If this reduction is partially recessive, as is plausible, then its beneficial effect in hemizygous males could outweigh the deleterious effects on females for mutations on the X chromosome, but not the autosomes, leading to a faster-X effect (see fig. 6 of Vicoso and Charlesworth 2009).
One hypothesis to explain this observation is a difference in the efficiency of purifying selection in removing recessive deleterious mutations in hemizygous males, a phenomenon which can often lead to a faster-X effect (Charlesworth et al. 1987; Vicoso and Charlesworth 2009a).
We detected both a Faster-X effect and an effect of male-biased gene expression.
Drosophila protein sequence divergence data show a general trend toward faster-X effects, with some exceptions (Presgraves 2008); studies of divergence in gene expression in Drosophila also show a faster-X effect (Kayserili et al. 2012; Meisel et al. 2012a).
> -wrap-foot> How, then, can we explain a faster-X effect that occurs regardless of sex-specific selection?
This then raises the question of why a faster-X effect is detected for pse – per but not for the other pseudoobscura clade comparisons.
First, X-linkage does not systematically increase rates of molecular evolution; while some previous studies do support a faster-X model [ 54, 55], the most recent and most comprehensive test of this model shows no evidence for faster-X evolution [ 56].
Surprisingly, however, we find a faster-X effect in the melanogaster clade that is as strong for female-biased genes as for other genes, whereas the standard theory predicts a lack of a faster-X effect for genes with female-specific fitness effects (Charlesworth et al. 1987).
By partitioning genes according to their levels of sex-biased gene expression, we confirm the existence of a Faster-X effect on adaptive evolution, as well as an effect of male-biased gene expression, but find no evidence for a strong association between Faster-X effects and sex-biased gene expression.
Comparisons between X-linked and autosomal loci inside and near inversions are also consistent with a faster-X effect, but nonsignificant, which is probably due to the smaller number of loci in these regions.
