Homologous Sex Determination across Octoploid Strawberry Species via a Conserved Gene with a Shifting Genomic Location

It is often unclear whether sex-determining mechanisms are conserved across species or have evolved independently. The wild octoploid strawberries Fragaria chiloensis, F. virginiana ssp. virginiana, and F. virginiana ssp. platypetala all exhibit distinct sexes with homomorphic ZW (female heterogametic) inheritance, yet the sex-determining locus maps to different regions of each genome. We sequenced the complete genomes of 9 females and 8-10 males/hermaphrodites in each of these three taxa. We identified 31-mers that were unique to females and assembled reads containing them into contigs. Remarkably, a short (~10kb) W-specific sequence is observed in all 27 females of all species and never in male-fertile plants. This sequence includes a putatively retroposed gene which may be the causal locus. We also observe this sequence in females of the related decaploid F. cascadensis and the putative octoploid hybrid F. × ananassa ssp. cuneifolia. However, it is only present in 1 of 3 females of F. virginiana ssp. glauca. Comparing the W-specific haplotypes across taxa reveals their history of translocations, largely but not perfectly concordant with a phylogeny of maternally-inherited chloroplast DNA. Sex determination is homologous across these Fragaria higher-order polyploids, and its rapidly changing genomic position may reflect sexual antagonism or escape from linked deleterious mutations.