Evolution of a plant sex chromosome driven by expanding pericentromeric recombination suppression

Recombination suppression around sex-determining gene(s) is a key step in evolution of sex chromosomes, but it is not well understood how it evolves. Recently evolved sex-linked regions offer an opportunity to understand the mechanisms of recombination cessation. This paper analyses such a region on Silene latifolia (Caryophyllaceae) sex chromosomes, where recombination was suppressed in the last 120 thousand years ("stratum 3"). Locating the boundaries of the stratum 3 in S. latifolia genome sequence revealed that this region is far larger than assumed previously-it is about 14 Mb long and includes 202 annotated genes. A gradient of X:Y divergence detected in the stratum 3, with divergence increasing proximally, indicates gradual recombination cessation, possibly caused by expansion of pericentromeric recombination suppression (PRS) into the pseudoautosomal region. Expansion of PRS was also the likely cause for the formation of the older stratum 2 on S. latifolia sex chromosomes. The role of PRS in sex chromosome evolution has been underappreciated, but it may be a significant factor, especially in the species with large chromosomes where PRS is often extensive.