The rate of W chromosome degeneration across multiple avian neo-sex chromosomes

When sex chromosomes evolve recombination suppression, the sex-limited chromosome (Y/W) commonly degenerate by losing functional genes. The rate of Y/W degeneration is believed to slow down over time as the most essential genes are maintained by purifying selection, but supporting data are scarce especially for ZW systems. Here, we study W degeneration in Sylvioidea songbirds where multiple autosomal translocations to the sex chromosomes, and multiple recombination suppression events causing separate evolutionary strata, have occurred during the last ~ 28.1-4.5 million years (Myr). We show that the translocated regions have maintained 68.3-97.7% of their original gene content, compared to only 4.2% on the much older ancestral W chromosome. By mapping W gene losses onto a dated phylogeny, we estimate an average gene loss rate of 1.0% per Myr, with only moderate variation between four independent lineages. Consistent with previous studies, evolutionarily constrained and haploinsufficient genes were preferentially maintained on W. However, the gene loss rate did not show any consistent association with strata age or with the number of W genes at strata formation. Our study provides a unique account on the pace of W gene loss and reinforces the significance of purifying selection in maintaining essential genes on sex chromosomes.