High Density Mapping Provides Genomic Insights Into The Evolution Of Sex Determination In Strawberries

Gynodioecy, the coexistence of females and hermaphrodites, has been a crucial transition point for many taxa in the evolution of entirely separate sexes (dioecy) from a hermaphroditic ancestor. This key sexual system exists in the diploid wild strawberry Fragaria vesca ssp. bracteata (Rosaceae), a member of a clade with dioecious species, making this subspecies an ideal model in which to study sexual transitions. To identify the genetic mechanism for sex determination in F. v. bracteata, we crossed two outbred parents from the same population and genotyped 48 F1 offspring at thousands of informative polymorphisms. We first sequenced the complete genomes of the parents at 5x coverage and identified >6000 heterozygous sites. We then designed custom biotinylated RNA baits to target the 200bp surrounding these sites for sample enrichment and sequencing, yielding genotypes at 98% of targeted sites with mean coverage >100x, plus over 600kb high-coverage non-targeted sequence. With the resulting high-density linkage map, we show that male-sterility is controlled by a single locus where females are heterozygous and hermaphrodites are homozygous, consistent with past work in strawberries. Sex determination maps to a narrow (<1Mb) genomic region of a chromosome that is not syntenic with the sex regions in the closely related polyploid species F. chiloensis and F. virginiana, suggesting either independent origins or translocation. The candidate genes in the region suggest possible biochemical mechanisms and evolutionary origins of the suppression of male function.