Isoenzymatic Variation in Zea (Gramineae)

An average of 14(7-34) plants each for 61 different collections of Zea (maize and its wild relatives, the teosintes) were studied for 12 enzyme systems coded for by 21 loci. Principal component and cluster analyses based on allele frequencies showed Zea can be divided into two major groups: 1) sect. Luxuriantes, including Z. perennis, Z. diploperennis, and Z. luxurians; and 2) sect. Zea (in part), including Z. mays subsp. mays, var. parviglumis, and subsp. mexicana. Zea mays var. huehuetenangensis (Huehuetenango teosinte) is isoenzymatically distinct from both sections, but shows its closest relationship to Z. mays var. parviglumis of sect. Zea. Populations of Z. mays subsp. mexicana and var. parviglumis grade isoenzymatically from one into the other without any clear break, but without any overlap either. Five populations of Z. mays subsp. mays are all isoenzymatically very similar to Z. mays var. parviglumis. The isozyme data are consistent with the theory that Mexican annual teosinte is the ancestor of maize. With the exception of the unusual isoenzymatic nature of Z. mays var. huehuetenangensis, the isoenzymatic data agree with previous studies of tassel morphology and cytoplasm DNAs. A comparison of sympatric populations of maize and teosinte suggests that teosintes are not greatly affected by introgression from maize. Zea mays var. parviglumis and Z. diploperennis have considerable within and among population variation; Z. luxurians has much less. Race Central Plateau of Z. mays subsp. mexicana has greater among population variation than any other taxon in Zea. Zea tends to show greater within population heterozygosity and more variance within species and subspecies than most other plants that have been studied isoenzymatically. In the research reported below, enzyme electrophoresis was employed to assess phylogenetic relationships, measure the degree of genetic variation, and determine the extent of gene exchange among populations of Zea. This study builds upon three previous studies of isozyme variation in Zea (Senadhira 1976; Mastenbroek et al. 1981; Smith et al. 1984), but differs from previous work by including a substantially larger sample of the perennial members of the genus and a larger number of loci. Also included are some recently discovered annual populations from Jalisco and Durango, Mexico, that were not previously surveyed. One purpose of this paper is to examine taxonomic relationships among the populations sampled. These populations occur over an extensive geographic region from Honduras to Chihuahua, Mexico. They are adapted to a broad range of environments, ranging from the seasonally arid savanna of the Mexican Central Plateau to the mesic montane forest of western Guatemala. They include both annuals and perennials, and, except for a few of the perennial populations, all are diploids. These populations have been treated taxonomically in two different fashions (table 1) by Wilkes (1967) and by Iltis and Doebley (1980). Wilkes recognized two sections: sect. Zea for the cultigen maize (Z. mays) and sect. Euchlaena for the wild taxa, commonly called teosintes. He recognized two wild species: 1) Z. mexicana, a widespread, diploid (n = 10) annual, and 2) Z. perennis, a tetraploid perennial found only in Jalisco, Mexico. The emphasis in Wilkes's classification, however, was on his division of Z. mexicana into six allopatric races. In agreement with previous authors (Collins 1931; Longley 1941a, b), Wilkes noted that his race Guatemala of southeastern Guatemala is distinct from all other teosintes. He considered race Huehuetenango of western Guatemala to be related to the Mexican annual teosintes. Wilkes also stated that races Chalco, Central Plateau, and Nobogame of central and northern Mexico are maize-introgressed forms of race Balsas of southwestern Mexico. Finally,