Origin and evolution of teosinte (Zea mexicana (Schrad.) Kuntze)

Summary

The tripartite hypothesis postulates that cultivated maize was derived from a wild podcorn which was once indigenous to the lowlands of South America; that this wild Z. mays is now extinct; that Z. mexicana (teosinte) originated from natural hybridization of Z. mays and a species of Tripsacum after cultivated maize was introduced into Central America; and that most modern races of maize resulted from introgression of primitive maize with teosinte, Tripsacum, or both. This hypothesis has been criticized primarily on the basis that it involves too many theories for the available facts.

The basic question that needs to be answered is whether maize was domesticated from a wild Z. mays or from Z. mexicana. The oldest known archaeological remains suggest that a wild Z. mays existed before Z. mexicana came on the scene. This question can only be settled by further archaeological research.

Accepting the assumption that a species of Zea which is distinctly different from Z. mexicana gave rise to maize, however, does not necessarily lead to accepting a hybrid origin for teosinte. Judging from present day breeding behaviour, Z. mays and Z. mexicina are conspecific and only distantly related to Tripsacum. This cytogenetic similarity could be explained on the basis of extensive introgression between teosinte and maize. Introgression of germplasm from teosinte into maize is obvious wherever these taxa are sympatric. This pattern of a crop absorbing genes of its wild progenitor across a weedy race, is commonly encountered in all other cereals. Should this not be true in maize, it certainly will be a rare exception.

The assumption that teosinte originated as a hybrid between domesticated Z. mays and a species of Tripsacum remains an intriguing possibility. However, after weighing the pros and cons of the available morphological and genetical evidence, the level of confidence with which this hypothesis can be excepted appears rather low.