Fast accumulation of nonsynonymous mutations on the female-specific W chromosome in birds

Following cessation of recombination during sex chromosome evolution, the nonrecombining sex chromosome is affected by a number of degenerative forces, possibly resulting in the fixation of deleterious mutations. This might take place because of weak selection against recessive or partly recessive deleterious mutations due to permanent heterozygosity of nonrecombining chromosomes. Furthermore, population genetic processes, such as selective sweeps, background selection, and Muller's ratchet, result in a reduction in N-e, which increase the likelihood of fixation of deleterious mutations. Theory thus predicts that nonrecombining genes should show increased levels of nonsynonymous (d(N)) to synonymous substitutions (d(S)). We tested this in an avian system by estimating the ratio between d(N) and d(S) in six gametologous gene pairs located on the Z chromosome and the nonrecombining, female-specific W chromosome. In comparisons, we found a significantly higher d(N)/d(S) ratio for the W-linked than the Z-linked copy in three of the investigated genes. In a concatenated alignment of all six genes, the d(N)/d(S) ratio was six times higher for W-linked than Z-linked genes. By using human and mouse as outgroup in maximum likelihood analyses, W-linked genes were found to evolve differently compared with their Z-linked gametologues and outgroup sequences. This seems not to be a consequence of functional diversification because d(N)/d(S) ratios between gametologous gene copies were consistently low. We conclude that deleterious mutations are accumulating at a high rate on the avian W chromosome, probably as a result of the lack of recombination in this female-specific chromosome.