Knocking down chitin synthase 2 by RNAi is lethal to the cotton boll weevil

Cotton plants overexpressing the Bacillus thuringiensis Cry23Aa and Cry37Aa binary-like toxins exhibit high resistance to the cotton boll weevil (Anthonomus grandis)

The cotton boll weevil (CBW, Anthonomus grandis) stands as one of the most significant threats to cotton crops (Gossypium hirsutum). Despite substantial efforts, the development of a commercially viable transgenic cotton event for effective open-field control of CBW has remained elusive. This study describes a detailed characterization of the insecticidal toxins Cry23Aa and Cry37Aa against CBW. Our findings reveal that CBW larvae fed on artificial diets supplemented exclusively with Cry23Aa decreased larval survival by roughly by 69%, while supplementation with Cry37Aa alone displayed no statistical difference compared to the control. However, the combined provision of both toxins in the artificial diet led to mortality rates approaching 100% among CBW larvae (LC50 equal to 0.26 PPM). Additionally, we engineered transgenic cotton plants by introducing cry23Aa and cry37Aa genes under control of the flower bud-specific pGhFS4 and pGhFS1 promoters, respectively. Seven transgenic cotton events expressing high levels of Cry23Aa and Cry37Aa toxins in flower buds were selected for greenhouse bioassays, and the mortality rate of CBW larvae feeding on their T0 and T1 generations ranged from 75% to 100%. Our in silico analyses unveiled that Cry23Aa displays all the hallmark characteristics of β-pore-forming toxins (β-PFTs) that bind to sugar moieties in glycoproteins. Intriguingly, we also discovered a distinctive zinc-binding site within Cry23Aa, which appears to be involved in protein-protein interactions. Finally, we discuss the major structural features of Cry23Aa that likely play a role in the toxin's mechanism of action. In view of the low LC50 for CBW larvae and the significant accumulation of these toxins in the flower buds of both T0 and T1 plants, we anticipate that through successive generations of these transgenic lines, cotton plants engineered to overexpress cry23Aa and cry37Aa hold promise for effectively managing CBW infestations in cotton crops.

RNAi turns 25:contributions and challenges in insect science

Since its discovery in 1998, RNA interference (RNAi), a Nobel prize-winning technology, made significant contributions to advances in biology because of its ability to mediate the knockdown of specific target genes. RNAi applications in medicine and agriculture have been explored with mixed success. The past 25 years of research on RNAi resulted in advances in our understanding of the mechanisms of its action, target specificity, and differential efficiency among animals and plants. RNAi played a major role in advances in insect biology. Did RNAi technology fully meet insect pest and disease vector management expectations? This review will discuss recent advances in the mechanisms of RNAi and its contributions to insect science. The remaining challenges, including delivery to the target site, differential efficiency, potential resistance development and possible solutions for the widespread use of this technology in insect management.

Molecular and functional analysis of chitin synthase genes in Chilo suppressalis (Lepidoptera: Crambidae)

The rice stem borer, Chilo suppressalis, has developed a high level of resistance to many of the compounds currently used for control. There is therefore an urgent need to develop novel control methods for C. suppressalis. Insect chitin synthases (CHS) have attracted interest as a potential target for insect pest management. However, to date, CHS have not been characterized in C. suppressalis. Two CHS genes (CsCHS1 and CsCHS2) were identified and cloned from C. suppressalis. Two transcript variants were identified for CsCHS1, CsCHS1a and CsCHS1b. Spatiotemporal expression profiling showed that both transcripts of CsCHS1 are most highly expressed on the last day of each larval instar stage and show the highest expression levels in the integument. In contrast, CsCHS2 is predominantly expressed during the larval feeding stages and shows the highest expression levels in the midgut. Knockdown of CsCHS1 by RNA interference significantly inhibited the molting and pupation of C. suppressalis, and knockdown of CsCHS2 significantly affected growth during the larval stage, but had no significant effect on the pupation. Moreover, knockout of CsCHS1 by CRISPR/Cas9 genome editing severely lowered the hatching rate, larval survivorship, pupation rate, and eclosion rate, but only larval survivorship at the G0 generation was lowered after the knockout of CsCHS2. These results demonstrate that CsCHS1 and CsCHS2 play vital roles in the growth and development of C. suppressalis, and so have potential as insecticidal targets for the control of this highly damaging pest.

