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Mexican study confirms GM contamination of maize
Fred Pearce . The Scientist . 21 may 2002

Mexican researchers are set to publish evidence supporting the hotly disputed claims of GM contamination of the Mexican maize crop. | By Fred Pearce

LONDON Mexican biologists believe they are close to confirming the hotly disputed finding of American researchers that the country's traditional varieties of maize are extensively contaminated with transgenic DNA from genetically modified maize.

The Mexican government has banned GM maize from its fields since 1998, while allowing imported GM foodstuffs. Last month, Jorge Soberon, the secretary of the country's National Commission on Biodiversity (CONABIO), told a conference in The Hague that a new study had found massive contamination among landraces grown close to the country's highways. "It is confirmed. There is no doubt about it," he said. The Scientist understands that his findings have now been confirmed by two laboratories using four different analytical techniques. They are set for publication within a few weeks.

The findings are being greeted with special concern because the apparent genetic 'pollution' had occurred in a centre of major biodiversity for the wild relatives of maize. "This is the world's worst case of GM contamination," Soberon told journalists in The Hague, attending a meeting of parties to the Convention on Biological Diversity.

The controversy began last September when Nature first reported that Ignacio Chapela, a Mexican plant molecular biologist now working at the University of California at Berkeley, had found transgenic DNA in cobs of native maize landraces growing in a remote region of Oaxaca. The state, in the south of the country, is a heartland for wild relatives of maize. The preliminary findings had been leaked by government officials.

When Chapela's findings were formally published in Nature in November they caused a storm of criticism that some colleagues say amounts to an orchestrated campaign to denigrate the scientist. The attacks centred as much on his support for a 1999 call for a moratorium on GM crops and his opposition to a $50-million research deal struck between UC Berkeley and Novartis, a producer of GM crops, as on the science.

Last month, Nature published some of the criticisms, and a response from Chapela and his co-author David Quist, in which they stood by their findings and augmented them with further data. Editor Philip Campbell said "in the light of diverse advice received the evidence available is not sufficient to justify the publication of the original paper." He hoped readers would "judge the science for themselves".

There have been two key criticisms of the research. The first questions the reliability of the technique, known as inverse polymerase chain reaction (i-PCR), used by Chapela to identify transgenic DNA in traditional landraces. This method is known to carry the risk of generating false positives, and a review of the work in the journal Transgenic Research in February concluded there was "no credible scientific evidence to support claims that gene flow between transgenic maize and traditional maize landraces has taken place." But Chapela argues that the "consistent performance" of his controls, which did not yield false positives, discounts this as a major factor in the findings.

The second criticism is of the claim by Chapela that the transgenic DNA constructs appeared to have fragmented and been scattered through the countryside. This led Chapela to claim: "The probability is high that diversity is going to be crowded out by these genetic bullies." But Matthew Metz of the University of Washington in Seattle claimed in Nature that Chapela's data did not support this conclusion. "Transgenic corn may or may not be hybridized to traditional maize cultivars in Mexico," he wrote. "But so far there is no evidence of transgenes fragmenting and scattering throughout genomes." Chapela's response concedes that more work is needed on this.

The new Mexican research attempts to get round the criticisms of methodology by adopting four different genetic techniques for spotting the rogue genes, and sending the samples to two independent laboratories. PCR, ELISA and Southern blot tests all yielded positive results, Soberon told journalists. Of more than 1800 maize seedlings analysed, around 10% contained transgenic DNA. Contamination in individual fields ranged from 1 to 35%. The plants contained three transgenic sequences, including the cauliflower mosaic virus 35S promoter, which is widely added to GM crops to get new genes to express themselves.

Soberon also claims to have found a pattern in the contamination, with fields close to major roads much more likely to exhibit the signs. This suggests that the findings are not an artefact of the analysis techniques. It also supports the idea that the contamination has been caused by farmers planting GM maize imported from the US and sold to villagers as an ingredient for food.

Soberon hopes to have a paper ready for publication within a month, he told delegates in The Hague. But much may hang on the attitude of the Mexican government, whose farming and trading lobbies back GM.

Many plant scientists are not unduly surprised by the findings of contamination. After the initial report from Chapela, Guy Poppy of CropGen, an association backed by the British biotech industry, praised the report and said "it is better to acknowledge that a minimum of cross-pollination cannot be avoided and not to panic."

But as Soberon said in The Hague: "It is one thing to prove there are transgenic sequences in the landraces of the people and another to find out what the effects are going to be if any." Luis Herrera-Estrella, a plant scientist from the Center for Research and Advanced Studies in Irapuato, Mexico, said "There is no scientific basis for believing that out-crossing from biotech crops could endanger maize biodiversity."

But many activists are now demanding a moratorium on imports of GM products to centres of diversity for wild relatives of key crops. "We think the same threat exists for potatoes in South America, rice in Asia, wheat in the middle east and even rape in Europe," says Patrick Mulvany of the Intermediate Technology Development Group. "GM rape is being imported to Europe from Canada and it is only a matter of time before it escapes into the wild rape genome in northern Europe."

The new findings from the Mexican researchers make it seem increasingly certain that transgenic DNA has jumped from GM maize into traditional landraces in Mexico. What remains far less clear is how much that matters.

Links for this article
Guist D and Chapela IH: Transgenic DNA introgressed into traditional maize landraces in Oaxaca, Mexico. Nature 2001, 414:541-543. 

National Commission on Biodiversity 

Convention on Biological Diversity 

University of California at Berkeley 

Palevitz, BA: Corn goes pop, then kaboom. Scientist 2002, 16:18. 


Christou P: No credible scientific evidence is presented to support claims that transgenic DNA was introgressed into traditional maize landraces in Oaxaca, Mexico. Transgenic Res 2002, 11:3-5. 

University of Washington in Seattle 


Intermediate Technology Development Group