A genetic linkage map of tetraploid orchardgrass (Dactylis glomerata L.) and quantitative trait loci for heading date.

Orchardgrass (Dactylis glomerata L.), or cocksfoot, is indigenous to Eurasia and northern Africa, but has been naturalized on nearly every continent and is one of the top perennial forage grasses grown worldwide. To improve the understanding of genetic architecture of orchardgrass and provide a template for heading date candidate gene search in this species, the goals of the present study were to construct a tetraploid orchardgrass genetic linkage map and identify quantitative trait loci associated with heading date. A combination of SSR markers derived from an orchardgrass EST library and AFLP markers were used to genotype an F1 population of 284 individuals derived from a very late heading Dactylis glomerata subsp. himalayensis parent and an early to mid-heading Dactylis glomerata subsp. aschersoniana parent. Two parental maps were constructed with 28 cosegregation groups and seven consensus linkage groups each, and homologous linkage groups were tied together by 38 bridging markers. Linkage group lengths varied from 98 to 187 cM, with an average distance between markers of 5.5 cM. All but two mapped SSR markers had homologies to physically mapped rice (Oryza sativa L.) genes, and six of the seven orchardgrass linkage groups were assigned based on this putative synteny with rice. Quantitative trait loci were detected for heading date on linkage groups 2, 5, and 6 in both parental maps, explaining between 12% and 24% of the variation.