Impact of agrochemical pollutant mixtures on the selection of insecticide resistance in the malaria vector Anopheles gambiae. Insights from experimental evolution and transcriptomics

Abstract

Abstract Context: There are several indications that pesticides used in agriculture contribute to the emergence and spread of resistance of mosquitoes to vector control insecticides. However, the impact of such indirect selection pressure has rarely been quantified and the molecular mechanisms involved are still poorly characterised. In this context, experimental selection with different agrochemical mixtures was conducted in An. gambiae and the impact on insecticide resistance was evaluated by phenotypic and molecular approaches. Methods: Mosquito larvae were selected for 30 generations with three different mixtures of agrochemicals containing i) insecticides, ii) non-insecticides compounds and iii) both agrochemical types. Each five generations, the resistance of adults to deltamethrin and bendiocarb was monitored using bioassays. The frequencies of the kdr west (L1014F) and Ace1 (G119S) target-site mutations were evaluated every 10 generations. Gene expression and polymorphism variations associated with each selection regime were investigated after 30 generations by RNA-seq. Results: Larval selection with agrochemical mixtures did not affect bendiocarb resistance and did not select for Ace1 mutation. Contrastingly, an increased deltamethrin resistance was observed in the three selected lines as compared to the non-selected line. Such increased resistance was associated with an increased frequency of the Kdr west mutation in the insecticide and non-insecticide selected lines. RNA-seq identified 63 candidate resistance genes over-transcribed in at least one selected line as compared to the non-selected line. These include genes coding for detoxification enzymes or cuticular proteins previously associated with insecticide resistance, and other genes potentially associated with chemical stress response. Polymorphism analyses identified several genes under selection in each line across multiple genomic loci supporting a multigenic adaptive response to agrochemical mixtures. Conclusion: This study supports the role of agrochemical contaminants as a significant larval selection pressure favouring insecticide resistance in malaria vectors. Such selection pressures impact Kdr mutations, detoxification enzymes but also other more generalist resistance mechanisms which could potentially lead to cross-tolerance to unrelated insecticide compounds. Such indirect effect of global landscape pollution on mosquito resistance to public health insecticides deserves further attention since it can affect the nature and dynamics of resistance alleles circulating in malaria vectors and impact the efficacy of control vector strategies.