Fitness costs of key point mutations that underlie acaricide target-site resistance in the two-spotted spider mite Tetranychus urticae

The frequency of insecticide/acaricide target-site resistance is increasing in arthropod pest populations and is typically underpinned by single point mutations that affect the binding strength between the insecticide/acaricide and its target-site. Theory predicts that although resistance mutations clearly have advantageous effects under the selection pressure of the insecticide/acaricide, they might convey negative pleiotropic effects on other aspects of fitness. If such fitness costs are in place, target-site resistance is thus likely to disappear in the absence of insecticide/acaricide treatment, a process that would counteract the spread of resistance in agricultural crops. Hence, there is a great need to reliably quantify the various potential pleiotropic effects of target-site resistance point mutations on arthropod fitness. Here, we used near-isogenic lines of the spider mite pest Tetranychus urticae that carry well-characterized acaricide target-site resistance mutations to quantify potential fitness costs. Specifically, we analyzed P262T in the mitochondrial cytochrome b, the combined G314D and G326E substitutions in the glutamate-gated chloride channels, L1024V in the voltage-gated sodium channel, and I1017F in chitin synthase 1. Five fertility life table parameters and nine single-generation life-history traits were quantified and compared across a total of 15 mite lines. In addition, we monitored the temporal resistance level dynamics of populations with different starting frequency levels of the chitin synthase resistant allele to further support our findings. Three target-site resistance mutations, I1017F and the co-occurring G314D and G326E mutations, were shown to significantly and consistently alter certain fitness parameters in T. urticae. The other two mutations (P262T and L1024V) did not result in any consistent change in a fitness parameter analyzed in our study. Our findings are discussed in the context of the global spread of T. urticae pesticide resistance and integrated pest management.

Plant-mediated RNAi of grain aphid CHS1 gene confers common wheat resistance against aphids

BACKGROUND

Chitin is an important component of the insect exoskeleton and peritrophic membrane. Chitin synthase 1 (CHS1) is a key enzyme in the chitin synthesis pathway, and has a role in insect molting and growth. Plant-mediated RNA interference (RNAi) has been used as a more target-specific and environmentally safe approach to prevent and control agricultural insects. The aims of this study were to use grain aphid (Sitobion avanae) CHS1 as the target gene and to produce transgenic wheat lines for aphid control via plant-mediated RNAi.

RESULTS

Expression levels of CHS1 changed at different developmental stages. After feeding on the representative T₃ transgenic lines Tb5-2 and Tb10-3, CHS1 expression levels in grain aphid decreased by 50.29% and 45.32%, respectively; and total and molting aphid numbers reduced significantly, compared with controls. Consistent with this, aphid numbers in mixed natural populations reduced significantly in the respective T₄ and T₅ transgenic lines under field conditions, and T5 transgenic lines had higher grain weight compared with the unsprayed insecticide wild-type and insecticide-sprayed wild-type.

CONCLUSION

These results indicate that plant-mediated RNAi of the grain aphid CHS1 gene confers common wheat resistance against aphids. © 2018 Society of Chemical Industry.

Chitin synthase 1 and five cuticle protein genes are involved in serosal cuticle formation during early embryogenesis to enhance eggshells in Nilaparvata lugens

Many holo- and hemimetabolous insects enhance their eggshells during embryogenesis by forming a serosal cuticle (SC). To date, scholarly understanding of the SC composition and SC-related gene functions has been limited, especially for hemimetabolous insects. In this study, we initially performed transmission electron microscopic (TEM) observation and chitin staining of the SC in Nilaparvata lugens, a hemimetabolous rice pest known as the brown planthopper (BPH). We confirmed that the SC was a chitin-rich lamellar structure deposited gradually during the early embryogenesis. Parental RNA interference (RNAi) against Nilaparvata lugens chitin synthase 1 (NlCHS1) in newly emerged and matured females resulted in decreases of egg hatchability by 100% and 76%, respectively. Ultrastructural analyses revealed loss of the lamellar structure of the SC in dsNlCHS1-treated eggs. According to temporal expression profiles, five cuticle protein coding genes, NlugCpr1/2/3/8/90, were specifically or highly expressed during the SC formation period, and NlugCpr1/2/3/90 were further detected in 72 h eggshells extract by ultra-performance liquid chromatography-tandem mass spectrometry/mass spectrometry. NlugCpr2/3/90 were likely three SC-specific cuticle proteins. TEM observations of the SC following parental RNAi against NlugCpr1/2/3/8/90 demonstrated that NlugCpr3/8/90 were essential for SC formation. The study provided an understanding of the SC formation process and SC-related cuticle proteins in BPHs, which offer potential targets for pest control in the egg stage as well.

