Contrasting Patterns Of Selection Define The Carica Papaya X and Y Chromosomes

The sex chromosomes of the tropical crop papaya (Carica papaya) are young and consequently allow for the examination of evolutionary mechanisms that shape early sex chromosome formation.  Previously, we found striking differences in the patterns of polymorphism and divergence of the papaya X and Y chromosome compared to other young sex chromosome systems.  Specifically, from a survey of four X/Y gene pairs from regions that span the sex-determining region, we found evidence of selective sweeps driving reduced diversity on the X while the Y has significantly elevated polymorphism.  We have expanded our study to include over 20 X/Y gene pairs spanning ~250 kb intervals across the gene enriched regions of the X and Y.  We also have increase our survey of autosomal diversity to include over 30 loci, with a particular focus on pericentromeric loci, as the papaya sex determining region spans a centromere. We present our analysis of polymorphism and divergence in these loci and show that the X chromosome is defined by large footprints of selective sweeps, while the Y is defined by extended regions of elevated polymorphism punctuated by genes with little or no polymorphism.   We suggest that the broad distribution of these patterns across the X and Y are affected by their unique pericentromeric location and the low recombination rates that characterize this region.