The peculiar properties of pericentromeric regions of genomes and their influence on sex chromosome evolution

Results from recent genome sequences support the idea that the lack of recombination across extensive sex-linked regions (SLRs) in some species may be a pre-existing feature, due mainly to the sex-determining locus being located near a centromere. This would cause it to be affected by the “centromere-effect”, which ensures low recombination rates near centromeres. In such cases, it is unnecessary to invoke any active recombination suppression process, or to debate ideas about the types of selection involved. In this review, I outline the properties of genome regions that recombine rarely, including pericentromeric regions of chromosomes, and discuss SLRs that overlap with pericentromeric regions. Unsurprisingly, it is difficult to distinguish special features of such SLRs that would suggest they have undergone a recombination suppression process. The property most likely to be specific to completely SLRs is major gene loss from Y- or W-linked regions of genes found in their respective X- or Z-linked counterparts, and this asymmetry is observed in many species. However, if pericentromeric regions are characterised by ephemeral gene content and low gene expression, asymmetry between pericentromeric SLRs might be less pronounced than in SLRs in other genome regions, and similar Y and X degeneration levels are indeed documented in several species. Additionally, some such SLRs may not be completely sex-linked, which could explain findings of extensive SLRs with low Y-X (or W-Z) divergence.