Widespread Recombination Suppression Facilitates Plant Sex Chromosome Evolution

1 Summary Classical models suggest recombination rates on sex chromosomes evolve in a stepwise manner to localize the inheritance of sexually antagonistic variation in the sex where it is beneficial, thereby lowering rates of recombination between X and Y chromosomes. However, it is also possible that sex chromosome formation occurs in regions with pre-existing recombination suppression. To evaluate these possibilities, we constructed linkage maps and a chromosome-scale genome assembly for the dioecious plant Rumex hastatulus , a species with a young neo-sex chromosome found in part of its geographical range. We found that the ancestral sex-linked region is located in a large region characterized by low recombination. Furthermore, comparison between the recombination landscape of the neo-sex chromosome and its autosomal homologue indicates that low recombination rates preceded sex linkage. Our findings suggest that ancestrally low rates of recombination have facilitated the formation and evolution of heteromorphic sex chromosomes.