Evolutionary dynamics of sex chromosomes of palaeognathous birds

Standard models of sex chromosome evolution propose that recombination suppression leads to the degeneration of the heterogametic chromosome, as is seen for the Y chromosome in mammals and the W chromosome in most birds. Unlike other birds, palaeognaths (ratites and tinamous) possess large non-degenerate regions on their sex chromosomes (PARs or pseudoautosomal regions), despite sharing the same sex determination region as neognaths (all other birds). It remains unclear why the large PARs of palaeognaths are retained over more than 100 MY of evolution, and the impact of these large PARs on sex chromosome evolution. To address this puzzle, we analysed Z chromosome evolution and gene expression across 12 palaeognaths, several of whose genomes have recently been sequenced.