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Research published today (9 August) says that there is no evidence to support controversial claims made in 2001 that genetically modified (GM) maize had 'contaminated' local varieties of the crop in Mexico.

In 2001, Nature published research showing that genes from GM maize had entered wild maize in the Mexican state of Oaxaca despite the country not allowing GM maize to be grown at the time (see GM maize found 'contaminating' wild strains).

Although the journal later disowned the paper, its authors, David Quist and Ignacio Chapela of the University of California at Berkeley, stood by their claim that one per cent of wild maize cobs contained genes from GM crops (see Nature backtracks over GM maize controversy).

The following year, the Mexican government confirmed that genes from GM plants had indeed contaminated wild varieties (see Mexico confirms GM maize contamination).

But in the first peer-reviewed follow-up to Quist and Chapela's study, researchers say that they found no evidence of genes from GM maize in more than 150,000 seeds taken from 870 plants in Oaxaca in 2003 and 2004.

The authors, led by Allison Snow of Ohio State University, United States, sampled seeds from 125 fields in Oaxaca.

"We conclude that transgenic maize seeds were absent or extremely rare in the sampled field," they write in today's online edition of the Proceedings of the National Academy of Sciences.

One of Snow's co-authors is Exequiel Ezcurra, of Mexico's Environment and Natural Resources Secretariat. In 2002, Ezcurra told the Mexican newspaper La Reforma that "genetic contamination of wild Mexican varieties is taking place".

At the time it was thought that GM maize imported from the United States and planted in Mexico without authorisation was the source of the genes.

Fears arose that this 'contamination' would threaten the genetic diversity of wild maize varieties, for which Mexico is the origin and centre of diversity.

Snow and colleagues (including Ezcurra) now write, however, that their results "suggest that many concerns about unwanted or unknown effects of this process can be discounted at present, at least within the sampled region".

They accept that GM genes might have been present in 2001 but say they might have since disappeared.

Chapela says he welcomes the research but says it raises more questions than it gives answers.

"It is very difficult to believe that the contamination we found in 2001 had gone by 2003-2004," he told SciDev.Net. "I don't believe that is something that happens in biology — ever."

Snow's team points out that "evidence that genes are rare or absent in the sampled area should not be extrapolated to other regions of Mexico without quantitative data, nor is the current situation likely to remain static".

Reference: Proceedings of the National Academy of Sciences doi:10.1073/pnas.0503356102 (2005)

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