Origin matters: Lessons from the search for the wild ancestor of maize

The wild ancestry of maize has long been a puzzle. Maize shows extraordinary phenotypic and genetic diversity with no obvious morphological similarity to any of its wild relatives. Beadle's teosinte hypothesis, which regards teosinte as the sole wild ancestor of maize, has become widely accepted as the most probable model of maize evolution based on taxonomic, genetic, and other types of evidence. Molecular studies have refined the teosinte hypothesis by identifying a particular form of teosinte, Zea mays ssp. parviglumis, as the direct ancestor of maize. Maize and teosinte therefore provide a typical case in which modern molecular genetic analyses, in the absence of obvious morphological similarity, have been critical for distinguishing the exact ancestor of a crop from other close wild relatives. Furthermore, a recent microsatellite-based study shows that all extant pre-Columbian maize landraces arose from Z. mays ssp. parviglumis roughly 9,000 years ago through a single domestication event in the central Balsas River drainage (southern Mexico). That model, showing that maize is a domesticated form of Z mays ssp. parviglumis, provides 9 logical, practical framework for investigations of the genetic mechanisms that drove maize domestication and diversification. This points up the importance of knowing exact origins in studies that use crops and their wild relatives as models. An overview of the progress in genetic and evolutionary studies is presented herein to clarify the identity of the wild progenitor of maize. Implications of recent findings on the origin of maize diversity are discussed.