Parallel divergence and degradation of the avian W sex chromosome

Sex chromosomes are ubiquitous in birds but our understanding of how they originated and evolved has remained incomplete. Recent work by Tsuda et A on tinamou and ratite birds suggests that, although all bird sex chromosomes evolved from the same pair of autosomes, the Z and W sex chromosomes have diverged from one another several times independently. This parallel evolution of the avian W presents a means for comparison in studies of sex chromosome evolution, which could help us understand more about the general forces that shape the development of all types of sex chromosome.