Evolution of sex chromosomes in Fragaria: insights from BAC sequences and phylogenomics

Fragaria includes 21 wild species with a diversity of sexual systems and ploidy levels. In particular the two North American octoploid strawberries (Fragaria virginiana, Fragaria chiloensis) and their natural hybrid Fragaria ×ananassa subsp. cuneifolia provide an excellent opportunity to understand the early stages in the evolution of sex chromosomes. Recent genetic linkage mapping using SSR markers in all three octoploid species have pinpointed linkage groups within homoeologous group VI as incipient sex chromosomes. Although plastome sequences suggest a common origin of the octoploids, the location of the HG VI sex determining region differs among the species. To characterize the sex determining region and uncover the unique features that accompany the earliest stages in the evolution of sex chromosomes, we constructed a bacterial artificial chromosome (BAC) library from genomic DNA of the female parent of our F. virginiana linkage map. The BAC libraries were probed using sex region-linked SSR markers on the existing map to selectively sequence a subset of BACs to identify additional markers to facilitate chromosome walking and fine mapping of the sex region. At present, we are genotyping a set of BAC facilitated markers to screen for sex segregation in a subset of our expanded F. virginiana mapping population. We will also use a phylogenomic approach with diploid and octoploid genome sequences to identify the ancestry of the homoeologs in the sex determining region of F. virginiana.