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 chrom osomes offer a potential means to study the molecular evolution of the same gene exposed to different genomic environments. We have analyzed the molec ular 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) s ex chromosomes, CHD17 and CHD1W. We show here that these two genes evolve i ndependently but are highly conserved at nucleotide as well as amino acid l evels, thus not indicating a female-specific role of the CHD1W gene. From c omparisons of sequence data from three avian lineages, the frequency of non synonymous substitutions (K-a) was round to be higher for CHD1W (1.55 per 1 00 sites) than for CHD1Z (0.81), while the opposite was found for synonymou s substitutions (K-a, 13.5 vs. 22.7). We argue that the lower effective pop ulation size and the absence of recombination on the MT chromosome will gen erally imply that nonsynonymous substitutions accumulate faster on this chr omosome than on the Z chromosome. The same should be true for the Y chromos ome relative to the X chromosome in XY systems. Our data are compatible wit h a male-biased mutation rate, manifested by the faster rate of neutral evo lution (synonymous substitutions) on the Z chromosome th;ln on the female-s pecific W chromosome.