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

Recombining regions of sex chromosomes, pseudoautosomal regions (PARs), are essential for proper segregation of the sex chromosome pair during meiosis. Yet, the evolutionary dynamics of PARs remain poorly understood, particularly in relation to chromosomal translocation and fusion events. Using comparative analyses of enlarged neo-sex chromosomes across Sylvioidea songbirds, we show that the ancestrally conserved songbird PAR has independently ceased recombining multiple times, each coinciding with the emergence of novel PAR in newly sex-linked regions. These PAR transitions were associated with lineage-specific fusions, but were driven by subsequent recombination suppression events rather than the fusions themselves. Our data further support hypotheses predicting that recombination suppression expansions are preceded by sex-associated sequence divergence near the PAR boundary. Together, our results reveal remarkable PAR diversity among birds and demonstrate how chromosome fusions relax constraints on existing PARs, facilitating recombination suppression and enabling PAR transitions along evolving sex chromosomes.