Molecular evolution of the avian CHD1 genes on the Z and W sex chromosomes

Genes shared between the nonrecombining parts of the two types of sex chromosomes offer a potential means to study the molecular evolution of the same gene exposed to different genomic environments. We have analyzed the molecular evolution of the coding sequence of tho first pair. of genes found to be shared by the avian Z, (present in both sexes) and W (female-specific) sex chromosomes, CHD17 and CHD1W. We show here that these two genes evolve independently but are highly conserved at nucleotide as well as amino acid levels, thus not indicating a female-specific role of the CHD1W gene. From comparisons of sequence data from three avian lineages, the frequency of nonsynonymous substitutions (K-a) was round to be higher for CHD1W (1.55 per 100 sites) than for CHD1Z (0.81), while the opposite was found for synonymous substitutions (K-a, 13.5 vs. 22.7). We argue that the lower effective population size and the absence of recombination on the MT chromosome will generally imply that nonsynonymous substitutions accumulate faster on this chromosome than on the Z chromosome. The same should be true for the Y chromosome relative to the X chromosome in XY systems. Our data are compatible with a male-biased mutation rate, manifested by the faster rate of neutral evolution (synonymous substitutions) on the Z chromosome th;ln on the female-specific W chromosome.