Contrasting levels of nucleotide diversity on the avian Z and W sex chromosomes

Sex chromosomes may provide a context for studying the local effects, of mu tation rate on molecular evolution, since the two types of sex chromosomes are generally exposed to different mutational environments in male and fema le germ lines. Importantly, recent studies of some vertebrates have provide d evidence for a higher mutation rate among males than among females. Thus, in birds, the Z chromosome, which spends two thirds of its time in the mal e germ line, is exposed to more mutations than the female-specific W chromo some. We show here that levels of nucleotide diversity are drastically high er on the avian Z chromosome than in paralogous sequences on the W chromoso me. In fact, no intraspecific polymorphism whatsoever was seen in about 3.4 kb of CHDIW intron sequence from a total of > 150 W chromosome copies of s even different bird species. In contrast, the amount of genetic variability in paralogous sequences on the Z chromosome was significant, with an avera ge pairwise nucleotide diversity (delta) of 0.0020 between CHDIZ introns an d with 37 segregating sites in a total of 3.8 kb of Z sequence. The contras ting levels of genetic variability on the avian sex chromosomes are thus in a direction predicted from a male-biased mutation rate. However, although a low gene number, as well as some other factors, argues against background selection and/or selective sweeps shaping the genetic variability of the a vian W chromosome, we cannot completely exclude selection as a contributor to the low levels of variation on the W chromosome.