Host-induced silencing of the CpCHI gene resulted in developmental abnormalities and mortality in maize stem borer (Chilo partellus)

RNAi-based insecticides for crop protection have witnessed rapid improvement over the years. However, their potential to efficiently control maize stem borer (Chilo partellus) pests has remained underexplored. In this study, double-stranded C. partellus chitinase (dsCHI) toxicity was investigated in C. partellus larvae. Furthermore, we developed transgenic maize lines expressing dsRNA targeted against C. partellus chitinase transcripts and performed detached leaf insect feeding bioassays. Our results revealed that C. partellus chitinase transcript expression was significantly downregulated by 57% and 82% in the larvae. Larvae exhibited various phenotypic distortion levels across developmental stages, and 53% mortality occurred in transgenic fed larvae compared to those fed on nontransgenic leaves. In conclusion, we have identified the C. partellus chitinase gene as a potential target for RNAi-mediated control and demonstrated that oral delivery via bacteria and plant-mediated delivery are viable means of achieving C. partellus RNAi-mediated control.

Gene silencing for invasive paper wasp management: Synthesized dsRNA can modify gene expression but did not affect mortality

Invasive paper wasps such as Polistes dominula are a major pest and problem for biodiversity around the globe. Safe and highly targeted methods for the control of these and other social wasp populations are needed. We attempted to identify potentially-lethal gene targets that could be used on adult paper wasps in a gene silencing or RNA interference (RNAi) approach. Double-stranded RNA (dsRNA) was designed to target genes for which silencing has proven lethal in other insects. dsRNA was provided either orally to foragers or directly injected into the wasps. We also provided the dsRNA unprotected or protected from degradation by gut nucleases in two different forms (lipofectamine and carbon quantum dots). The effects of oral delivery of 22 different gene targets to forager wasps was evaluated. The expression of five different genes was successfully reduced following dsRNA ingestion or injection. These gene targets included the FACT complex subunit spt16 (DRE4) and RNA-binding protein fusilli (FUSILLI), both of which have been previously shown to have potential as lethal targets for pest control in other insects. However, we found no evidence of significant increases in adult wasp mortality following ingestion or injection of dsRNA for these genes when compared with control treatments in our experiments. The methods we used to protect the dsRNA from digestive degradation altered gene expression but similarly did not influence wasp mortality. Our results indicate that while many of the same gene targets can be silenced and induce mortality in other insects, dsRNA and RNAi approaches may not be useful for paper wasp control.

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

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

Current Scenario of Exogenously Induced RNAi for Lepidopteran Agricultural Pest Control: From dsRNA Design to Topical Application

Invasive insects cost the global economy around USD 70 billion per year. Moreover, increasing agricultural insect pests raise concerns about global food security constraining and infestation rising after climate changes. Current agricultural pest management largely relies on plant breeding-with or without transgenes-and chemical pesticides. Both approaches face serious technological obsolescence in the field due to plant resistance breakdown or development of insecticide resistance. The need for new modes of action (MoA) for managing crop health is growing each year, driven by market demands to reduce economic losses and by consumer demand for phytosanitary measures. The disabling of pest genes through sequence-specific expression silencing is a promising tool in the development of environmentally-friendly and safe biopesticides. The specificity conferred by long dsRNA-base solutions helps minimize effects on off-target genes in the insect pest genome and the target gene in non-target organisms (NTOs). In this review, we summarize the status of gene silencing by RNA interference (RNAi) for agricultural control. More specifically, we focus on the engineering, development and application of gene silencing to control Lepidoptera through non-transforming dsRNA technologies. Despite some delivery and stability drawbacks of topical applications, we reviewed works showing convincing proof-of-concept results that point to innovative solutions. Considerations about the regulation of the ongoing research on dsRNA-based pesticides to produce commercialized products for exogenous application are discussed. Academic and industry initiatives have revealed a worthy effort to control Lepidoptera pests with this new mode of action, which provides more sustainable and reliable technologies for field management. New data on the genomics of this taxon may contribute to a future customized target gene portfolio. As a case study, we illustrate how dsRNA and associated methodologies could be applied to control an important lepidopteran coffee pest.

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.

