Large-scale adaptive hitchhiking upon high recombination in Drosophila simulans.

Natural selection is expected to leave a characteristic footprint on neighboring nucleotide variation through the effects of genetic linkage. The size of the region affected is proportional to the strength of selection and greatly reduced with the recombinational distance from the selected site. Thus, the genomic footprint of selection is generally believed to be restricted to a small DNA stretch in normal and highly recombining regions. Here, we study the effect of selection on linked polymorphism (hitchhiking effect) by surveying nucleotide variation across a highly recombining approximately 88-kb genomic fragment in an African population of Drosophila simulans. We find a core region of up to 38 kb with a major haplotype at intermediate frequency. The extended haplotype structure that gradually vanishes until disappearing is unusual for a highly recombining region. Both the presence in the structured genomic domain of a single major haplotype depleted of variability and the detected spatial pattern of variation along the approximately 88-kb fragment are incompatible with neutral predictions in a panmictic population. A major role of demographic effects could also be discarded. The observed pattern of variation clearly provides evidence that directional selection has acted recently on this region, sweeping out variation around a strongly adaptive mutation. Our findings suggest a major role of positive selection in shaping DNA variability even in highly recombining regions.