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January 14, 2002
BERKELEY, Calif.--Some varieties of corn native to rural Oaxaca,Mexico, have been found to contain genetically modified (GM) DNA, according toresearchers at the University of California (UC) here, who reported theirfindings in the Nov. 29 issue of Nature (414:541-43, 2001) (www.nature.com).The biotech DNA was discovered as researchers compared indigenous corn withsamples of GM-free and GM corn varieties.
"This is very serious," said Ignacio Chapela, assistant professorof microbial ecology in UC Berkeley's department of environmental science,policy and management. "Because the region where our samples were taken isknown for its diverse varieties of native corn."
Mexico imposed a moratorium in 1998 on new plantings of transgenic maize. Theclosest region where bioengineered corn was ever known to have been planted is60 miles away from the Sierra Norte de Oaxaca fields, according to Chapela.
The Mexican corn samples, called criollo, were taken from fields inSierra Norte de Oaxaca. Non-GM control samples were obtained from blue maizegrown in the Cuzco Valley of Peru and from a collection of seeds from the SierraNorte de Oaxaca region taken in 1971. GM-positive control samples--obtained fromleftover stock of the U.S. 2000 planting season--included bulk grain samples ofYieldgard Bacillus thuringiensis (Bt)-maize (Bt1; Monsanto Corp.) andRoundup-Ready maize (RR1; Monsanto Corp.). Scientists studied all samples usingpolymerase chain reaction (PCR)-tests, checking for elements of transgenic DNAconstructs that are used when bioengineered genes are introduced into a plantgenome.
Results indicated that the Peruvian corn and the 1971 corn seeds were clearof transgenic DNA. However, in four out of six samples of the criollocorn, clear evidence of p-35S (a promoter from the cauliflower mosaic virusoften used in GE crops) was found. When scientists sequenced the DNA of thetransgenic-positive plants, they found similarities to commercial transgeniccrops.
In addition, two of the six criollo samples tested positive for thenopaline synthase terminator sequence (T-NOS) from Agrobacterium tumefasciens,which is another sign of transgenic contamination. An additional criollosample also tested positive for the cry-1A gene of Bacillus thuringeiensis (Bt),the insecticidal bacterium that kills pests feeding on corn.
"I repeated the tests at least three times to make sure I wasn't gettingfalse-positives," said David Quist, lead author and UC Berkeley graduatestudent in environmental science, policy and management. "It was initiallyhard to believe that corn in such a remote region would have testedpositive."
Chapela and Quist hypothesized that the contamination came from multiplepollinations over a period of time. They identified the DNA fragments flankingthe CMV promoter sequence through inverse PCR tests. Those fragments werediverse, which suggested random contamination as opposed to intentional geneticmodification.
"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 criollogenome," Quist said. "Instead, we're finding it at different pointsalong the genome."
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