Tuesday, January 13, 2015

Vegetable Corn Domestication Evolves in the Span of Ten Millennia Producing the Modern Varieties We See Today



""The United States is a country of corn. But 4000 years ago, the crop was an import, spreading up from Mexico and taking hold in the American Southwest. New research retraces the route the crop may have traveled and reconstructs the genetic changes it went through on the way to becoming modern maize.

The wild ancestor of maize is teosinte, a species of grass that grows in warm, damp environments of central Mexico, where it was first domesticated sometime between 6000 and 10,000 years ago. About 4000 years ago, maize started showing up in what is now the southwestern United States. Scientists have two main ideas about how it made the trip: It might have spread up the Pacific coast of Mexico, or it might have moved first into the cooler, drier highlands of central Mexico before eventually being planted in the American Southwest.

The wild ancestor of maize is teosinte, a species of grass that grows in warm, damp environments of central Mexico, where it was first domesticated sometime between 6000 and 10,000 years ago. About 4000 years ago, maize started showing up in what is now the southwestern United States. Scientists have two main ideas about how it made the trip: It might have spread up the Pacific coast of Mexico, or it might have moved first into the cooler, drier highlands of central Mexico before eventually being planted in the American Southwest.




To help figure out which route maize traveled, M. Thomas Gilbert, who studies paleogenomics at the University of Copenhagen, and colleagues compared the DNA of 32 ancient corn cobs, some from Mexico and some from the U.S. Southwest. They discovered that the oldest maize samples from the southwestern United States—dating to more than 3000 years ago—are more genetically similar to maize from Mexico’s highlands than to varieties that grew along the coast, the team reports today in Nature Plants.

The younger the samples get, however, the more genes they share with coastal varieties. That suggests that maize “initially went up the highland route,” Gilbert says. But the crop, which was still in its early stages of domestication, probably wasn’t thriving at high altitudes or in the dry American Southwest. As the centuries passed, Gilbert says, farmers “were crossing in varieties from elsewhere to keep improving it through time.”

Gilbert’s team was able to reconstruct the directed evolution of modern maize. The first trait the farmers bred out of the crop was shattering, a process by which wild plants scatter their seeds in all directions—a “very, very irritating feature” if you want to collect seeds to plant the next season, Gilbert says. Then, over the course of several centuries, the farmers bred drought-tolerant maize, which allowed the plant to thrive in the dry conditions of the American Southwest""




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