ISOZYME VARIATION IN THE RACES OF MAIZE FROM MEXICO

Twelve plants each of 94 collections of maize (Zea mays L. subsp. mays) representing 34 races from Mexico were analyzed for 13 enzyme systems encoded by 23 loci. This analysis revealed an exceptionally high level of variation within and among the races. We recorded an average of 7.09 alleles/locus and an expected heterozygosity of 0.182. Seventy-two percent of the isozyme variation resided within collections, and 27% among collections. Races from northern and northwestern Mexico tend to possess higher levels of variation than those from the south. Variation for some isozyme alleles is strongly correlated with altitude. Maize is among the most variable species that have been studied isoenzymatically. Maize has levels of variation comparable to those found in its wild relatives, the teosintes. Principal component and cluster analyses showed continuous variation among the races with no well-defined racial complexes; however, three weakly differentiated groups were apparent: 1) the high-elevation Mexican pyramidal races, 2) the northern and northwestern races, and 3) most remaining races including the southern and western low-elevation dent and flour corns. of Mexico from the desert northwest to the tropical lowlands of Chiapas. Over this ecogeographic landscape, maize has diversified into a multitude of forms, each adapted to the demands of its environment and the desires of its cultivators. Because the morphological-ecological variation among these forms is continuous in nature, the classification of Mexican maize into discrete categories is difficult. Despite these difficulties, Wellhausen, Roberts and Hernandez (1952) produced a useful morphological classification of Mexican maize including the description of 29 races and subraces. This work and the more recent description of five additional Mexican maize races by Hernandez and Alanis (1970) provide an understanding of the racial diversity in Mexican maize. In this paper, we examine the pattern of isozyme variation among and within the races of maize of Mexico. The isozyme data are used to examine relationships among the races and to assess the amount of genetic variation within each race. Further, the correlations between isozyme variation and ecological-geographical parameters are discussed and the amount of genetic variation in Mexican maize, teosinte, and other plants compared. MATERIA1S AND METHODSWe examined 12 plants each from 93 collections of maize from Mexico and 1 collection from Guatemala (TaMAIZE (Zea mays L. subsp. mays) is the third most important cereal worldwide and the single most important food plant in Latin America. In Mexico and other Latin American countnes, maize has a central role in the native cultures, being displayed with prominence and reverence in both religious ceremonies and secular festivals (Anderson, 1952; Mangelsdorf 1974). The cultural importance of maize in Mexican society testifies to its long history of cultivation in this country. The oldest known archaeological maize ears were discovered near Puebla, Mexico, and were dated to the fifth millenium B.C. (MacNeish, 1967). This and other evidence have established Mexico as the cradle of maize domestication (Mangelsdorf, 1974). Not surprisingly, Mexico is also the homeland of the only close relative and probable wild ancestor of maize, teosinte (Wilkes, 1967; Doebley, 1983). Maize is cultivated in nearly every district ' Received for publication 1 1 June 1984; revision accepted 2 October 1984. Paper No. 9615 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC. This investigation was supported, in part, by a grant from Pioneer Hi-Bred International of Johnston, Iowa, and by NIH Research Grant No. GM11546 from the National Institute of General Medical Sciences of the U.S.A. We wish to thank Drs. Peter Bretting, Ted Emigh, Paul Sisco, David Timothy, and Jon Wendel for providing helpful criticisms on this paper.