Natural selection acts across species boundaries in Drosophila simulans and D. sechellia. Daniel Garrigan, Sarah Kingan, Cara Brand. Department of Biology, University of Rochester, Rochester, NY.

   Until recently, it was believed that Drosophila simulans and D. sechellia diverged in allopatry. However, a whole-genome resequencing study reveals that at least 5% of these two genomes has experienced gene flow after their initial divergence. Here, we resequence a 20 kb region of chromosome 3R that shows the most pronounced evidence for recent gene flow. We find a haplogroup that introgressed from D. simulans to D. sechellia approximately 10,000 years ago and occurs at high frequency in both species. In D. sechellia, a core region of the introgressed haplogroup is fixed in our sample, while in D. simulans, the core region is nearly fixed and the haplogroup varies in length among our samples. Patterns of both allele frequencies and linkage disequilibrium support the hypothesis that positive natural selection has recently elevated the introgressed haplogroup to high frequency in both species. Interestingly, the target of natural selection centers on a cluster of transcription factor binding sites adjacent to a homeobox transcription factor gene. However, the introgressed haplogroup also harbors two genes involved in olfaction that have hitchhiked to high frequency in D. sechellia. While we currently cannot detail the phenotype being targeted by natural selection, the resequence data are unique in that they show evidence for an adaptive mutation that is globally advantageous across divergent genomic backgrounds and ecological conditions.