CARACTERIZACIÓN Y DISMINUCIÓN EN LA EXPRESIÓN DE UNA QUITINA SINTASA MEDIADA POR ARNi EN Hypothenemus hampei (CURCULIONIDAE)

La broca del café, Hypothenemus hampei, es un insecto plaga que causa daños significativos al grano de café y grandes pérdidas económicas a los productores en todo el mundo. Al igual que otros insectos, la broca de café requiere de quitina sintasas (CHS) para la biosíntesis de la quitina, componente principal del exoesqueleto del insecto, y de vital importancia para su crecimiento y desarrollo. En este estudio, el gen CHS1 de la broca del café (HhCHS1) fue identificado, caracterizado y posteriormente silenciado mediante el uso de ARNi, mecanismo que permite degradar el ARNm e interrumpir la expresión de proteínas de interés en un organismo. Los perfiles de expresión del gen HhCHS1, medidos por RT-qPCR, mostraron niveles de expresión diferencial en las diferentes etapas del desarrollo del insecto. Los niveles más altos de expresión se encontraron en larvas de segundo estadio (L2) y machos adultos. El ARNcd administrado por vía oral, a concentraciones de 2 µg/100 µL, generó un silenciamiento efectivo del gen HhCHS1 (84 %) después de 7 días de tratamiento. Estos resultados sugieren que el gen HhCHS1 desempeña un papel importante en el desarrollo del insecto, y que, por ende, podría usarse como objetivo para desarrollar nuevas estrategias de manejo de este insecto plaga, mediante el uso de ARNi.  Palabras Clave: ARNcd, café, insecto plaga, quitina-sintasa, RT-qPCR.

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.

Increasing Anthonomus grandis susceptibility to Metarhizium anisopliae through RNAi-induced AgraRelish knockdown: a perspective to combine biocontrol and biotechnology

BACKGROUND

The hemolymph and insect gut together have an essential role in the immune defense against microorganisms, including the production of antimicrobial peptides (AMP). AMPs are mainly induced by two specific signaling pathways, Toll and immune deficiency (IMD). Here, we characterize the expression profile of four genes from both pathways and describe the importance of AgraRelish in the immune defense of Anthonomus grandis against the entomopathogenic fungus Metarhizium anisopliae by RNA interference (RNAi).

RESULTS

To characterize the pathway that is activated early during the A. grandis-M. anisopliae interaction, we assessed the expression profiles of AgraMyD88 and AgraDorsal (Toll pathway), AgraIMD and AgraRelish (IMD pathway), and several AMP genes. Interestingly, we found that IMD pathway genes are upregulated early, and Toll pathway genes are upregulated just 3 days after inoculation (DAI). Furthermore, nine AMPs were upregulated 24 h after fungus inoculation, including attacins, cecropins, coleoptericins, and defensins. AgraRelish knockdown resulted in a reduction in median lethal time (LT₅₀ ) for M. anisopliae-treated insects of around 2 days compared to control treatments. In addition, AgraRelish remained knocked down at 3 DAI. Finally, we identified that AgraRelish knockdown increased fungal loads at 2 DAI compared to control treatments, possibly indicating a faster infection.

CONCLUSIONS

Our data indicate the influence of the IMD pathway on the antifungal response in A. grandis. Combining biocontrol and RNAi could significantly improve cotton boll weevil management. Hence, AgraRelish is a potential target for the development of biotechnological tools aimed at improving the efficacy of M. anisopliae against A. grandis.

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.

The Critical Role of Small RNAs in Regulating Plant Innate Immunity

Plants, due to their sessile nature, have an innate immune system that helps them to defend against different pathogen infections. The defense response of plants is composed of a highly regulated and complex molecular network, involving the extensive reprogramming of gene expression during the presence of pathogenic molecular signatures. Plants attain proper defense against pathogens through the transcriptional regulation of genes encoding defense regulatory proteins and hormone signaling pathways. Small RNAs are emerging as versatile regulators of plant development and act in different tiers of plant immunity, including pathogen-triggered immunity (PTI) and effector-triggered immunity (ETI). The versatile regulatory functions of small RNAs in plant growth and development and response to biotic and abiotic stresses have been widely studied in recent years. However, available information regarding the contribution of small RNAs in plant immunity against pathogens is more limited. This review article will focus on the role of small RNAs in innate immunity in plants.

