Excess of amino acid substitutions relative to polymorphism between X-linked duplications in Drosophila melanogaster

We have obtained sequence polymorphism data from 13 genes belonging to 5 gene families in Drosophila melanogaster where the K(a)/K(s) between copies is greater than 1. Twelve of these 13 loci are X-linked. In general, there is evidence of purifying selection in all families, as inferred both from levels of silent and replacement variation and insertion/deletion variation, suggesting that the loci are likely functional. Shared polymorphisms indicative of gene conversion between paralogs are rare among the X-linked families, in contrast to available data from autosomal duplicates. McDonald-Kreitman tests between duplicates reveal an excess of amino-acid fixations between copies in the X-linked families, suggesting that the divergence between these loci was driven by positive selection. In contrast, available data from autosomal duplicates show a deficit of fixations, consistent with gene conversion being a strong homogenizing force.