Assessment of genetic relationships in Cucurbita pepo (Cucurbitaceae) using DNA markers

Cucurbita pepo (pumpkin, squash, gourd), an economically important species of the Cucurbitaceae, is extremely variable in fruit characteristics. The objective of the present study was to clarify genetic relationships across a broad spectrum of the C. pepo gene pool, with emphasis on domesticates, using AFLP, ISSR and SSR markers. Forty-five accessions were compared for presence or absence of 448 AFLP, 147 ISSR, and 20 SSR bands, their genetic distances (GDs) were estimated and UPGMA cluster analysis was conducted. The results obtained from these three marker systems were highly correlated (P << 0.001). Clustering was in accordance with the division of C. pepo into three subspecies, fraterna, texana and pepo, with the first two less distant to one another than to the last one. Within the clusters, sub-clustering occurred in accordance with fruit shape and size. The subsp. texana cluster consisted of six sub-clusters, one each for the representatives of its five cultivar-groups (Acorn, Crookneck, Scallop, Straightneck and Ovifera Gourd) and wild gourds. Within the subsp. pepo cluster, the representatives of two cultivar-groups (Zucchini and Orange Gourd) formed distinct sub-clusters and the representatives of two other groups (Cocozelle and Vegetable Marrow) tended to sub-cluster separately from one another but formed an assemblage with the representatives of the remaining group (Pumpkin). Within-group GDs were less than corresponding between-group GDs in nearly all comparisons. The smallest-fruited accession, 'Miniature Ball', appears to occupy a genetically central position within C. pepo.