paper describing the research was published in
the June 20, 2006 issue of the
Proceedings of the U.S. National Academy of Sciences.
"We chose samples across the entire range of
rice and looked for DNA sequences that were shared by both wild
and domesticated types," said Londo. "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."
Schaal said that she was surprised and
"delighted" by their results. "People have moved rice around so
much and the crop crosses with its wild ancestors pretty
readily, so I was fully prepared to see no domestication signal
whatsoever,," Schaal said.
"I would have expected to see clustering of
the cultivated rice, but I was delighted to see geographical
clustering of the wild rice. I was thrilled that there was even
genetic structure in the wild rice."
In contrast to rice, other staple crops such
as wheat, barley and corn appears to have been domesticated just
once in history.
Rice is the largest staple crop for human
consumption, supplying 20 percent of caloric content for the
world.
By finding the geographic origins of rice,
researchers can consider ways to improve the crop's nutritional
value and disease resistance, which in turn can help
impoverished populations in Asia and elsewhere that rely heavily
on the crop.
A third type of rice might have originated
independently in India, but the researchers can't be certain,
said Londo, because "with two of the gene networks we see sharp
similarities, but with a third one that emerges from the data we
don't have enough resolution."
Londo expects to find even more evidence for
differing geographic domestication. He said that by using the
database that they've gathered, they could design a sampling to
target specialty rices such as the aromatic rices basmati and
jasmine.
For instance, one direction that the
researchers are going is Thailand, where the Karen tribe has
been using multiple landraces of rice for many hundreds of
years, Landraces are localized varieties of rice that have been
cultivated by traditional methods and have been passed down many
generations, Schaal said. "We're going to try to find out how
landrace varieties change after domestication. These landraces
are ancient varieties, which are high in genetic diversity, thus
valuable to breeders looking for new traits."
Jason
Londo, Washington University in Arts & Sciences biology doctoral
candidate, and his adviser, Barbara A. Schaal, Ph.D., (photo)
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). 