (GNAF). Then, its ability to increase dsRNA uptake and transfection efficiency in lepidopteran midgut cells was evaluated, as well as its ability to protect and promote the RNAi response in the beet armyworm Spodoptera exigua. Confocal microscopy analysis showed that GNAF-complexed dsRNA was internalized faster in Choristoneura fumiferana midgut CF1 cells (1 min) compared to naked dsRNA (>1 h). The faster uptake was also correlated with an increased RNAi efficiency in these CF1 cells. In vivo feeding bioassays with GNAF-complexed dsRNA led to an increased mortality in S. exigua compared to the controls. By targeting the essential gene V-ATPase A, we observed that the mortality increased to 48% in the GNAF-dsRNA treatment compared to only 8.3% and 6.6% in the control treatments with the naked dsRNA and the GNAF, respectively.

Molecular characterization and RNA interference responses of the lethal giant larvae gene in Diabrotica virgifera virgifera adults

High specificity for silencing target genes and single-copy target genes that yield clear phenotypes are two important factors for the success of RNA interference (RNAi). The lethal giant larvae (Lgl) gene appears to be an ideal gene for RNAi because RNAi can effectively suppress its expression and results in molting defects and mortality in Tribolium castaneum. To investigate the suitability of this gene for RNAi in other insects, we identified and characterized DvLgl from the western corn rootworm, Diabrotica virgifera virgifera, a species exhibiting high RNAi efficiency. DvLgl was expressed in all developmental stages and tissues investigated. The deduced DvLgl protein showed high amino-acid sequence identities and similar domain architecture to Lgls from other insect species. Despite many similarities among insect Lgls, RNAi-mediated suppression of DvLgl failed to produce a phenotype in D. v. virgifera adults. The difference in developing phenotypes could be attributed greatly to the level of gene suppression and the insect developmental stages for RNAi. These results highlight the variability in RNAi response among insects and showcase the importance of screening multiple target genes when conducting RNAi studies. Our findings are expected to help the design of future RNAi studies and future investigations of Lgl in insects.

Development of Catechin, Poly-l-lysine, and Double-Stranded RNA Nanoparticles

Developing strategies to optimize double-stranded RNA (dsRNA) delivery remains a significant challenge in improving RNA interference (RNAi) in insects. Nanoformulations may provide an avenue for the safe and effective delivery of dsRNA. We investigated nanoparticle-mediated gene silencing using biodegradable polymers, poly-l-lysine (PLL), and polyphenol (-)-epigallocatechin gallate (EGCG) for dsRNA delivery into Spodoptera frugiperda (Sf9) cells. Negatively charged cores were formed by EGCG and dsRNA complexes, and PLL was used to encapsulate the cores. The nanoparticles were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy-dispersive spectrometry (EDS) analysis. The stability of the nanoparticles was assessed by incubating them in nuclease-containing Sf9 cell conditioned media. The effectiveness of the nanoparticles was investigated in Sf9 cells stably expressing the luciferase gene. The results revealed that the nanoparticles formed were small and spherical. The PLL/EGCG/dsRNA nanoparticles exhibited better stability compared to that of PLL/dsRNA or naked dsRNA. Nanoparticles prepared with dsRNA targeting the luciferase gene induced an efficient knockdown (66.7%) of the target gene. In Sf9 cells, nanoparticles prepared with Cy3- or CyPHer-5E-labeled dsRNA showed higher cellular uptake and endosomal escape, respectively, than the naked dsRNA. The improvement in uptake and cytosolic delivery may have helped to increase the knockdown efficiency. In Sf9 cells, the nanoparticles prepared with dsRNA targeting the inhibitor of apoptosis gene induced apoptosis by knocking down its expression. In conclusion, we demonstrate that PLL/EGCG/dsRNA nanoparticles are stable, highly efficient, and effective in dsRNA delivery and knockdown of the target gene.

Synergistic action of the gut microbiota in environmental RNA interference in a leaf beetle

BACKGROUND

RNA interference (RNAi) has emerged as an efficient tool to control insect pests. When insects ingest double-stranded RNAs (dsRNAs) targeted against essential genes, strong gene silencing and mortality can be induced. To exert their function, dsRNA molecules must pass through the insect's gut and enter epithelial cells and/or the hemolymph. Gut bacteria are known to play multifarious roles in food digestion and nutrition, and confer protection against pathogens and parasites. Whether there is a cross talk between gut bacteria and ingested dsRNAs and whether the microbiome affects RNAi efficiency are unknown.

