Natural selection shapes genome-wide patterns of copy-number polymorphism in Drosophila melanogaster

The role that natural selection plays in governing the locations and early evolution of copy- number mutations remains largely unexplored. We used high- density full- genome tiling arrays to create a fine- scale genomic map of copy- number polymorphisms ( CNPs) in Drosophila melanogaster. We inferred a total of 2658 independent CNPs, 56% of which overlap genes. These include CNPs that are likely to be under positive selection, most notably high- frequency duplications encompassing toxin- response genes. The locations and frequencies of CNPs are strongly shaped by purifying selection, with deletions under stronger purifying selection than duplications. Among duplications, those overlapping exons or introns, as well as those falling on the X chromosome, seem to be subject to stronger purifying selection.