Contemporary evolution of a Lepidopteran species, Heliothis virescens, in response to modern agricultural practices
Adaptation to human-induced environmental change has the potential to profoundly influence the genomic architecture of affected species. This is particularly true in agricultural ecosystems, where anthropogenic selection pressure is strong. Heliothis virescens feeds on cotton in its larval stages and US populations have been declining since the widespread planting of transgenic cotton, which endogenously express proteins derived Bacillus thuringiensis (Bt). No physiological adaptation to Bt toxin has been found in the field, so adaptation to this altered environment could involve: 1) shifts in host plant selection mechanisms to avoid cotton, 2) changes in detoxification mechanisms required for cotton-feeding versus feeding on other host plants, or 3) loss of resistance to previously used management practices including insecticides. Here we begin to address the question of whether such changes occurred in H. virescens populations between the years 1997 and 2012. As part of our study, we produced an H. virescens genome assembly and used this in concert with a ddRAD-seq enabled genome scan to identify loci with significant allele frequency changes over the 15 year period after Bt crops became widespread in the agricultural landscape. Confirmation that a known genetic change at a previously described H. virescens target of selection was detectable in our genome scan increased our confidence in this methodology. We quantified the strength of selection required to elicit the observed allele frequency changes at loci under selection. Potential contributions of genes near the loci under selection to adaptive phenotypes in the H. virescens cotton system are discussed.