Revisiting the Origin of the Octoploid Strawberry

The cultivated strawberry, Fragaria ×ananassa , originated in France approximately 270 years ago via hybridization between two wild species introduced from North and South America. Both the cultivated strawberry and its parental species are octoploids with 2 n = 8x =56 chromosomes. In the recent publication of the genome of the cultivated strawberry, the authors present a novel phylogenetic hypothesis, proposing that each of the four subgenomes originated from a different 2 n =2 x =14 diploid progenitor. They further suggest that the hexaploid species Fragaria moschata was a direct ancestor of the strawberries. We reanalyzed the four octoploid subgenomes in a phylogenomic context, and found that only two extant diploids were progenitors, a result that is consistent with several previous studies. We also conducted a phylogenetic analysis of genetic linkage-mapped loci in the hexaploid F. moschata , and resolved its origin as independent of the octoploids. We identified assumptions in their tree-searching algorithm that prevented it from accepting extinct or unsampled progenitors, and we argue that this is a critical weakness of their approach. Correctly identifying their diploid progenitors is important for understanding and predicting the responses of polyploid plants to climate change and associated environmental stress.