A microsatellite linkage map for the cultivated strawberry (Fragaria × ananassa) suggests extensive regions of homozygosity in the genome that may have resulted from breeding and selection

The linkage maps of the cultivated strawberry, Fragaria × ananassa (2n = 8x = 56) that have been reported to date have been developed predominantly from AFLPs, along with supplementation with transferrable microsatellite (SSR) markers. For the investigation of the inheritance of morphological characters in the cultivated strawberry and for the development of tools for marker-assisted breeding and selection, it is desirable to populate maps of the genome with an abundance of transferrable molecular markers such as microsatellites (SSRs) and gene-specific markers. Exploiting the recent release of the genome sequence of the diploid F. vesca, and the publication of an extensive number of polymorphic SSR markers for the genus Fragaria, we have extended the linkage map of the 'Redgauntlet' × 'Hapil' (RG × H) mapping population to include a further 330 loci, generated from 160 primer pairs, to create a linkage map for F. × ananassa containing 549 loci, 490 of which are transferrable SSR or gene-specific markers. The map covers 2140. 3 cM in the expected 28 linkage groups for an integrated map (where one group is composed of two separate male and female maps), which represents an estimated 91% of the cultivated strawberry genome. Despite the relative saturation of the linkage map on the majority of linkage groups, regions of apparent extensive homozygosity were identified in the genomes of 'Redgauntlet' and 'Hapil' which may be indicative of allele fixation during the breeding and selection of modern F. × ananassa cultivars. The genomes of the octoploid and diploid Fragaria are largely collinear, but through comparison of mapped markers on the RG × H linkage map to their positions on the genome sequence of F. vesca, a number of inversions were identified that may have occurred before the polyploidisation event that led to the evolution of the modern octoploid strawberry species