Mexico: Outcrossing of genetically modified maize:

A threat to biological diversity?

In November 2001 the renowned scientific journal Nature published a short article: two Californian scientists had been studying maize from a remote region of Mexico. They claimed to have found DNA from genetically modified maize in their samples. The article triggered a heated debate amongst scientists and the general public alike.

Forefather of maize. Maize is descended from the wild grass teosinte.

Maize diversity. Maize varieties belonging to a farmer from central Mexico. - Mexico is the centre of biological diversity for maize.

(Photographs: Hugh Iltis)

Background: Mexico as the centre of biological diversity for maizeMexico is a special region for maize

Using hybridisation and selection, the indigenous people of Mexico developed the precursors of modern maize varieties from the grass teosinte over thousands of years. As Mexican farmers mostly still use traditional breeding methods today, there are countless local varieties with a vast genetic diversity. Mexico is a centre of biological diversity for maize.

One of the objectives of the Convention on Biological Diversity signed in Rio in 1992 is to protect and preserve centres such as this. Signatories pledge to avoid environmental effects associated with genetically modified organisms which could harm the preservation and sustainable use of biological diversity.

There has been a moratorium on the cultivation of genetically modified maize in Mexico since 1998. The importation of agricultural products, however, is allowed. Every year, several million tonnes of maize are imported from the USA, where in 2001 approximately eight million hectares of genetically modified maize was grown.

Foreign genes in native varieties. How do they get there?

Ignazio Chapela, an ecologist at the University of California at Berkeley, and his PhD student David Quist studied six maize samples which they had collected in Oaxaca, a mountainous region in southern Mexico.

Chapela and Quist found gene sequences typically used in transgenic plants in four of the six samples. This genetic information occurs for instance in insect- and herbicide-resistant maize varieties such as those grown in the USA.

Assuming that Chapela und Quist’s findings are true, how can genes from genetically modified maize be found in local varieties in the remote region of Oaxaca? The cultivation of transgenic maize is not permitted in Mexico, so that can be ruled out as an explanation.

There are two possible hypotheses:

  • Transgenic pollen was carried by wind and air streams from the USA to Mexico, where it crossed into native varieties.
    Opinions vary as to how far maize pollen can be transported by the wind. Although theoretical calculations suggest distances of several hundred kilometres per day, in practice however, a study conducted in the USA found no incrossings at just 300 metres from a field of transgenic maize.
    Maize pollen is also comparatively heavy and only viable for approximately 24 hours. This makes the pollen drift theory seem fairly unlikely.
  • Mexican farmers have used genetically modified maize imported from the USA for crop-breeding.
    Seed produced by farmers in Mexico could therefore contain genes from GMmaize. It is conceivable that outcrossings to local varieties could arise from maize plants grown from this seed.

Incrossing: scientifically proven or junk science?

Irrespective of the search for possible causes, many scientists have expressed considerable doubts about the findings of Chapela and Quist. Their flawed investigative methods have been particularly criticised.

  • Back in March 2001, for example, Dr. Urs Pauli from the Swiss Federal Office of Public Health had already pointed out that the data was insufficiently supported by control studies.
  • The International Maize and Wheat Improvement Centre in Mexico (CIMMYT) made several references to studies of samples from its own gene bank: foreign genes were not found either in older seed bank samples or in those collected from the Oaxaca region in 2000.
  • In August 2002 scientists from the USA and Canada stated that the test probes (primers) used by Chapela and Quist to detect the foreign DNA were unsuitable for clearly distinguishing between natural maize sequences and transgenic DNA.
  • Other studies commissioned by the National Institute of Ecology (INE) in Mexico confirm Chapela and Quist’s findings. But these studies have yet to be published in a recognised scientific journal. The president of the INE reported that Nature magazine had rejected the article on conflicting grounds, just as they had done with similar studies from other Mexican scientists.

A threat to biodiversity?

Did Chapela and Quist actually provide evidence for the encroachment of transgenic DNA into local varieties - or was it shoddy science (junk science)? It is now a seasoned debate, which is almost impossible to keep track of and riddled with suspicions on both sides. The Commission for Environmental Corporation of North America (CEC) has made a fresh attempt to clarify the situation: in June 2002 it commissioned a working group to examine the possible gene transfer from genetic maize varieties to Mexican native breeds and teosinte. It is also looking at the economic and health implications and the impact on biodiversity. The final report is expected to be finished by mid 2004.

For many scientists there is no question that outcrossings of transgenes to local maize varieties are possible in principle. More detailed studies of the consequences are more important than pursuing the argument.

Can the new genes become permanently established in the genetic pool of regional native breeds? Do the transgenes confer a selection advantageon the plants compared with other members of the same species? Does this threaten the biodiversity, the vast genetic variety of Mexican native species? What consequences might this have?

Gene flow, the exchange of genes between cultivated and wild varieties, is a natural process. Genes from conventional cultivated varieties migrate to native breeds and wild populations. It is not just outcrossings between plants in neighbouring fields that are involved in this process – traditional methods of crop breeding also play a part. In Mexico in particular farmers tend to breed their local maize varieties.

Consequently genes from different cultivated and native breeds get mixed up. The genetic makeup of Mexican native breeds has always been in constant flux.

According to the International Maize and Wheat Improvement Centre in Mexico (CIMMYT), incrossings from cultivated varieties do not on their own cause a decline in the diversity of maize species in Mexico. Mexico was not a region of biodiversity for maize simply because many varieties had been “found” there.

The diversity was rather a consequence of the crop-breeding skills of farmers, who set great store by growing different varieties. The varieties are subject to a continuous process of change. Whether genes from conventional high-performance strains or transgenic strains can become permanently established in native breeds and thereby decrease the biodiversity ultimately depends on whether they confer a selection advantage on the progeny. It is the genes rather than the breeding technique which determine whether there is a potential threat to biodiversity.

‘Contamination’ of the seed bank?

Those opposed to genetic engineering see things differently. They regard every incrossing of transgenes as harmful and a threat to biodiversity. The Non-Governmental Organisations Committee (NGOC) of the Consultative Group on International Agricultural Research (CGIAR) stated for example that the CGIAR and its centres had failed to act against the ‘genetic contamination’ of the Mexican centre of origin for maize biodiversity. The CGIAR is a major amalgamation of public and private donors which finances sixteen agricultural research centres, including the CIMMYT. According to the NGOC, the CIMMYT was called upon to verify the risks of contamination, but chose instead to await further scientific evidence.

The CIMMYT, on the other hand, maintains that extensive safety precautions have been taken. Since last year the gene banks set up to safeguard the genetic diversity of Mexican native breeds have only accepted tested seed containing no transgenic sequences. When propagating these seeds, crossing with unknown maize varieties in the environment is prevented in the field by time-consuming hand fertilisation, safety margins and plant cordons.