Uncovering the footprint of positive selection on the X chromosome of Drosophila melanogaster.

A usual approach to detect the spatial footprint left by recent adaptive events has been to follow up putative candidates emerging from multilocus scans of variation by sequencing additional fragments. We have used a similar experimental and analytical approach to study variation at 15 independently evolving and randomly chosen regions of the X chromosome of Drosophila melanogaster. These incompletely sequenced regions, each extending over approximately 40 kb, were subjected to two tests of positive selection that take into account the spatial distribution of nucleotide variation. Our analysis of variation at these genomic regions in a European population of D. melanogaster has allowed us to uncover a candidate region for positive selection and to empirically evaluate the comparative performance of the two tests of selection under a bottleneck scenario. Moreover, the boundaries here estimated for both the rate of adaptive substitution (delta) and the average selection coefficient (s) would support previous estimates obtained by maximum likelihood that suggest rather strong but uncommon positive selection.