Use of SSR Markers to Assess Identity, Pedigree, and Diversity of Cultivated Muscadine Grapes

The North American muscadine grape (Muscadinia rotundifolia Small) is a valuable source of resistance to powdery mildew (Uncinula necator (Schw.) Burr), root-knot nematode (Meloidogyne Goeldi), dagger nematode (Xiphinema index Thorne and Allen), grape phylloxera (Daktulosphaira vitifoliae Fitch), and Pierce's disease (Xylella fastidiosa Wells et al.). Efforts to breed muscadine grapes commenced in the early 1900s and have generated a large number of cultivars and a limited number of hybrids withVitis vinifera L. and otherVitis L. species. Collections of this germplasm are currently maintained with accession identity based on declared identity when collected, breeding records, and comparisons of morphological traits. This study reports on the first use of DNA-based simple sequence repeat (SSR) marker profiles to authenticate M. rotundifolia cultivars and hybrids. A total of 57 accessions (39 M. rotundifolia cultivars, 3 V. vinifera cultivars, 3 Vitis spp. hybrids, and 12 V. vinifera · M. rotundifolia (VR) hybrids) from collections at the U.S. Department of Agriculture National Clonal Germplasm Repository and the University of California (Davis) Department of Viticulture and Enology were analyzed with 14 SSR markers. The fingerprint profiles were used to verify published breeding records of 31 M. rotundifolia cultivars and hybrids by comparing the shared alleles of parents and progeny. Marker data indicated that four cultivars were incorrectly identified; their alleles did not match respective parent/progeny relationships at more than five loci. Two M. rotundifolia accessions had the same fingerprint profile as a third accession at all 14 markers, implicating a likely planting error. The M. rotundifolia cultivars exhibited 88 unique alleles that were not present in a database of more than 600 V. vinifera cultivars. Muscadine grape is genetically and morphologically distinct from species within the genus Vitis. The most obvious genetic difference between these two taxa is the number of somatic chromosomes: Muscadinia Small species have 40, and Vitis species have 38. Muscadinia species also differ from Vitis species in their seed, bark, tendril, and cluster morphology. There is disagreement as to whether the differences between Muscadinia and Vitis warrant generic status (Olmo, 1995) or whether these taxa are best considered subgenera or sections (Liberty Hyde Bailey Horitorium, 1976). Muscadinia species possess very strong resistance to grape pests and diseases (Olmo, 1986), which prompted efforts to cross Muscadinia rotundifolia with Vitis vinifera cultivars. AlthoughVitis species hybridize freely, Vitis · Muscadinia crosses are difficult, and hybrids are rare and normally sterile, with 39 chromosomes. Crop improvement within M. rotundifolia began with the selection of high-quality pistillate vines from within the native range (southeastern and south-central United States) of this dioecious species. Muscadine grapes were cultivated in the 16th century by European settlers, and they likely used high-quality forms selected by Native Americans before this date. 'Eden', 'Flowers', 'James', 'Memory', 'Mish', 'Scuppernong', and 'Thomas' are among the oldest cultivars and were all collected from the wild (Husmann and Dearing, 1913; Reimer and Detjen, 1914). Since then, there have been three primary objectives in muscadine breeding programs: 1) the develop- ment of hermaphroditic, self-fertile cultivars with enhanced