A single domestication for maize shown by multilocus microsatellite genotyping

Abstract

There exists extraordinary morphological and genetic diversity among the maize landraces that have been developed by pre-Columbian cultivators. To explain this high level of diversity in maize, several authors have proposed that maize landraces were the products of multiple independent domestications from their wild relative (teosinte). We present phylogenetic analyses based on 264 individual plants, each genotyped at 99 microsatellites, that challenge the multiple-origins hypothesis. Instead, our results indicate that all maize arose from a single domestication in southern Mexico about 9,000 years ago. Our analyses also indicate that the oldest surviving maize types are those of the Mexican highlands with maize spreading from this region over the Americas along two major paths. Our phylogenetic work is consistent with a model based on the archaeological record suggesting that maize diversified in the highlands of Mexico before spreading to the lowlands. We also found only modest evidence for postdomestication gene flow from teosinte into maize.

Figure 1

Geographic distribution of maize and teosinte used in this study. Core Andean maize characterized by hand-grenade-shaped ears (22 samples), other South American maize (47), Guatemalan and southern Mexican maize (31), Caribbean maize (6), lowland western and northern Mexican maize (15), highland Mexican maize (20), eastern and central U.S. maize (24), southwestern U.S. maize (22), northern Mexican maize (6), ssp. parviglumis (34), and ssp. mexicana (33). Inset shows the distribution of the 34 populations of ssp. parviglumis in southern Mexico with the populations that are basal to maize in Fig. 2 (represented as asterisks). The blue line is the Balsas River and its major tributaries.

Figure 2

Phylogenies of maize and teosinte rooted with ssp. huehuetenangensis based on 99 microsatellites. Dashed gray line circumscribes the monophyletic maize lineage. Asterisks identify those populations of ssp. parviglumis basal to maize, all of which are from the central Balsas River drainage. (a) Individual plant tree based on 193 maize and 71 teosinte. (b) Tree based on 95 ecogeographically defined groups. The numbers on the branches indicate the number of times a clade appeared among 1,000 bootstrap samples. Only bootstrap values greater than 900 are shown. The arrow indicates the position of Oaxacan highland maize that is basal to all of the other maize.

Figure 3

Graph of the first two axes from a principal component analysis of 193 maize and 71 teosinte individual plants. The first component explains 3.5% and the second 2.6% of the total variation.