Search

Highlights: [•] We tested how natural variation affects the outcomes of RNAi treatments on insects. [•] Three field western corn rootworm populations and two genes were tested. [•] Effect of RNAi targeting a cysteine protease gene varied across populations. [•] RNAi of an immune gene caused opposite effects among populations feeding on soybean. [•] Natural variation among populations can alter the outcome of RNAi applications. [Copyright &y& Elsevier]

Herbivorous insects encounter a variety of toxic environmental substances ranging from ingested plant defensive compounds to human-introduced insecticidal agents. Dietary antioxidants are known to reduce the negative physiological impacts of toxins in mammalian systems through amelioration of reactive oxygen-related cellular damage. The analogous impacts to insects caused by multigenerational exposure to pesticides and the effects on adaptive responses within insect populations, however, are currently unknown. To address these research gaps, we used Drosophila as a model system to explore adaptive phenotypic responses to acute dichlorodiphenyltrichloroethane (DDT) exposure in the presence of the dietary antioxidant vitamin C and to examine the structural genomic consequences of this exposure. DDT resistance increased significantly among four replicates exposed to a low concentration of DDT for 10 generations. In contrast, dietary intake of vitamin C significantly reduced DDT resistance after mutigenerational exposure to the same concentration of DDT. As to the genomic consequences, no significant differences were predicted in overall nucleotide substitution rates across the genome between any of the treatments. Despite this, replicates exposed to a low concentration of DDT without vitamin C showed the highest number of synonymous and non-synonymous variants (3196 in total), followed by the DDT plus vitamin C (1174 in total), and vitamin C alone (728 in total) treatments. This study demonstrates the potential role of diet (specifically, antioxidant intake) on adaptive genome responses, and thus on the evolution of pesticide resistance within insect populations. Unlabelled Image • Dietary intake of vitamin C significantly decreased DDT resistance following multigenerational low-dose DDT selection. • Dietary vitamin C impacted directional selection, decreasing the number of genomic synonymous and non-synonymous variants. • This study reveals that dietary antioxidants may, under some circumstances, impact the evolution of pesticide resistance. [ABSTRACT FROM AUTHOR]

Insecticide resistance is an ongoing challenge in agriculture and disease vector control. Here, we demonstrate a novel strategy to attenuate resistance. We used genomics tools to target fundamental energy-associated pathways and identified a potential "Achilles' heel" for resistance, a resistance-associated protein that, upon inhibition, results in a substantial loss in the resistance phenotype. Specifically, we compared the gene expression profiles and structural variations of the insulin/insulin-like growth factor signaling (IIS) pathway genes in DDT-susceptible (91-C) and -resistant (91-R) Drosophila melanogaster (Drosophila) strains. A total of eight and seven IIS transcripts were up- and down-regulated, respectively, in 91-R compared to 91-C. A total of 114 nonsynonymous mutations were observed between 91-C and 91-R , of which 51.8% were fixed. Among the differentially expressed transcripts, phosphoenolpyruvate carboxykinase (PEPCK), down-regulated in 91-R , encoded the greatest number of amino acid changes, prompting us to perform PEPCK inhibitor–pesticide exposure bioassays. The inhibitor of PEPCK, hydrazine sulfate, resulted in a 161- to 218-fold decrease in the DDT resistance phenotype (91-R) and more than a 4- to 5-fold increase in susceptibility in 91-C. A second target protein, Glycogen synthase kinase 3β (GSK3β-PO), had one amino acid difference between 91-C and 91-R , and the corresponding transcript was also down-regulated in 91-R. A GSK3β-PO inhibitor, lithium chloride, likewise reduced the resistance but to a lesser extent than did hydrazine sulfate for PEPCK. We demonstrate the potential role of IIS genes in DDT resistance and the potential discovery of an "Achilles' heel" against pesticide resistance in this pathway. Unlabelled Image • 114 nonsynonymous mutations, for genes within the insulin signally pathway, were observed between 91-C and 91-R. • PEPCK had the greatest number of amino acid differences identified between 91-C and 91-R. • Inhibitor of PEPCK (hydrazine sulfate) resulted in a 161- to 218-fold decrease in 91-R. • The nutritional status of flies influenced the toxicity of DDT and the effectiveness of inhibitors on resistance levels. [ABSTRACT FROM AUTHOR]

4,4'-dichlorodiphenyltrichloroethane (DDT) has been re-recommended by the World Health Organization for malaria mosquito control. Previous DDT use has resulted in resistance, and with continued use resistance will increase in terms of level and extent. Drosophila melanogaster is a model dipteran that has many available genetic tools, numerous studies done on insecticide resistance mechanisms, and is related to malaria mosquitoes allowing for extrapolation. The 91-R strain of D. melanogaster is highly resistant to DDT (>1500-fold), however, there is no mechanistic scheme that accounts for this level of resistance. Recently, reduced penetration, increased detoxification, and direct excretion have been identified as resistance mechanisms in the 91-R strain. Their interactions, however, remain unclear. Use of UAS-RNAi transgenic lines of D. melanogaster allowed for the targeted knockdown of genes putatively involved in DDT resistance and has validated the role of several cuticular proteins ( Cyp4g1 and Lcp1 ), cytochrome P450 monooxygenases ( Cyp6g1 and Cyp12d1 ), and ATP binding cassette transporters ( Mdr50, Mdr65 , and Mrp1 ) involved in DDT resistance. Further, increased sensitivity to DDT in the 91-R strain after intra-abdominal dsRNA injection for Mdr50, Mdr65 , and Mrp1 was determined by a DDT contact bioassay, directly implicating these genes in DDT efflux and resistance. [ABSTRACT FROM AUTHOR]

Highlights: [•] The DDT-resistant 91-R is highly resistant to DDT. [•] Resistance is polyfactorial. [•] Cuticular penetration of DDT is significantly reduced in the 91-R strain. [•] Excretion of unaltered DDT is significantly increased in the 91-R strain. [•] Oxidative, reductive and conjugative xenobiotic metabolisms are increased in the 91-R strain. [Copyright &y& Elsevier]