Chitin Synthases Are Critical for Reproduction, Molting, and Digestion in the Salmon Louse (Lepeophtheirus salmonis)

Chitin synthase (CHS) is a large transmembrane enzyme that polymerizes Uridine diphosphate N-acetylglucosamine into chitin. The genomes of insects often encode two chitin synthases, CHS1 and CHS2. Their functional roles have been investigated in several insects: CHS1 is mainly responsible for synthesizing chitin in the cuticle and CHS2 in the midgut. Lepeophtheirus salmonis is an ectoparasitic copepod on salmonid fish, which causes significant economic losses in aquaculture. In the present study, the tissue-specific localization, expression, and functional role of L. salmonis chitin synthases, LsCHS1 and LsCHS2, were investigated. The expressions of LsCHS1 and LsCHS2 were found in oocytes, ovaries, intestine, and integument. Wheat germ agglutinin (WGA) chitin staining signals were detected in ovaries, oocytes, intestine, cuticle, and intestine in adult female L. salmonis. The functional roles of the LsCHSs were investigated using RNA interference (RNAi) to silence the expression of LsCHS1 and LsCHS2. Knockdown of LsCHS1 in pre-adult I lice resulted in lethal phenotypes with cuticle deformation and deformation of ovaries and oocytes in adult lice. RNAi knockdown of LsCHS2 in adult female L. salmonis affected digestion, damaged the gut microvilli, reduced muscular tissues around the gut, and affected offspring. The results demonstrate that both LsCHS1 and LsCHS2 are important for the survival and reproduction in L. salmonis.

Drosophila, Chitin and Insect Pest Management

Insects are a great menace in agriculture and vectors of human diseases. Hence, controlling insect populations is an important issue worldwide. A common strategy to control insects is the application of insecticides. However, insecticides entail three major problems. First, insecticides are chemicals that stress ecosystems and may even be harmful to humans. Second, insecticides are often unspecific and also eradicate beneficial insect species like the honeybee. Third, insects are able to develop resistance to insecticides. Therefore, the efficient generation of new potent insecticides and their intelligent delivery are the major tasks in agriculture. In addition, acceptance or refusal in society is a major issue that has to be considered in the application of a pest management strategy. In this paper, we unify two issues: 1) we illustrate that our molecular knowledge of the chitin synthesis and organization pathways may offer new opportunities to design novel insecticides that are environmentally harmless at the same time being specific to a pest species; and 2) we advocate that the fruit fly Drosophila melanogaster may serve as an excellent model of insect to study the effects of insecticides at the genetic, molecular and histology level in order to better understand their mode of action and to optimize their impact. Especially, chitin synthesis and organization proteins and enzymes are excellently dissected in the fruit fly, providing a rich source for new insecticide targets. Thus, D. melanogaster offers a cheap, efficient and fast assay system to address agricultural questions, as has been demonstrated to be the case in bio-medical research areas.

RNAi-Mediated Suppression of Laccase2 Impairs Cuticle Tanning and Molting in the Cotton Boll Weevil (Anthonomus grandis)

The cotton boll weevil, Anthonomus grandis, is the most economically important pest of cotton in Brazil. Pest management programs focused on A. grandis are based mostly on the use of chemical insecticides, which may cause serious ecological impacts. Furthermore, A. grandis has developed resistance to some insecticides after their long-term use. Therefore, alternative control approaches that are more sustainable and have reduced environmental impacts are highly desirable to protect cotton crops from this destructive pest. RNA interference (RNAi) is a valuable reverse genetics tool for the investigation of gene function and has been explored for the development of strategies to control agricultural insect pests. This study aimed to evaluate the biological role of the Laccase2 (AgraLac2) gene in A. grandis and its potential as an RNAi target for the control of this insect pest. We found that AgraLac2 is expressed throughout the development of A. grandis with significantly higher expression in pupal and adult developmental stages. In addition, the immunolocalization of the AgraLac2 protein in third-instar larvae using specific antibodies revealed that AgraLac2 is distributed throughout the epithelial tissue, the cuticle and the tracheal system. We also verified that the knockdown of AgraLac2 in A. grandis resulted in an altered cuticle tanning process, molting defects and arrested development. Remarkably, insects injected with dsAgraLac2 exhibited defects in cuticle hardening and pigmentation. As a consequence, the development of dsAgraLac2-treated insects was compromised, and in cases of severe phenotypic defects, the insects subsequently died. On the contrary, insects subjected to control treatments did not show any visible phenotypic defects in cuticle formation and successfully molted to the pupal and adult stages. Taken together, our data indicate that AgraLac2 is involved in the cuticle tanning process in A. grandis and may be a promising target for the development of RNAi-based technologies.