RESULTS

Here, using a leaf beetle gut microbiota system, we investigated whether gut bacteria interact with dsRNA molecules and how the gut microbiota affects RNAi responses in insects. We first showed that the leaf beetle Plagiodera versicolora (Coleoptera) is highly susceptible to RNAi. We then demonstrated that ingestion of dsRNAs by non-axenic P. versicolora larvae results in (i) significantly accelerated mortality compared with axenic larvae, and (ii) overgrowth and dysbiosis of the gut microbiota. The latter may be caused by bacterial utilization of dsRNA degradation products. Furthermore, we found that Pseudomonas putida, a gut bacterium of P. versicolora, acts as major accelerator of the death of P. versicolora larvae by transitioning from commensal to pathogenic lifestyle.

CONCLUSIONS

The present study illuminates the complex interplay between lethal dsRNA, the insect host, and its gut microbiota. The ingestion of dsRNA by the leaf beetle caused a dysbiosis of gut bacterial community, and the dsRNA degradation products by host insect preferentially promoted the growth of an entomopathogenic bacterium, which accelerated dsRNA lethality to the insect. Our findings reveal a synergistic role of the gut microbiota in dsRNA-induced mortality of pest insects, and provide new insights in the mechanisms of RNAi-based pest control. Video abstract.

Involvement of Methoprene-tolerant and Krüppel homolog 1 in juvenile hormone-signaling regulating the maturation of male accessory glands in the moth Agrotis ipsilon

Male accessory glands (MAGs) produce seminal fluid proteins that are essential for the fertility and also influence the reproductive physiology and behavior of mated females. In many insect species, and especially in the moth Agrotis ipsilon, juvenile hormone (JH) promotes the maturation of the MAGs but the underlying molecular mechanisms in this hormonal regulation are not yet well identified. Here, we examined the role of the JH receptor, Methoprene-tolerant (Met) and the JH-inducible transcription factor, Krüppel homolog 1 (Kr-h1) in transmitting the JH signal that upregulates the growth and synthetic activity of the MAGs in A. ipsilon. We cloned two full length cDNAs encoding Met1 and Met2 which are co-expressed with Kr-h1 in the MAGs where their expression levels increase with age in parallel with the length and protein content of the MAGs. RNAi-mediated knockdown of either Met1, Met2, or Kr-h1 resulted in reduced MAG length and protein amount. Moreover, injection of JH-II into newly emerged adult males induced the transcription of Met1, Met2 and Kr-h1 associated to an increase in the length and protein content of the MAGs. By contrast, JH deficiency decreased Met1, Met2 and Kr-h1 mRNA levels as well as the length and protein reserves of the MAGs of allatectomized old males and these declines were partly compensated by a combined injection of JH-II in operated males. Taken together, our results highlighted an involvement of the JH-Met-Kr-h1 signaling pathway in the development and secretory activity of the MAGs in A. ipsilon.

Exogenous administration of dsRNA for the demonstration of RNAi in Maruca vitrata (lepidoptera: crambidae)

The polyphagous spotted pod borer, Maruca vitrata is an important agricultural pest that causes extensive damage on various food crops. Though the pest is managed by synthetic chemicals, exploration of biotechnological approaches for its control is important. RNAi-based gene silencing is one such tool that has been extensively used for functional genomics and is highly variable in insects. In view of this, we have attempted to demonstrate RNAi in M. vitrata through exogenous double-stranded RNA (dsRNA) administration targeting seven genes associated with midgut, chemosensory, cell signalling and development. Two modes of exogenous dsRNA delivery by either haemolymph injection and/or ingestion into third and late third instar larval stages respectively exhibited efficient silencing of specific transcripts. Furthermore, dsRNA injection into the haemolymph showed significant reduction of target gene expression compared to negative controls establishing this mode of delivery to be more efficient. Interestingly, haemolymph injection required lesser dsRNA and led to higher reduction of transcript level vis-à-vis ingestion as demonstrated in dsRNA Serine Protease 33 (ds-SP33)-fed larvae. Over-expression of key RNAi component DICER and detection of siRNA authenticated the presence of RNAi in M. vitrata. Additionally, we have identified inhibitor molecules like morpholine, piperidine, carboxamide and piperidine-carboxamide through in silico analysis for blocking the function of SP33 to demonstrate the utility of functional genomics. Thus, the present study establishes the usefulness of injection and ingestion approaches for exogenous dsRNA delivery into M. vitrata larvae for effective RNAi.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02741-8.

