Homoploid Hybridization Resolves the Origin of Octoploid Strawberries

Abstract

The identity of the diploid progenitors of octoploid cultivated strawberry (Fragaria × ananassa) has been subject to much debate. Past work identified four subgenomes and consistent evidence for F. californica (previously named F. vesca subsp. bracteata) and F. iinumae as donors for subgenomes A and B, respectively, with conflicting results for the origins of subgenomes C and D. Here, reticulate phylogeny and admixture analysis support hybridization between F. viridis and F. vesca in the ancestry of subgenome A, and between F. nipponica and F. iinumae in the ancestry of subgenome B. Using an LTR-age-distribution-based approach, we estimate that the octoploid and its intermediate hexaploid and tetraploid ancestors emerged approximately 0.8, 2, and 3 million years ago, respectively. These results provide an explanation for previous reports of F. viridis and F. nipponica as donors of the C and D subgenomes and unify conflicting hypotheses about the evolutionary origin of octoploid Fragaria.