Abstract
Sex chromosomes play a prominent role in development and evolution and have several characteristic features that distinguish them from autosomes. Across diverse taxa, recombination is typically suppressed at the sex-determining region (SDR) and proportionally elevated in the remainder of the chromosome or pseudoautosomal region (PAR). However, in most model taxa the sex chromosomes are ancient and highly differentiated from autosomes, and thus little is known about recombination dynamics of homomorphic sex chromosomes with incipient sex-determining mechanisms. Here we examine male function (pollen production) and female function (fruit production) in crosses of the dioecious octoploid strawberry Fragaria chiloensis in order to map the small and recently evolved SDR controlling both traits and to examine recombination patterns on the young ZW chromosome. The SDR occurs in a narrow 280kb window, in which the maternal recombination rate is lower than in the orthologous paternal region and the genome-wide average rate, but within the range of autosomal rate variation. In contrast to the SDR, the ZW recombination rate in the PAR is much higher than the rates of the ZZ or autosomal linkage groups, substantially overcompensating for the SDR rate. By extensively sequencing sections of the SDR vicinity in several crosses and unrelated plants, we show that W-specific divergence is elevated within a portion of the SDR and find only a single SNP to be in high linkage disequilibrium with sex, suggesting that any W-specific haplotype protected from recombination is not large. We hypothesize that selection for recombination suppression within the small SDR may be weak, but that fluctuating sex ratios could favor elevated recombination in the PAR to remove deleterious mutation on the W. Thus these results illuminate the recombination dynamics of a nascent sex chromosome with a modestly diverged SDR, which could be typical of other dioecious plants.
Author Summary In many species, the sex-determining chromosomes have distinct features relative to the rest of the genome, including unusual recombination rates. The wild strawberry Fragaria chiloensis possesses a recently evolved sex chromosome that illuminates the early stages of sex chromosome differentiation and adjustment of recombination rates. We examined the phenotypes of both male function (pollen production) and female function (fruit production) among hundreds of strawberry offspring resulting from several crosses, which allowed us to identify a narrow chromosomal region influencing both of these traits. We then examined and compared maternal and paternal recombination rates in the vicinity of this sex-determining region and all along the ZW sex chromosome. A region of suppressed recombination surrounding the sex gene(s) is relatively small, but substantial differences in recombination rate among the parents are seen on the sex chromosome outside of this region. These results help us to understand how sex-determining regions appear, are maintained, and impact the rest of the genome.
