Evolutionary dynamics of enlarged sex chromosomes and novel pseudoautosomal regions in Sylvioidea songbirds

Sex chromosomes often contain recombining pseudoautosomal regions (PARs). Uncovering the dynamics of PAR is crucial for understanding sex chromosome evolution. We analyse PAR dynamics in Sylvioidea songbird neo-sex chromosomes formed by multiple sex chromosome–autosome fusions. We show that the otherwise highly conserved ancestral songbird PAR has stopped recombining independently multiple times when novel PARs formed in the new sex-linked regions. Such PAR transitions were associated with PAR-end fusions, but resulted from subsequent recombination suppression events rather than the fusions themself. We provide support for the hypothesis that episodes of recombination suppression are preceded by sex-associated sequence divergence close to the PAR boundary, and suggest that PAR-end fusions have released the ancestral PAR from constraints in evolving recombination suppression and allowed PAR transitions along the neo-sex chromosomes. This study reveals exceptional PAR diversity among birds and provides novel insights into the process of recombination suppression.