RNAi-based gene silencing in Phenacoccus solenopsis and its validation by in planta expression of a double-stranded RNA

BACKGROUND

Cotton is a cash crop majorly affected by many hemipteran pests, among them the cotton mealybug, Phenacoccus solenopsis. Cotton mealybug attack has a devastating effect on cotton production and causes huge yield losses.

RESULTS

In this study, 25 potential RNA interference (RNAi) target genes were selected from the iBeetle database and a transcriptome data set for P. solenopsis. To assess the effectiveness of the selected target genes, three methods were utilized to deliver double-stranded (ds)RNA (ingestion, artificial diet bioassay and transient gene silencing). dsRNA molecules at different concentrations were fed to insects and insect mortality was recorded for each target gene. Based on the mortality data, three genes, Krüppel homologue-1, ADP-ATP/Translocase and IDGF-1, were selected for further gene expression studies using a reduced concentration of dsRNA (5 μg/ml). Of the three genes, Krüppel homologue-1 showed significantly downregulated expression (by 70.81% and 84.33%) at two different time points (8 and 14 days). An RNAi silencing construct was designed for Krüppel homologue-1 under control of the double enhancer CamV35S promoter in the plant binary vector. Significant downregulation of gene expression, by 66.69% and 81.80%, was found for Krüppel homologue-1 using transient gene silencing at the same time intervals.

CONCLUSION

This work provides the first evidence for targeting the Krüppel homologue-1 gene in a hemipteran pest, P. solenopsis, using RNAi technology through oral delivery and in planta-based transient gene silencing methods. © 2020 Society of Chemical Industry.

The Ras/MAPK pathway is required for regenerative growth of wing discs in the black cutworm Agrotis ypsilon

Regeneration is a common phenomenon in various organisms by which tissues restore the damaged or naturally detached parts. In insects, appendage regeneration takes place during the embryonic, larval and pupal stages for individual survival. The wing disc of black cutworm Agrotis ypsilon has the capacity of regeneration after ablation, but understanding of molecular mechanisms in wing disc regeneration is still limited. After ablation of partial or whole wing discs before the fifth instar larval stage, the adult wings appeared to be normal. In the last two larval stages, ablation of the left wing disc led to smaller corresponding adult wing. Cell proliferation was reduced in the ablated wing disc but was gradually recovered two days post ablation. Transcriptome analysis found that genes in the mitogen-activated protein kinase (MAPK) pathway were upregulated. Repression of gene expression in this pathway, including Ras oncogene at 64B (Ras64B), Downstream of raf1 (Dsor1), and cAMP-dependent protein kinase catalytic subunit 3 (Pka-C3) by RNA interference after ablation, led to diminishment of both adult wings, suggesting that the MAPK signaling is essential for wing growth. Additionally, cell proliferation was still decelerated by injecting Ras64B, Dsor, or Pka-C3 dsRNA two days after ablation, indicating that the MAPK signaling-regulated cell proliferation is essential for growth. These results provide molecular clues to the regulation of cell proliferation during regeneration in lepidopteran insects.

