Male-driven evolution among eoaves? A test of the replicative division hypothesis in a heterogametic female (ZW) system

Because avian females are heterogametic, the reverse of mammals, avian sex chromosomes undergo significantly different patterns and numbers of DNA rep lications than do those in mammals. This makes the W (female-specific) and the Z chromosomes an excellent model system for the study of the replicativ e division hypothesis, which purports that DNA substitution rate is determi ned by the number of germline replications. The sex-specific chromosome in birds (the W) is predicted to change at the slowest rate of all avian chrom osomes because it undergoes the fewest rounds of replication per unit of ev olutionary time. Using published data on gametogenesis from a variety of so urces, we estimated the ratio of male-to-female germline replications (c) i n galliforms and anseriforms to be approximately 4.4. The value of c should predict the value of the ratio of male-to-female mutation rates (alpha(m)) if the replicative division hypothesis is true. Homologous DNA sequences i ncluding an intron and parts of two exons of the CHD gene were obtained fro m the W and the Z chromosomes in ostrich, sage grouse, canvasback duck, tun dra swan, and snow goose. The exons show significantly different nucleotide composition from the introns, and the W-linked exons show evidence of rela xed constraint. The Z-linked intron is diverging approximate to 3.1 times f aster than the W-linked intron. From this, alpha(m) was calculated to be ap proximately 4.1, with a confidence interval of 3.1 to 5,1. The data support the idea that the number of replicative divisions is a major determinant o f substitution rate in the Eoavian genome.