Genes from genetically-engineered corn have been found in traditional crop strains in Mexico, according to a new study likely to reignite a bitter controversy over biotech maize.
The paper, by scientists from Mexico, the United States and the Netherlands, backs a 2001 probe that sparked a row over the safety of genetically-modified (GM) crops.
Green activists say GM crops are a potential hazard, arguing that their genes could spread to related plants through cross-pollination.
Their campaign has helped drive bans on GM crops in some countries, including Mexico itself, the ancestral home of maize, as corn is also called.
In the 2001 study, published in the prestigious British journal Nature, researchers reported finding transgenes in samples of corn taken from the Sierra Juarez region of Oaxaca.
But this study was blasted for technical inaccuracy and choice of samples. In an exceptional slap, Nature distanced itself from the paper, saying the evidence had not been strong enough to warrant publication.
This damning verdict was underscored by a further study, carried out in 2005 by a different team, that was unable to replicate the results.
But new research now says the original study was right.
A team led by Elena Alvarez-Buylla of the National Autonomous University in Mexico City looked at nearly 2,000 samples from 100 fields in the region from 2001 and 2004, and found that around one percent of the samples had genes that had jumped from GM varieties.
"We confirmed that there was contamination in 2001 and also found contamination in 2004, which means that it either persisted in the local maize that we sampled or that it was reintroduced, which is less likely," Alvarez-Buylla told AFP.
She said the difference between previous studies and her research lay in the samples chosen for gene sequencing and in the molecular technique for decrypting the DNA.
The investigators looked for two specific genes that had escaped from biotech corn, and found them in some fields but not in others.
Alvarez-Buylla said the evidence shed stark light on the failure of efforts to shield Mexico from unauthorised GM corn.
The country imposed a moratorium on the planting of transgenic maize in 1998 in order to protect genetic diversity. It is the home of about 60 traditional domesticated strains, also called landraces, as well as several wild strains.
Transgenic seeds are entering the country, most probably from the United States, and getting mixed with local seeds in trade among small farmers, Alvarez-Buylla believed.
"It is very hard to avoid gene flow from transgenic maize to non-transgenic maize in Mexico, even though there has been a moratorium," she said.
"It is really worrying that the government of Mexico has not been efficient enough in biosecurity monitoring," she said, accusing watchdogs of failing to establish rigorous molecular monitoring that was independent of data provided by biotech giants.
Alvarez-Buylla's team did not explore the impact of the escaped genes on the native corn, on the local environment or human health, nor did it test whether the foreign genes passed on to progeny plants.
The study appears in the latest issue of Molecular Ecology, a peer-reviewed journal published by Britain's Blackwell group. It has been endorsed by a lead author of the 2005 paper.
GM crops have had genes inserted into them to produce benefits for farmers. For instance, they exude natural toxins that kill off pests, or are resistant to herbicides, enabling a farmer to spray a field in one go and not kill the crop.
GM producers say there is no evidence of any threat to human health or the environment. The overwhelming view of scientists is that, so far, this is true.
But suspicions remain strong in many countries, especially Europe, where several governments retain safeguard measures against GM corn despite EU-wide approval.
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