Genetic relationships and evolution in Cucurbita as viewed with simple sequence repeat polymorphisms: the centrality of C. okeechobeensis

Genetic relationships among 88 accessions from nine of the dozen species of Cucurbita (Cucurbitaceae) were assessed from polymorphisms at 74 SSR (simple sequence repeat) loci originating from C. pepo and C. moschata, yielding a total of 315 alleles distributed among 17 linkage groups, with an average of 4.3 alleles per locus. Genetic distance (GD) values were calculated, a principal coordinate analysis conducted, and a dendrogram constructed. Average within-species genetic distance values ranged from 0.07 for C. ecuadorensis and C. ficifolia to 0.46 for C. pepo. Each species was clearly defined, as all mean within-species GD values were lower than the respective mean between-species GD values. C. okeechobeensis had the most central position in the genus Cucurbita, with the lowest average GD to the other species, 0.61. C. foetidissima, the only xerophytic species examined, was the most distant, with a mean GD of 0.73 to the other species. C. pepo and C. ficifolia were the most outlying of the mesophytic species. Mean across-species GDs generally corresponded with crossability. However, there were some outstandingly low GD values between particular accessions of Cucurbita pepo, the economically most important species, and disease-resistant wild species, particularly C. okeechobeensis but also C. foetidissima. The results suggest that more intensive search and collection of C. okeechobeensis populations would likely yield genotypes that are more compatible with C. pepo. Moreover, successful application of genetic resources in the genus Cucurbita might be facilitated by using GD values obtained from SSR polymorphisms as a guide in choosing parents for interspecific crossing.