The study by the trilateral Commission for Environmental Cooperation said gene transfers could damage Mexico's vast storehouse of native corn, whose wild ancestral genes might one day be needed to help commercial crops overcome diseases or adverse conditions.
The report, presented at a corn symposium in the colonial city of Oaxaca, is still in draft form and must be approved during a commission meeting in June.
It does not provide data on the prevalence of genetically modified corn in the Mexican countryside, but Amanda Galvez, head of the Mexican government's interagency group on biosafety and genetically modified organisms, said a federally sponsored study had confirmed instances of massive gene transfer.
In 1998, Mexico declared a moratorium on genetically modified corn, making it illegal to grow anywhere outside licensed laboratories. Still, in a study of 188 corn-growing communities across Oaxaca state, Galvez said that 7.6 percent of plants tested positive for genetic modification in 2001.
Galvez said officials warned farmers about the possibility that they were planting genetically modified seeds, helping to reduce the number of plants testing positive to 0.11 percent in later studies. But the rate of unaffected plants will never drop to zero, she said.
"We can try to reduce the penetration of these plants, but we can't go back and stop their spread now," said Galvez.
Aldo Gonzalez, head of a group representing subsistence farmers, complained the findings were incomplete.
"I would like to ask if the amount of genetically modified plants really has dropped or if the lower amounts detected simply mean the scientific community can't detect transgenic effects in second-generation corn," said Gonzalez, who addressed the Environmental Cooperation symposium with a dead stalk of corn by his side.
The commission report said that gene transfer so far has been "insignificant from a biological point of view." It also says, however, that the uncontrolled spread of genetically modified corn could one day make it impossible to find corn not manipulated by science.
"We don't know to what extent these genetically modified planets could just take over and cause other species of corn to die off," said Chantal Line Carpentier, the report's coordinator. "But that possibility is out there."
Gene migration is a hotly debated subject. Some scientists say it has not yet been proven to occur in corn. Others maintain that negative characteristics caused by gene splicing will cause modified plants to die before they can reproduce, and they say that any positive effects would help native species survive.
Much U.S. corn is altered to produce a naturally occurring toxin known as Bacillus Thuringiensis, or Bt, to ward off pests. It was that Bt-producing gene that was found in the Mexican government study, Galvez said.
Farmers in Mexico first bred modern corn some 6,000 to 8,000 years ago. The country is home to at least 59 species of maize, from the protein-rich variety used to make tortilla chips to softer grain mashed for use in tamales.
Corn was born when farmers began crossbreeding teosinte, a plant with a jumble of sharp, dark-green leaves that look like corn stalks but grow out instead of up. Teosinte is still found in Mexican fields, but is considered a weed.
Today, due in part to the North American Free Trade Agreement, 70 percent of Mexico's corn — some 5 million tons a year — is imported from the United States. Between 30 and 50 percent of that is genetically modified. While much of the imported corn is intended for use as animal feed, some was planted — and spread its pollen.
"The government aid program to food-depressed areas is the most likely culprit for disseminating genetically modified maize," said the report, which will be about 400 pages upon completion.
Olga Toro's corn fields in the mountains of northern Oaxaca, was the first place in Mexico where scientists detected genetically modified corn growing in the wild.
The 2001 study about alleged contamination in Toro's five-acre (two-hectare) plot was published in the journal Nature. But the magazine later noted there was evidence that the researchers had not proven conclusively that contamination had occurred.
Toro said she unknowingly planted modified seeds she received from a local food bank. The results were plants that shot up to heights two and three times their normal size, produced double the amount of corn and grew with next to no water. But she won't plant them again.
"They modified the genes and we got plants that lasted only one season," said the 43-year-old mother of six. "Regular plants last longer without help from anyone."
By Will Weissert