Thursday, January 15, 2015

Origin of Maize


Fig.1. Maize, known as corn in the United States, comes in many varieties. This colorful array started out as a teosinte plant in Mexico about 10,000 years ago (Creative Commons: Miguel Vieira, 2011)

Maize (Zea mays L.) is one of the most important cereal crops in the world. It originated from the wild grass teosinte, endemic to Mexico and Central America. The evolution from small, hard seeds to a full ear of corn has captivated scientists. Unlike other grains, which evolved slowly, maize appeared rather suddenly in the archaeological record around 10,000 years ago. Although the teosinte ancestor has been accepted, the path from a warmer and wetter Mexico into the arid U.S. Southwest remained controversial. A recent study, however, found how maize traveled and adapted so quickly to a vastly different environment.

The journey north

After domestication from teosinte in southern Mexico, maize spread through the Americas and north into modern day United States about 4,100 years ago. Maize’s journey from Mexico to the U.S. was long disputed as either a coastal route along the Gulf of California or a highland route through the mountains. This study, done by a team of researchers from UC Davis, University of Copenhagen, and the Smithsonian’s National Museum of Natural History, found a compromise between the two paths.

Maize took the highland route 4,100 years ago and returned again 2,100 years later along the coast into the United States. In addition to determining the movement of maize, these researchers also provided clues to how and when maize adapted to certain environmental attributes. Indeed, extreme aridity, cultivators’ diets, and temperature changes forced maize to adapt quickly.

 

A window to the past

In order to understand when, where, and how maize evolved, scientists analyzed the DNA of corn cobs dating back over 4,000 years. They examined genetic similarities between ancient samples from the U.S. Southwest and traditional maize varieties in Mexico. Multiple stratigraphic layers of the Tularosa cave in New Mexico allowed researchers to compare genetic data from samples in different time periods.

“These unique data allowed us to follow the changes occurring in individual genes through time,” said lead author Rute da Fonseca of the University of Copenhagen.

Most studies on plant and animal domestication identify key genetic changes. This research provided a much more nuanced approach to the history of maize and certainly went beyond the simple distinction of “wild” and “domesticated.” This paleogenomic approach utilized stored scientific collections of Mexican varieties, as well as archaeological material, to create a detailed understanding of maize’s evolution. These types of studies and collections allow us to prepare for how maize and other crops might adapt in a changing environment.



References:

da Fonseca, R.R., Smith, B.D., Wales, N., Cappellini, E., Skoglund, P., Fumagalli, M., Samaniego, J.A., Caroe, C., Avila-Arcos, M.C., Hufnagel, D.E., Korneliussen, T.S., Vieira, F.G., Jakobsson, M., Arriaza, B., Willerslev, E., Nielsen, R., Huffor, M.B., Albrechtsen, A., Ross-Ibarra, J., & Gilbert., M.T.P. (2015). The origin and evolution of maize in the Southwestern United States. Nature Plants, 1. ​doi:10.1038/nplants.2014.3

Genetic Science Learning Center (2014, June 22) The Evolution of Corn. Learn.Genetics. Retrieved January 14, 2015, from http://learn.genetics.utah.edu/content/selection/corn/

University of California, Davis. (2015) Ancient maize followed two paths into the Southwest [Press release]. Retrieved from http://www.eurekalert.org/pub_releases/2015-01/uoc--amf010715.php.

Zeder, M.A., Emshwiller, E., Smith, B.D., & Bradley, D.G. Documenting domestication: the intersection of genetics and archaeology.(2006). Trends Genet., 22 (3): 139-155.  doi:10.1016/j.tig.2006.01.007

Glossary

endemic
Restricted or native to a particular region
domestication
A process of increasing mutual dependence between human societies and the plant and animal populations they target (Zeder et al., 2006)
stratigraphy
A branch of geology which studies rock layers. Various rock layers can pinpoint changes in time and the environment.
paleogenomics
A discipline that maps the genetic code (genome) of organisms and that organism’s evolution over time, specifically using ancient DNA.

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