Cis and trans-acting variants contribute to survivorship in a naive Drosophila melanogaster population exposed to ryanoid insecticides
Insecticide resistance is a paradigm of microevolution and insecticides are responsible for the strongest cases of recent selection in the genome of Drosophila melanogaster. Here we use a naive population and a novel insecticide class to examine the ab initio genetic architecture of a potential selective response. Genome wide association studies of chlorantraniliprole susceptibility reveal variation in a gene of major effect, Stretchin Myosin light chain kinase (Strn-Mlck), which we validate with linkage mapping and transgenic manipulation of gene expression. We propose that allelic variation in Strn-Mlck alters sensitivity to the calcium depletion attributable to chlorantraniliproles mode of action. Genome-wide association studies also reveal a network of genes involved in neuromuscular biology. In contrast, phenotype to transcriptome associations identify differences in constitutive levels of multiple transcripts regulated by cnc, the homologue of mammalian Nrf2. This suggests that genetic variation acts in trans to regulate multiple metabolic enzymes in this pathway. The most outstanding association is with the transcription level of Cyp12d1 which is also affected in cis by copy number variation. Transgenic overexpression of Cyp12d1 reduces susceptibility to both chlorantraniliprole and the closely related insecticide cyantraniliprole. This systems genetics study reveals multiple allelic variants segregating at intermediate frequency in a population that is completely naive to this new insecticide chemistry and it adumbrates a selective response among natural populations to these chemicals.\n\nSignificanceAround the world insecticides are being deregistered and banned, as their environmental costs are deemed too great or their efficacy against pest insects is reduced through the evolution of insecticide resistance. With the introduction of replacement insecticides comes the responsibility to assess the way new insecticides perturb various levels of biological systems; from insect physiology to ecosystems. We used a systems genetics approach to identify genetic variants affecting survivorship of Drosophila melanogaster exposed to chlorantraniliprole. The study population was completely naive to this insecticide chemistry and yet we find associations with variants in neuromuscular genes and co-regulated detoxification genes. We predict that these variants will increase in populations of this sentinel species as these insecticides are applied in the environment.