Evidence for a Selective Sweep in the wapl Region of Drosophila melanogaster

A scan of the X chromosome of a European Drosophila melanogaster population revealed evidence for the recent action of positive directional selection at individual loci. In this study we analyze one such region that showed no polymorphism in the genome scan (located in cytological division 2C10-2E1). We detect a 60.5-kb stretch of DNA encompassing the genes ph-d, ph-p, CG3835, bcn92, Pgd, wapl, and Cyp4d1, which almost completely lacks variation in the European sample. Loci flanking this region show a skewed frequency spectrum at segregating sites, strong haplotype structure, and high levels of linkage disequilibrium. Neutrality tests reveal that these data are unlikely under both the neutral equilibrium model and the simple bottleneck scenarios. In contrast, newly developed maximum-likelihood ratio tests suggest that strong selection has acted recently on the region under investigation, causing a selective sweep. Evidence that this sweep may have originated in an ancestral population in Africa is presented.