The other species, Oryza glaberrima, comes from Africa.
Carolina Gold is now considered an heirloom variety of rice that is no longer grown widely, and its species origins are unknown.
Historical accounts suggest that Carolina Gold first reached America on a cargo ship from Madagascar, indicating that the rice variety was picked up on the African island country.
But the variety could also have had Asian origins, picked up en route at an earlier port of call.
(Related news: "Stone Age Rice Fields Discovered in China Swamp" [September 26, 2007].)
To find Carolina Gold's origins, McClung and colleague Robert Fjellstrom examined the rice variety's DNA and found that it has a unique marker associated with a gene that controls starch content.
"It's not just like any other rice," McClung explained.
This marker is akin to a red flag, making it possible for the researchers to look for a similar red flag in the DNA of rice varieties from all over the world.
The pair searched for the marker in about 1,600 varieties randomly selected from more than 20,000 with known origins housed by the U.S. Department of Agriculture.
They found the marker in less than one percent of the samples.
To further narrow their search, the scientists hunted for varieties that were described to have other traits similar to Carolina Gold that came from plausible source regions: Indonesia, Madagascar, and West Africa.
They grew those varieties side-by-side with Carolina Gold to look for similar physical characteristics, for example a golden-hued husk. This narrowed the search down to 13 varieties.
Next, the team identified 43 additional genetic markers for Carolina Gold and searched for matches across the 13 varieties. One variety shared 42 of the markers.
"So, where did that [one] come from? According to our database in this library, it says—and this is where my heart went 'bumpedty-wow'—it says it came from West Africa," McClung said.
According to the U.S. Department of Agriculture's records, the Crops Research Institute of Ghana donated the variety, called Bankorum, to the U.S. collection in 1972.
The USDA records say the rice was derived from the domesticated African species Oryza glaberrima.
Judith Carney is a geographer at the University of California, Los Angeles, and author of the 2001 book Black Rice: The African Origins of Rice Cultivation in the Americas.
"If true, [the new finding] neatly fills the historical gap about the appearance of rice cultivation in South Carolina in tandem with the founding generations of enslaved African rice in the colony," she wrote in an email.
Carney has argued for an African origin for Carolina Gold in her own work, saying it arrived in the colony as leftover subsistence food for slaves.
Verifying the Find
McClung cautions that her research is preliminary and not yet published in a journal.
The possibility exists, for example, that Bankorum was brought to Ghana after the emergence of Carolina Gold from the U.S. and deposited in a seed bank there.
Edda Fields-Black studies early and precolonial African history at Carnegie Mellon University in Pittsburgh, Pennsylvania.
She noted that the history of Asian rice varieties in Africa probably goes back to the 1500s. So Carolina Gold could also have distant origins in Asia.
"We still need to know more," she said.
USDA's McClung is currently trying to contact the Crops Research Institute of Ghana to get the organization's history for Bankorum.
"There's just all this detective work that still has to be done," she said, "but it's very intriguing at this point."
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