Copy number variants underlie the major selective sweeps in insecticide resistance genes inAnopheles arabiensisfrom Tanzania

Abstract

To keep ahead of the evolution of resistance to insecticides in mosquitoes, national malaria control programmes must make use of a range of insecticides, both old and new, while monitoring resistance mechanisms. Knowledge of the mechanisms of resistance remains limited inAnopheles arabiensis, which in many parts of Africa is of increasing importance because it is apparently less susceptible to many indoor control interventions. Furthermore, comparatively little is known in general about resistance to non-pyrethroid insecticides such as pirimiphos-methyl (PM), which are crucial for effective control in the context of resistance to pyrethroids. We performed a genome-wide association study to determine the molecular mechanisms of resistance to deltamethrin (commonly used in bednets) and PM, inAn. arabiensisfrom two regions in Tanzania. Genomic regions of positive selection in these populations were largely driven by copy number variants (CNVs) in gene families involved in resistance to these two insecticides. We found evidence of a new gene cluster involved in resistance to PM, identifying a strong selective sweep tied to a CNV in theCoeae2g-Coeae6gcluster of carboxylesterase genes. Using complementary data fromAn. coluzziiin Ghana, we show that copy number at this locus is significantly associated with PM resistance. Similarly, for deltamethrin, resistance was strongly associated with a novel CNV allele in theCyp6aa/Cyp6pcluster. Against this background of metabolic resistance, target site resistance was very rare or absent for both insecticides. Mutations in the pyrethroid target siteVgscwere at very low frequency in Tanzania, yet combining these samples with threeAn. arabiensisindividuals from West Africa revealed a startling diversity of evolutionary origins of target site resistance, with up to 5 independent origins ofVgsc-995 mutations found within just 8 haplotypes. Thus, despite having been first recorded over 10 years ago,Vgscresistance mutations in TanzanianAn. arabiensishave remained at stable low frequencies. Overall, our results provide a new copy number marker for monitoring resistance to PM in malaria mosquitoes, and reveal the complex picture of resistance patterns inAn. arabiensis.