"This is very serious because the region where our samples were taken are known for their diverse varieties of native corn, which is something that absolutely needs to be protected," said Ignacio Chapela, assistant professor of microbial ecology in the Department of Environmental Science, Policy & Management at UC Berkeley's College of Natural Resources.

In the study, published Thursday (Nov. 29) in the journal Nature, Chapela and David Quist, lead author and UC Berkeley graduate student in environmental science, policy and management, compared indigenous corn with samples known to be free from genetic engineering as well as with genetically modified varieties.

The native corn, or "criollo," samples were taken from four fields in the remote, mountainous region of Sierra Norte de Oaxaca. Control samples that had not been genetically modified came from blue maize grown in the Cuzco Valley in Peru, and also from a collection of seeds from the Sierra Norte de Oaxaca region taken in 1971, before the advent of transgenic crops.

Using highly sensitive polymerase chain reaction (PCR)-based tests, the researchers checked for various elements of transgenic DNA constructs used when bioengineered genes are introduced into a plant genome.

They found no signs of transgenic DNA in the Peru and 1971 seed collection. In the criollo samples, however, four out of six samples tested showed weak but clear evidence of p-35S, a promoter from the cauliflower mosaic virus widely used in transgenic crops. When they sequenced the DNA of the transgenic-positive criollo samples, the researchers found that the CMV promoter matched those used in commercial transgenic crops.

The presence of the nopaline synthase terminator sequence (T-NOS) from Agrobacterium tumefasciens, another telltale sign of transgenic contamination, was detected in two of the six criollo samples tested. One criollo sample tested positive for the actual cry-1A gene of Bacillus thuringiensis (Bt), the insecticidal bacterium that kills pests feeding on corn.

"I repeated the tests at least three times to make sure I wasn't getting false-positives," said Quist. "It was initially hard to believe that corn in such a remote region would have tested positive."

Chapela and Quist said the contamination likely came from multiple pollinations over time. They were able to identify the DNA fragments flanking the CMV promoter sequence through inverse PCR tests. Those fragments were diverse, suggesting a random insertion of the transgenic sequence into the maize genome.

"If this contamination was the result of a single gene transfer event, we would expect to find the transgenic DNA in a consistent location on the criollo genome," said Quist. "Instead, we're finding it at different points along the genome."

The researchers first detected the transgenic DNA in October 2000 while working with the Mycological Facility in Oaxaca, a locally-run biological laboratory where Chapela serves as the scientific director.

Soon after the initial discovery of the transgenic contamination, Chapela alerted the Mexican government, which then proceeded to conduct its own tests. Reporting the results in a September press release, Mexico's Ministry of the Environment and Natural Resources found transgenic DNA in three to 10 percent of the Sierra Norte de Oaxaca maize, supporting the results of the UC Berkeley researchers.

Just how the contamination occurred remains a puzzle. Agricultural experts and proponents of biotech crops maintain that corn pollen is characteristically heavy, so it doesn't blow far from corn fields by the wind. Chapela said this assumption may need to be reevaluated in light of the recent findings in Mexico.

In addition, Mexico imposed a moratorium in 1998 on new plantings of transgenic maize. The closest region where bioengineered corn was ever known to have been planted is 60 miles away from the Sierra Norte de Oaxaca fields, said Chapela.

"It's not clear if the moratorium was poorly enforced, or if the contamination occurred before the moratorium was enacted," said Chapela. While new plantings are banned in Mexico, it is still legal to import biotech corn into the country. "Whatever the source, it's clear that genes are somehow moving from bioengineered corn to native corn," he said.

Such a prospect is almost certain to fuel the already contentious debate over the use of genetically modified crops. Proponents of transgenic agriculture say biotechnology helps to increase crop yields for feeding a rapidly growing world population, improve the food's nutritional value and reduce the use of pesticides.

Opponents say not enough is known about the health and ecological effects of biotech crops and that the risks outweigh the benefits.

To date, more than 30 million hectares of transgenic crops have been grown, according to "Transgenic Plants and World Agriculture," a white paper published in 2000 by a group of seven national science academies around the world, including the U.S. National Academy of Sciences and the Royal Society of London.

Genes from genetically modified crops that spread unintentionally can threaten the diversity of natural crops by crowding out native plants, said Chapela. A wealth of maize varieties has been cultivated over thousands of years in the Sierra Norte de Oaxaca region, providing an invaluable "bank account" of genetic diversity, he said. Chapela added that genetically diverse crops are less vulnerable to disease, pest outbreaks and climatic changes.