A political row has erupted over a scientific paper by authors who claim to have found transgenic DNA from genetically modified (GM) maize in local varieties of the crop in Mexico.

Calls from environmental groups to halt the planting of transgenic crops in Mexico and elsewhere followed hot on the heels of the paper's publication in Nature last November (Nature 414, 541–543; 2001). But some researchers have since raised questions over the study's validity.

The paper was written by Ignacio Chapela and David Quist of the University of California, Berkeley. In it they report that a promoter sequence of DNA that originated in the cauliflower mosaic virus has shown up in creole maize varieties in two remote mountain areas of Mexico. The viral DNA is used in a variety of GM maize to enhance the activity of the introduced genes.

Their results led the authors to ask whether such gene flow would threaten native species of maize in Mexico, the centre of origin for the crop.

But an editorial in this month's Transgenic Research (11, 3–5; 2002) says that “the data presented in the published article are mere artifacts resulting from poor experimental design and practices”. The article was written on behalf of the journal's editorial board by the editor, Paul Christou, who is director of the molecular biotechnology unit at the John Innes Centre in Norwich, UK. It concludes that “no credible scientific evidence is presented in the paper to support claims made by the authors”.

Philip Campbell, editor of Nature, declines to discuss the matter in detail. “We treat all submissions as confidential and so are not willing to comment on specific cases,” he says. “Our policy in general is to consider criticisms received after publication as promptly as possible.”

But on 19 February, 144 non-governmental organizations, led by the Canada-based Action Group on Erosion, Technology and Concentration (ETC), issued a statement alleging that “pro-industry academics are engaging in a highly unethical mud-slinging campaign against the Berkeley researchers”.

The statement calls on the UN Food and Agriculture Organization and the Consultative Group on International Agricultural Research (CGIAR), which runs 16 agricultural research laboratories around the world, to “propose an immediate moratorium on the shipment of GM seed or grain in countries or regions that form part of the centre of origin or centre of genetic diversity for the species”.

Alex Avery, director of research at the Virginia-based Center for Global Food Issues, which supports agricultural biotechnology, dismisses the ETC's statement as “some last-minute damage control before the Quist and Chapela study is thoroughly refuted”. He claims that the ETC “doesn't care about the scientific debate — it is just trying to sway reporters into bolstering the credentials of Chapela and Quist”.

Both sides of the argument are hoping to influence imminent decisions about the regulation of transgenic crops — in particular, the fate of the existing European Union moratorium on their commercial use. The United States, which views the ban as protectionism, would like to see its removal discussed at the next meeting of European leaders in Barcelona on 15–16 March. But according to one official at the European Commission, a decision to lift the moratorium is unlikely until the end of the year at the earliest.

Some experts say that the debate on the Mexico findings is, in any case, somewhat beside the point. Because maize is wind-pollinated and varieties cross readily, almost everyone agrees that genes from GM maize will cross to local varieties if they are grown close together.

What really matters is the ecological impact of such gene flow. Local maize varieties are not very stable, and farmers have long crossbred them with other varieties. “Gene flow is a constant,” says Tim Reeves, director of the CGIAR International Maize and Wheat Improvement Center (CIMMYT) in Mexico. “The real question is whether it makes any difference if one of the genes that flows in is a transgene.”

Scientists are divided on that question. Some argue that the transgenes will reduce genetic diversity, whereas others contend that they could either have a neutral effect or actually enhance diversity.