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.