Isozyme Variation and Phylogenetic Relationhips among Open-Pollinated Sweet Corn Cultivars

Sweet corn (Zea mays L.) is one of the most important vegetables in the USA. But, little is known regarding the phylogeny and diversity among open-pollinated sweet corn cultivars. To determine the relationship and diversity among sweet corn cultivars, isozyme variability among 60 North American cultivars and four Mexican and two Peruvian sugary1 (sul) populations were compared by principal components analysis (PCA) and cluster analysis (CA). Four major groups were identified: yellow-kernel sweet corn (‘Golden Bantam’ type), whitekernel sweet corn, Mexican and Amerindian sweet corns, and Peruvian populations. Two cultivars ‘Early June’ and ‘Country Gentleman’ did not fall into any group. The first two groups originated in the northeastern USA. To determine the relationship among sweet corn germplasm and the major North American races of corn, PCA and CA were performed on the isozyme data from the 66 cultivars and populations and mean allele frequencies of the races ‘Northern Flint’ (NF), ‘Southern Dent’ (SD) and ‘Corn Belt Dent’ (CBD). The mean frequencies from NF, SD, and CBD were obtained from previously published data. Northern Flint was grouped with the white and yellow kernel sweet corns, while SD and CBD were grouped with the Mexican and Amerindian cultivars. Fifty-three of the 60 North American sweet corn cultivars were grouped closely together and could be considered a discrete race, Northeastern Sweets. But, due to the commercial importance of Country Gentleman and the introgression of non-sweet germplasm into modern sweet corn, we believe that sweet corn should be defined based on its use as a vegetable and on the presence of one or more genes that increase sugar levels in the endosperm. Research supported by the College of Agric. and Life Sci. Univ. of Wisconsin-Madison and the Comisión Interministerial de Ciencia y Tecnología, Madrid, Spain.