Selection for high levels of resistance to double-stranded RNA (dsRNA) in Colorado potato beetle (Leptinotarsa decemlineata Say) using non-transgenic foliar delivery

Insecticidal double-stranded RNAs (dsRNAs) silence expression of vital genes by activating the RNA interference (RNAi) mechanism in insect cells. Despite high commercial interest in insecticidal dsRNA, information on resistance to dsRNA is scarce, particularly for dsRNA products with non-transgenic delivery (ex. foliar/topical application) nearing regulatory review. We report the development of the CEAS 300 population of Colorado potato beetle (Leptinotarsa decemlineata Say) (Coleoptera: Chrysomelidae) with > 11,100-fold resistance to a dsRNA targeting the V-ATPase subunit A gene after nine episodes of selection using non-transgenic delivery by foliar coating. Resistance was associated with lack of target gene down-regulation in CEAS 300 larvae and cross-resistance to another dsRNA target (COPI β; Coatomer subunit beta). In contrast, CEAS 300 larvae showed very low (~ 4-fold) reduced susceptibility to the Cry3Aa insecticidal protein from Bacillus thuringiensis. Resistance to dsRNA in CEAS 300 is transmitted as an autosomal recessive trait and is polygenic. These data represent the first documented case of resistance in an insect pest with high pesticide resistance potential using dsRNA delivered through non-transgenic techniques. Information on the genetics of resistance and availability of dsRNA-resistant L. decemlineata guide the design of resistance management tools and allow research to identify resistance alleles and estimate resistance risks.

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.

Current Status and Potential of RNA Interference for the Management of Tomato Spotted Wilt Virus and Thrips Vectors

Tomato spotted wilt virus (TSWV) is the type member of the genus Orthotospovirus in the family Tospoviridae and order Bunyavirales. TSWV, transmitted by several species of thrips, causes significant disease losses to agronomic and horticultural crops worldwide, impacting both the yield and quality of the produce. Management strategies include growing virus-resistant cultivars, cultural practices, and managing thrips vectors through pesticide application. However, numerous studies have reported that TSWV isolates can overcome host-plant resistance, while thrips are developing resistance to pesticides that were once effective. RNA interference (RNAi) offers a means of host defence by using double-stranded (ds) RNA to initiate gene silencing against invading viruses. However, adoption of this approach requires production and use of transgenic plants and thus limits the practical application of RNAi against TSWV and other viruses. To fully utilize the potential of RNAi for virus management at the field level, new and novel approaches are needed. In this review, we summarize RNAi and highlight the potential of topical or exogenous application of RNAi triggers for managing TSWV and thrips vectors.

Target verification of allyl isothiocyanate on the core subunits of cytochrome c oxidase in Sitophilus zeamais by RNAi

BACKGROUND

Allyl isothiocyanate (AITC) is a volatile organic compound with a potent insecticidal activity to the stored-grain pest Sitophilus zeamais Motschulsky, which severely damages grain storage and container transport worldwide. Our previous study showed that mitochondrial complex IV was the primary target of AITC in adult Sitophilus zeamais. To further verify the targets of AITC, we employed RNA interference (RNAi) by using double-stranded RNA (dsRNA) to knockdown three core subunits of cytochrome c oxidase (COX)-I, -II and -III in 18-day-old larvae prior to their exposure to AITC to detect susceptibility changes.

RESULTS

The susceptibility of dsRNACOX-I and -II injection treatments to AITC significantly increased at 72 h while the mortality reached up to 85.56% and 67.78%, respectively, and dsRNACOX-I and dsRNACOX-II injection showed the same subcellular structural characteristics showing vacuolization and vague mitochondrial cristae and decrease of COX activity during AITC fumigation treatment, suggesting the potential of COX-I and COX-II as the targets of AITC. High mortality reached up to 75.55%, 71.88% and 82.22%, respectively, and the phenotype of larvae turning from milky white to dark brown in the thorax and death eventually was confirmed after dsRNACOX-I, -II and -III injection.

CONCLUSION

COX-I and -II were elucidated as the potential targets of AITC and dsRNACOX-I, -II and -III have the potential to be developed into nucleic acid pesticides for their robust lethal effects and are worth pursuing for improving AITC fumigation activity in Sitophilus zeamais control. © 2020 Society of Chemical Industry.

