Biologists from Washington University in St. Louis and their collaborators from Taiwan have examined the DNA sequence family trees of rice varieties and have determined that the crop was domesticated independently at least twice in various Asian locales.
Jason Londo, Washington University in Arts & Sciences biology doctoral candidate, and his adviser, Barbara A. Schaal, Ph.D., Washington University Spencer T. Olin Professor of Biology in Arts & Sciences, ran genetic tests of more than 300 types of rice, including both wild and domesticated, and found genetic markers that reveal the two major rice types grown today were first grown by humans in India and Myanmar and Thailand (Oryza sativa indica) and in areas in southern China (Oryza sativa japonica).
A paper describing the research was published June 9, 2006, in the on-line issue of the Proceedings of the U.S. National Academy of Science.
“We look where the genetic signature of clusters on a haplotype tree (family tree),” explained Londo. “We chose samples across the entire range of rice and looked for DNA sequences that were shared by both wild and domesticated types. These two major groups clustered out by geography.”
DNA is comprised of vast, varied combinations of chemical subunits known as base pairs. Londo, Schaal and their collaborators concentrated on finding genetic markers shared by both cultivated and wild rice types that ranged from 800 to 1,300 base pairs.
Cultivated rice has a genetic signature that defines it as cultivated, Schaal explained.
“What you do is go out and sample all the wild rice across regions and you look for that signature in the wild,” said Schaal, who has done similar work with cassava and jocote, a tropical fruit. “You find that the unique signature of cultivated rice is only found in certain geographic regions. And that’s how you make the determination of where it came from.”
Great site for a more in depth picture.
So far, the oldest domesticated rice grains have been found in Korea, at about 15,000 years old. The earliest found in China appear to be about 13,000 years old.