An SSR and AFLP molecular marker-based genetic map of white clover (Trifolium repens L.)

A framework genetic map of white clover (Trifolium repens L.) has been constructed using an F2 progeny set derived from the intercross of fourth and fifth generation inbred genotypes carrying a self-fertile mutation (Sf). White clover SSR (TRSSR) and AFLP markers were used to derive a map with 135 markers (78 TRSSR loci and 57 AFLP loci) assigned to 18 linkage groups (LGs) with a total map length of 825 cm. Sixteen of these LGs are presumed to correspond to the 16 chromosomes of the white clover karyotype. A limited number of multiple loci were detected. Substantial segregation distortion was observed for both mapped and unmapped loci, with a bias towards heterozygous types and a preponderance of distorted markers on certain LGs. This observation, along with a high degree of residual heterozygosity within the inbred parental genotypes, suggests that reduced individual fitness due to loss of heterosis is a major effect for white clover and will limit the applicability of F2 or backcross mapping strategies for this species. A core set of map-assigned co-dominant, single locus SSR markers has been defined for whole genome scans of genetic variability in white clover.