RNAi-Mediated Knockdown of Chitin Synthase 1 (CHS1) Gene Causes Mortality and Decreased Longevity and Fecundity in Aphis gossypii

Chitin is a vital part of the insect exoskeleton and peritrophic membrane, synthesized by chitin synthase (CHS) enzymes. Chitin synthase 1 (CHS1) is a crucial enzyme in the final step of chitin biosynthetic pathway and consequently plays essential role towards insect growth and molting. RNA interference (RNAi) is an agent that could be used as an extremely target-specific and ecologically innocuous tactic to control different insect pests associated with economically important crops. The sole purpose of the current study is to use CHS1 as the key target gene against the cotton-melon aphid, Aphis gossypii, via oral feeding on artificial diets mixed with dsRNA-CHS1. Results revealed that the expression level of CHS1 gene significantly decreased after the oral delivery of dsRNA-CHS1. The knockdown of CHS1 gene caused up to 43%, 47%, and 59% mortality in third-instar nymph after feeding of dsCHS1 for 24, 48, and 72 h, respectively, as compared to the control. Consistent with this, significantly lower longevity (approximately 38%) and fecundity (approximately 48%) were also found in adult stage of cotton-melon aphids that were fed with dsCHS1 for 72 h at nymphal stage. The qRT-PCR analysis of gene expression demonstrated that the increased mortality rates and lowered longevity and fecundity of A. gossypii were attributed to the downregulation of CHS1 gene via oral-delivery-mediated RNAi. The results of current study confirm that CHS1 could be an appropriate candidate target gene for the RNAi-based control of cotton-melon aphids.

Benzoylurea resistance in western flower thrips Frankliniella occidentalis (Thysanoptera: Thripidae): the presence of a point mutation in chitin synthase 1

We examined the susceptibility of field strains (BO-1, BO-2, TO-1, and YH-1) and one laboratory strain (H-1) of the western flower thrip, Frankliniella occidentalis, to benzoylureas. LC₅₀ values of novaluron were determined as 0.64 ppm against laboratory strain and 2.1-130 ppm against field strains. In the presence of piperonyl butoxide, a cytochrome P450 inhibitor, the insecticidal activity of novaluron tended to be enhanced. To examine whether point mutations in chitin synthase 1 (CHS1) discovered in an etoxazole-resistant strain of Tetranychus urticae and a benzoylurea-resistant strain of Plutella xylostella exist in F. occidentalis, the nucleotide sequence of CHS1 was analyzed. We found a nonsynonymous substitution that corresponded to the location of the mutations found in T. urticae and P. xylostella in the field strains of F. occidentalis but not in the laboratory strain, indicating that this point mutation might be associated with the benzoylurea resistance exhibited by the field strains.

Enantioselectivity Mechanism of Etoxazole Enantiomers in Tetranychus cinnabarinus: Acaricidal Activity, Metabolic Difference, and Target Binding

Etoxazole, a widely used mite growth inhibitor, contains a chiral center in its chemical structure, resulting in two mirror-image enantiomers. These enantiomers of etoxazole display significant differences in biological activity and environmental behavior. In bioassays conducted against Tetranychus cinnabarinus, it was observed that S-etoxazole demonstrated approximately 279.63 times greater activity against mite eggs compared to R-etoxazole, and its efficacy against adult mites was approximately 5 times higher than that of R-etoxazole. Furthermore, S-etoxazole was found to be more potent in inhibiting chitin synthesis. To elucidate the underlying mechanism for the observed differences in enantiomeric activity, we conducted metabolism assays using crude enzymes, revealing that R-etoxazole undergoes metabolism more readily than S-etoxazole. Subsequent molecular docking and Microscale Thermophoresis (MST) experiments showed that R-etoxazole exhibits weaker binding affinity to the target protein, chitin synthase 1, compared to S-etoxazole. This study thus demonstrates that the differential enantiomeric activity of etoxazole was attributable to a combination of factors, including detoxification metabolism and target protein binding.