RNA interference-mediated functional characterization of Group I chitin deacetylases in Holotrichia parallela Motschulsky

Chitin deacetylases (CDAs, EC 3.5.1.41) catalyze the N-deacetylation of chitin to produce chitosan, which is essential for insect survival. Hence, CDAs are promising targets for the development of novel insecticidal drugs. In this study, the putative Group I chitin deacetylase genes HpCDA1, HpCDA2-1 and HpCDA2-2 were identified from Holotrichia parallela. Conserved domain database search identified a chitin-binding peritrophin-A domain (ChBD), a low-density lipoprotein receptor class A domain (LDLa), and a putative CDA-like catalytic domain. RT-qPCR analysis showed that the Group I HpCDAs were expressed in various tissues and predominant in the integument. The developmental expression patterns from the first-instar to third-instar larvae showed that HpCDAs were highly expressed on the first day and gradually declined after molting. The functional characteristics of the Group I CDAs in cuticle organization were examined using RNA interference (RNAi) and transmission electron microscopy (TEM) methods. Administration of double-stranded HpCDA (dsHpCDA) through larval injection could suppress the expression levels of HpCDA1 and HpCDA2, thus resulting in abnormal or lethal phenotypes. TEM analysis revealed that RNAi of either HpCDA1 or HpCDA2 remarkably affected the cuticle integrity, as evidenced by cuticle disorganization and chitin laminae disruption, suggesting the crucial role of CDAs in chitin modification. These experimental results demonstrate the important contribution of putative key genes involved in chitin metabolism, and provide a foundation for developing new strategies to control H. parallela.

Comparison of strategies for enhancing RNA interference efficiency in Ostrinia nubilalis

BACKGROUND

Targeting insect-specific genes through post-transcriptional gene silencing with RNA interference (RNAi) is a new strategy for insect pest management. However, lepidopterans are recalcitrant to RNAi, which prevents application of novel RNAi technology to many notorious pests, including Ostrinia nubilalis (ECB). Strategies for enhancing RNAi efficiency, including large doses of double-stranded RNA (dsRNA), nuclease inhibitors, transfection reagents, and nanoparticles, have proved useful in other insects exhibiting substantial dsRNA degradation, a major mechanism limiting RNAi efficacy. To determine if similar strategies can enhance RNAi efficiency in ECB, various reagents were tested for their ability to enhance dsRNA stability in ECB tissues, then compared for their effectiveness in whole ECB.

RESULTS

Ex vivo incubation experiments revealed that Meta dsRNA lipoplexes, EDTA, chitosan-based dsRNA nanoparticles, and Zn²⁺ enhanced dsRNA stability in ECB hemolymph and gut content extracts, compared with uncoated dsRNA. Despite these positive results, the reagents used in this study were ineffective at enhancing RNAi efficiency in ECB in vivo. To reduce assay time and required dsRNA, midguts were dissected and incubated in tissue culture medium containing dsRNA with and without reagents. These experiments showed that RNAi efficiency varied between target genes, and nuclease inhibitors improved RNAi efficiency for only a portion of the refractory target genes investigated ex vivo.

CONCLUSION

These results indicate that enhancing dsRNA stability is insufficient to improve RNAi efficiency in ECB and suggests the existence of additional, complex mechanisms contributing to low RNAi efficiency in ECB.

Midgut Genes Knockdown by Oral dsRNA Administration Produces a Lethal Effect on Cotton Boll Weevil

The "cotton boll weevil" (Anthonomus grandis Boheman) is a key pest in America whose larval stage develops within the cotton flower bud. During its development, the larva uses the flower bud as food and as a shelter from predators. This behavior limits the effective control through conventional insecticide applications and biocontrol techniques. Increasing genetic information from insects has allowed the development of new control technologies based on the use of RNA interference (RNAi) to design orally delivered double-stranded RNA (dsRNA) strategies. In this study, we evaluated the effect of continuous oral administration of six specific dsRNA in order to identify an effective target gene for RNAi-mediated control of cotton boll weevil. First, six selected A. grandis gene fragments were amplified and cloned to perform in vivo synthesis of the specific dsRNA, and subsequently, larvae and adults were fed with this dsRNA for 2 weeks. Larvae mortality ranged from 40 to 60% depending on the targeted gene sequence. Indeed, α-amylase and cytochrome p450 dsRNAs were the most effective. Oral administration in adults caused smaller but still significant death rates (15-30%). Thus, the results demonstrated RNAi responses depend on life stages and target genes. The dsRNA ingestion was capable of providing knockdown mRNA levels in cotton boll weevil midgut and this effect was significantly higher in the larval stage. In this study, we present a new report of silencing of midgut genes in A. grandis larva induced by continuously feeding with dsRNA. This potential new tool should be further evaluated in cotton boll weevil control strategies.

