Genetic diversity of North American popcorn and its relationship with Mexican and South American popcorns
To determine the genetic diversity and relationships among representative popcorns of the New World, 56 accessions coming from the United States and nine Latin American countries, were characterized for 29 morphological traits, 18 isozyme loci and 31 simple sequence repeat (SSR) loci. Means, ranges, standard deviations and within-plot variances were estimated from morphological data, whereas, for molecular markers, expected heterozygosity, number of alleles per locus, proportion of polymorphic loci and F-statistics were estimated to determine genetic diversity and population structure. Principal component, cluster and discriminant analyses were performed using standardized morphological data and allelic frequencies from isozyme and SSR loci, to elucidate potential patterns of grouping among accessions, according to their geographical and historical relationships. Results of the three lines of evidence indicate that popcorn accessions from the United States have reduced genetic diversity in comparison with their counterparts of Latin America; however, genetic variability in the North American popcorns is not uniformly distributed, being the Yellow, pearl-type group of popcorns the least genetically diverse, followed by the Pointed and Early groups of popcorn. Three groups of popcorn, with distinctive morphological characteristics and genetic profiles, were identified in the North American populations. The first group includes the yellow, pearl-type varieties, which are currently the most important for commercial production. This group could be derived from introductions of the race Curagua from Chile into New England in the 19th century. The second group includes the pointed, rice-type varieties, which probably originated from the complex of traditional races of pointed popcorns from Latin America, such as Palomero Toluqueno, Confite Puntiagudo, Canguil and Pisankalla, which diffused from the highlands of central Mexico into northern Mexico and then into southwestern United States. The third group includes the early, short-statured varieties, which have marked influence of Northern Flint maize, from which they probably acquired earliness, and probably they also have influence from maize of northwestern Mexico and even from early European varieties of popcorn introduced in the 19th century. It is proposed that the groups of popcorn identified in this study can be equated to the category of races.