Joost van Heerwarden and co-workers1 have solved a problem in our understanding of maize domestication. Previous work had shown that maize originated from Balsas teosinte, Zea mays subspecies parviglumis, a wild species that occurs in low and mid-elevation regions of south-west Mexico2. This made the Rio Balsas area, where parviglumis occurs, the most likely area of maize domestication. This was corroborated by Piperno et al.‘s3 discovery of 8,700 years old maize remains in that area; the oldest evidence of maize unearthed to date.
The problem was that the maize land races genetically most similar to parviglumis are not found there. They occur in the Mexican highlands. And that’s awkward, particularly because highland maize has a rather different set of ecological adaptations than lowland maize.
Van Heerwaarden et al. say this is a paradox caused by the role of another wild species: Zea mays subspecies mexicana. This species occurs in the highlands, and it is inter-fertile with cultivated maize. The tricky thing is that because the two wild species, parviglumis and mexicana, both referred to as teosinte, are closely related, more closely to each other than to their cultivated cousin, geneflow from mexicana makes the genes of highland maize look more like those of parviglumis!
This means that you cannot directly identify the most ancestral maize populations from genetic similarity with their putative ancestor. Instead, Van Heerwaarden et al. estimated ancestral gene frequencies from cultivated maize populations, without direct reference to the wild species. And, Bingo! Western lowland populations are indeed more ancestral than the highland populations. Maize did originate in the lowlands, and from there it spread to the highlands and to other parts of the Americas.Footnotes:
- van Heerwaarden J, Doebley J, Briggs WH, Glaubitz JC, Goodman MM, de Jesus Sanchez Gonzalez J, & Ross-Ibarra J (2010). Genetic signals of origin, spread, and introgression in a large sample of maize landraces. Proceedings of the National Academy of Sciences of the United States of America PMID: 21189301 [↩]
- Matsuoka Y, Vigouroux Y, Goodman MM, Sanchez G J, Buckler E, & Doebley J (2002). A single domestication for maize shown by multilocus microsatellite genotyping. Proceedings of the National Academy of Sciences of the United States of America, 99 (9), 6080-4 PMID: 11983901 [↩]
- Piperno DR, Ranere AJ, Holst I, Iriarte J, & Dickau R (2009). Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico. Proceedings of the National Academy of Sciences of the United States of America, 106 (13), 5019-24 PMID: 19307570 [↩]