Short term transcriptional responses of P450s to phytochemicals in insects and mites

Cytochrome P450 monooxygenases (P450s) play a key role in the detoxification of phytochemicals in arthropod herbivores. We present here an overview of recent progress in understanding the breadth and specificity of gene expression plasticity of P450s in response to phytochemicals. We discuss experimental setups and new findings in mechanisms of P450 regulation. Whole genome transcriptomic analysis of arthropod herbivores, either after direct administration of phytochemicals or after host plant shifts, allowed to integrate various levels of chemical complexity and lead to the unbiased identification of responsive P450 genes. However, despite progress in identification of inducible P450s, the link between induction and metabolism is still largely unexplored, and to what extent the overall response is biologically functional should be further investigated. In the near future, such studies will be more straightforward as forward and reverse genetic tools become more readily available.

Characterization, expression patterns, and transcriptional responses of three core RNA interference pathway genes from Ostrinia nubilalis

RNA interference (RNAi) is commonly used in the laboratory to analyze gene function, and RNAi-based pest management strategies are now being employed. Unfortunately, RNAi is hindered by inefficient and highly-variable results when different insects are targeted, especially lepidopterans, such as the European corn borer (ECB), Ostrinia nubilalis (Lepidoptera: Crambidae). Previous efforts to achieve RNAi-mediated gene suppression in ECB revealed low RNAi efficiency with both double-stranded RNA (dsRNA) injection and ingestion. One mechanism that can affect RNAi efficiency in insects is the expression and function of core RNAi pathway genes, such as those encoding Argonaut 2 (Ago2), Dicer 2 (Dcr2), and a dsRNA binding protein (R2D2). To determine if deficiencies in these core RNAi pathway genes contribute to low RNAi efficiency in ECB, full-length complementary DNAs encoding OnAgo2, OnDcr2, and OnR2D2 were cloned, sequenced, and characterized. A comparison of domain architecture suggested that all three predicted proteins contained the necessary domains to function. However, a comparison of evolutionary distances revealed potentially important variations in the first RNase III domain of OnDcr2, the double-stranded RNA binding domains of OnR2D2, and both the PAZ and PIWI domains of OnAgo2, which may indicate functional differences in enzymatic activity between species. Expression analysis indicated that transcripts for all three genes were expressed in all developmental stages and tissues investigated. Interestingly, the introduction of non-target dsRNA into ECB second-instar larvae via microinjection did not affect OnAgo2, OnDcr2, or OnR2D2 expression. In contrast, ingestion of the same dsRNAs resulted in upregulation of OnDcr2 but downregulation of OnR2D2. The unexpected transcriptional responses of the core machinery and the divergence in amino-acid sequence between specific domains in each core RNAi protein may possibly contribute to low RNAi efficiency in ECB. Understanding the contributions of different RNAi pathway components is critical to adapting this technology for use in controlling lepidopteran pests that exhibit low RNAi efficiency.

Silencing of Double-Stranded Ribonuclease Improves Oral RNAi Efficacy in Southern Green Stinkbug Nezaraviridula

Variability in RNA-interference (RNAi) efficacy among different insect orders poses a big hurdle in the development of RNAi-based pest control strategies. The activity of double-stranded ribonucleases (dsRNases) in the digestive canal of insects can be one of the critical factors affecting oral RNAi efficacy. Here, the involvement of these dsRNases in the southern green stinkbug Nezaraviridula was investigated. First, the full sequence of the only dsRNase (NvdsRNase) in the transcriptome of N. viridula was obtained, followed by an oral feeding bioassay to evaluate the effect of NvdsRNase-silencing on oral RNAi efficacy. The NvdsRNase was first silenced in nymphs by NvdsRNase-dsRNA injections, followed by exposure to an artificial diet containing a lethal αCop-specific dsRNA. A significantly higher mortality was observed in the NvdsRNase-silenced nymphs when placed on the dsαCop-containing diet (65%) than in the dsGFP injected and dsαCop fed control (46.67%). Additionally, an ex vivo dsRNA degradation assay showed a higher stability of dsRNA in the saliva and midgut juice of NvdsRNase-silenced adults. These results provide evidence for the involvement of NvdsRNase in the reduction of oral RNAi efficacy in N. viridula. This information will be useful in further improving potential RNAi-based strategies to control this pest.