Insecticidal Gene Silencing by RNAi in the Neotropical Region

Insecticidal gene silencing by RNA interference (RNAi) involves a post-transcriptional mechanism with great potential for insect control. Here, we aim to summarize the progress on RNAi research toward control of insect pests in the Neotropical region and discuss factors determining its efficacy and prospects for pest management. We include an overview of the available RNAi information for Neotropical pests in the Lepidoptera, Coleoptera, Diptera, and Hemiptera orders. Emphasis is put on significant findings in the use of RNAi against relevant Neotropical pests, including diamondback moth (Plutella xylostella L.), Asian citrus psyllid (Diaphorina citri Kuwayama), and the cotton boll weevil (Anthonomus grandis Boheman). We also examine the main factors involved in insecticidal RNAi efficiency and major advances to improve screening of lethal genes, formulation, and delivery. Few studies detail resistance mechanisms to RNAi, demonstrating a need for more research. Advances in formulation, delivery, and resistance management tools for insecticidal RNAi in the Neotropics can provide a basis for efficient field application.

Mechanisms, Applications, and Challenges of Insect RNA Interference

The RNA interference (RNAi) triggered by short/small interfering RNA (siRNA) was discovered in nematodes and found to function in most living organisms. RNAi has been widely used as a research tool to study gene functions and has shown great potential for the development of novel pest management strategies. RNAi is highly efficient and systemic in coleopterans but highly variable or inefficient in many other insects. Differences in double-stranded RNA (dsRNA) degradation, cellular uptake, inter- and intracellular transports, processing of dsRNA to siRNA, and RNA-induced silencing complex formation influence RNAi efficiency. The basic dsRNA delivery methods include microinjection, feeding, and soaking. To improve dsRNA delivery, various new technologies, including cationic liposome-assisted, nanoparticle-enabled, symbiont-mediated, and plant-mediated deliveries, have been developed. Major challenges to widespread use of RNAi in insect pest management include variable RNAi efficiency among insects, lack of reliable dsRNA delivery methods, off-target and nontarget effects, and potential development of resistance in insect populations.

Using de novo transcriptome assembly and analysis to study RNAi in Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae)

Phenacoccus solenopsis is one of the major polyphagous crop pests in India. Inadequate genomic or transcriptomic resources have limited the molecular studies in this insect despite its huge economic importance. The existing molecular sequence resources of this insect were supplemented through RNA sequencing, de novo transcriptome assembly and analysis, which generated 12, 925 CDS from 23,643 contigs with an average size of 1077.5 bp per CDS and 85.1% positive BLAST hits with NCBI Non redundant (nr) database. Twenty three genes involved in RNAi machinery identified through BLASTx search against NCBI nr database suggested the existence of robust RNAi in mealybug. RNAi in P. solenopsis was demonstrated through knockdown of IAP (Inhibitor of Apoptosis), AQP (Aquaporin), CAL (Calcitonin), VATPase (V-type proton ATPase subunit F 1), bursicon, chitin synthase, SNF7 and α-amylase by injecting sequence specific dsRNA of respective genes in adult female. Additionally, feeding RNAi has been demonstrated in 2nd instar nymph through dsRNA uptake in plant. The knockdown of core RNAi machinery genes such as Dicer, Argonaute and Staufen significantly hampered RNAi efficiency in this insect. However, downregulation of dsRNases improved RNAi efficiency. Sequential studies for understanding RNAi in P. solenopsis using transcriptome sequences have also been reported. The present study provides a base for future research on developing RNAi as strategy for management of this pest.

Chitinous Structures as Potential Targets for Insect Pest Control

Chitinous structures are physiologically fundamental in insects. They form the insect exoskeleton, play important roles in physiological systems and provide physical, chemical and biological protections in insects. As critically important structures in insects, chitinous structures are attractive target sites for the development of new insect-pest-control strategies. Chitinous structures in insects are complex and their formation and maintenance are dynamically regulated with the growth and development of insects. In the past few decades, studies on insect chitinous structures have shed lights on the physiological functions, compositions, structural formation, and regulation of the chitinous structures. Current understanding of the chitinous structures has indicated opportunities for exploring new target sites for insect control. Mechanisms to disrupt chitinous structures in insects have been studied and strategies for the potential development of new means of insect control by targeting chitinous structures have been proposed and are practically to be explored.