A multispecies approach for comparing sequence evolution of X-linked and autosomal sites inDrosophila

Abstract

SummaryPopulation genetics models show that, under certain conditions, the X chromosome is expected to be under more efficient selection than the autosomes. This could lead to ‘faster-X evolution’, if a large proportion of mutations are fixed by positive selection, as suggested by recent studies inDrosophila. We used a multispecies approach to test this: Muller's element D, an autosomal arm, is fused to the ancestral X chromosome inDrosophila pseudoobscuraand its sister species,Drosophila affinis. We tested whether the same set of genes had higher rates of non-synonymous evolution when they were X-linked (in theD. pseudoobscura/D. affiniscomparison) than when they were autosomal (inDrosophila melanogaster/Drosophila yakuba). Although not significant, our results suggest this may be the case, but only for genes under particularly strong positive selection/weak purifying selection. They also suggest that genes that have become X-linked have higher levels of codon bias and slower synonymous site evolution, consistent with more effective selection on codon usage at X-linked sites.