Effectiveness of orally-delivered double-stranded RNA on gene silencing in the stinkbug Plautia stali

Development of a reliable method for RNA interference (RNAi) by orally-delivered double-stranded RNA (dsRNA) is potentially promising for crop protection. Considering that RNAi efficiency considerably varies among different insect species, it is important to seek for the practical conditions under which dsRNA-mediated RNAi effectively works against each pest insect. Here we investigated RNAi efficiency in the brown-winged green stinkbug Plautia stali, which is notorious for infesting various fruits and crop plants. Microinjection of dsRNA into P. stali revealed high RNAi efficiency-injection of only 30 ng dsRNA into last-instar nymphs was sufficient to knockdown target genes as manifested by their phenotypes, and injection of 300 ng dsRNA suppressed the gene expression levels by 80% to 99.9%. Knockdown experiments by dsRNA injection showed that multicopper oxidase 2 (MCO2), vacuolar ATPase (vATPase), inhibitor of apoptosis (IAP), and vacuolar-sorting protein Snf7 are essential for survival of P. stali, as has been demonstrated in other insects. By contrast, P. stali exhibited very low RNAi efficiency when dsRNA was orally administered. When 1000 ng/μL of dsRNA solution was orally provided to first-instar nymphs, no obvious phenotypes were observed. Consistent with this, RT-qPCR showed that the gene expression levels were not affected. A higher concentration of dsRNA (5000 ng/μL) induced mortality in some cohorts, and the gene expression levels were reduced to nearly 50%. Simultaneous oral administration of dsRNA against potential RNAi blocker genes did not improve the RNAi efficiency of the target genes. In conclusion, P. stali shows high sensitivity to RNAi with injected dsRNA but, unlike the allied pest stinkbugs Halyomorpha halys and Nezara viridula, very low sensitivity to RNAi with orally-delivered dsRNA, which highlights the varied sensitivity to RNAi across different species and limits the applicability of the molecular tool for controlling this specific insect pest.

Off-target effects of RNAi correlate with the mismatch rate between dsRNA and non-target mRNA

RNAi is a potent technique for the knockdown of target genes. However, its potential off-target effects limit the widespread applications in both reverse genetic analysis and genetic manipulation. Previous efforts have uncovered rules underlying specificity of siRNA-based silencing, which has broad applications in humans, but the basis for specificity of dsRNAs, which are better suited for use as insecticides, is poorly understood. Here, we investigated the rules governing dsRNA specificity. Mutational analyses showed that dsRNAs with >80% sequence identity with target genes triggered RNAi efficiently. dsRNAs with ≥16 bp segments of perfectly matched sequence or >26 bp segments of almost perfectly matched sequence with one or two mismatches scarcely distributed (single mismatches inserted between ≥5 bp matching segments or mismatched couplets inserted between ≥8 bp matching segments) also able to trigger RNAi. Using these parameters to predict off-target risk, dsRNAs can be designed to optimize specificity and efficiency, paving the way to the widespread, rational application of RNAi in pest control.

Functional conservation and diversification of yellow-y in lepidopteran insects

The diverse colors and patterns found in Lepidoptera are important for success of these species. Similar to the wings of adult butterflies, lepidopteran larvae exhibit diverse color variations to adapt to their habitats. Compared with butterfly wings, however, less attention has been paid to larval body colorations and patterns. In the present study, we focus on the yellow-y gene, which participates in the melanin synthesis pathway. We conducted CRISPR/Cas9-mediated targeted mutagenesis of yellow-y in the tobacco cutworm Spodoptera litura. We analyzed the role of S. litura yellow-y in pigmentation by morphological observation and discovered that yellow-y is necessary for normal black pigmentation in S. litura. We also showed species- and tissue-specific requirements of yellow-y in pigmentation in comparison with those of Bombyx mori yellow-y mutants. Furthermore, we found that almost none of the yellow-y mutant embryos hatched unaided. We provide evidence that S. litura yellow-y has a novel important function in egg hatching, in addition to pigmentation. The present study will enable a greater understanding of the functions and diversification of the yellow-y gene in insects.

Can the mammalian organoid technology be applied to the insect gut?

Mammalian intestinal organoids are multicellular structures that closely resemble the structure of the intestinal epithelium and can be generated in vitro from intestinal stem cells under appropriate culture conditions. This technology has transformed pharmaceutical research and drug development in human medicine. For the insect gut, no biotechnological platform equivalent to organoid cultures has been described yet. Comparison of the regulation of intestinal homeostasis and growth between insects and mammals has revealed significant similarities but also important differences. In contrast to mammals, the differentiation potential of available insect cell lines is limited and can not be exploited for in vitro permeability assays to measure the uptake of insecticides. The successful development of in vitro models could be a result of the emergence of molecular mechanisms of self-organization and signaling in the intestine that are unique to mammals. It is nevertheless considered that the technology gap is a consequence of vast differences in knowledge, particularly with respect to culture conditions that maintain the differentation potential of insect midgut cells. From the viewpoint of pest control, advanced in vitro models of the insect midgut would be very desirable because of its key barrier function for orally ingested insecticides with hemolymphatic target and its role in insecticide resistance. © 2020 Society of Chemical Industry.

Dissecting protein domain variability in the core RNA interference machinery of five insect orders

RNA interference (RNAi)-mediated gene silencing can be used to control specific insect pest populations. Unfortunately, the variable efficiency in the knockdown levels of target genes has narrowed the applicability of this technology to a few species. Here, we examine the current state of knowledge regarding the miRNA (micro RNA) and siRNA (small interfering RNA) pathways in insects and investigate the structural variability at key protein domains of the RNAi machinery. Our goal was to correlate domain variability with mechanisms affecting the gene silencing efficiency. To this end, the protein domains of 168 insect species, encompassing the orders Coleoptera, Diptera, Hemiptera, Hymenoptera, and Lepidoptera, were analysed using our pipeline, which takes advantage of meticulous structure-based sequence alignments. We used phylogenetic inference and the evolutionary rate coefficient (K) to outline the variability across domain regions and surfaces. Our results show that four domains, namely dsrm, Helicase, PAZ and Ribonuclease III, are the main contributors of protein variability in the RNAi machinery across different insect orders. We discuss the potential roles of these domains in regulating RNAi-mediated gene silencing and the role of loop regions in fine-tuning RNAi efficiency. Additionally, we identified several order-specific singularities which indicate that lepidopterans have evolved differently from other insect orders, possibly due to constant coevolution with plants and viruses. In conclusion, our results highlight several variability hotspots that deserve further investigation in order to improve the application of RNAi technology in the control of insect pests.

dsRNA Uptake in Plant Pests and Pathogens: Insights into RNAi-Based Insect and Fungal Control Technology

Efforts to develop more environmentally friendly alternatives to traditional broad-spectrum pesticides in agriculture have recently turned to RNA interference (RNAi) technology. With the built-in, sequence-specific knockdown of gene targets following delivery of double-stranded RNA (dsRNA), RNAi offers the promise of controlling pests and pathogens without adversely affecting non-target species. Significant advances in the efficacy of this technology have been observed in a wide range of species, including many insect pests and fungal pathogens. Two different dsRNA application methods are being developed. First, host induced gene silencing (HIGS) harnesses dsRNA production through the thoughtful and precise engineering of transgenic plants and second, spray induced gene silencing (SIGS) that uses surface applications of a topically applied dsRNA molecule. Regardless of the dsRNA delivery method, one aspect that is critical to the success of RNAi is the ability of the target organism to internalize the dsRNA and take advantage of the host RNAi cellular machinery. The efficiency of dsRNA uptake mechanisms varies across species, and in some uptake is negligible, rendering them effectively resistant to this new generation of control technologies. If RNAi-based methods of control are to be used widely, it is critically important to understand the mechanisms underpinning dsRNA uptake. Understanding dsRNA uptake mechanisms will also provide insight into the design and formulation of dsRNAs for improved delivery and provide clues into the development of potential host resistance to these technologies.