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Mexico: Traces of GM maize confirmed

Scientists at Mexico City University have detected transgenic DNA sequences in Mexican maize samples from the years 2001 to 2004, thereby confirming the results of a controversial study published in 2001. It is still not clear how genetically modified maize ended up on Mexican fields and whether the transgenes have become established in the gene pool of Mexican landraces. The Mexican government has now authorised experimental planting of genetically modified maize.

Mexico is regarded as the birthplace of maize. There are numerous landraces with great genetic diversity.

Key study findings
Study location: two of Mexico’s 31 states: Puebla and Oaxaca.

2001
Samples: Maize ears from household stocks in two localities in Puebla and 19 localities in Oaxaca. A total of 68 maize ears from 21 households.

Analysis: The maize grains were sown and DNA was extracted from the leaves of the young maize plants and analysed using PCR.

Result: Transgenic DNA sequences were found in 21 of the 1867 plants (1.1%). The maize grains from which these plants had grown came from three localities in Oaxaca.

2004
Samples: 300 leaves from each of 30 maize fields in two of the three localities that had tested positively in 2001.

Analysis: DNA was extracted from the leaves and analysed using PCR.

Result: Transgenic DNA sequences were detected in one locality on three out of 30 fields and in the second locality on eight out of 30 fields

At the end of 2001, Ignacio Chapela and David Quist of Berkeley University had published a study in Nature about traces of GMOs detected in Mexican maize varieties. The publication came under heavy criticism from several scientists because of methodological shortcomings. The main criticisms at the time do not apply to the new study by a working group led by Elena Alvarez-Buylla and published in Molecular Ecology. For instance, the results have been confirmed using a second method. High-profile ecologist Allison Snow, whose working group was unable to find any transgenic DNA sequences in maize samples from Mexico in 2003 and 2004, has said that the new study provides convincing proof that transgenes have crossed from genetically modified maize varieties into Mexican landraces.

The starting material for the new study (68 maize ears from 21 localities) consisted of samples that the National Biodiversity Council (CONABIO) and the National Ecology Institute (INE) had collected in 2001. Since 2002, representatives of these two Mexican authorities have claimed to have collected their own data that would confirm the findings of Chapela and Quist. However, the data has not been published in any scientific journal. It was known that Nature had rejected an article on the subject at the time. The authors of the latest study also had difficulties getting it published at first.

It remains unclear whether, and to what extent introgression has taken place, i.e. whether the transgenes have actually become established in the landrace gene pool. It would appear that native maize plants have crossed with genetically modified maize. However, it is not clear whether and to what extent the resultant plants – the F1 generation – are crossing with native plants and whether any resultant F2 generation plants will also cross with native plants, etc. These ‘back crosses’ are what would be required for the transgenes to become permanently established in the gene pool of native varieties This possibility is not assessed unanimously by scientists. Some are of the view that introgression of transgenes is to be expected only if they confer a selective advantage on the plants.

Environmental groups fear that the outcrossing of genetically modified maize varieties could lead to a reduction in genetic diversity and, in the worst case scenario, to the extinction of landraces. Introgression from modern high-yielding varieties does not necessarily represent a threat to landraces though, as is shown by a study from Italy that appears in the same issue of Molecular Ecology as the study by Elena Alvarez-Buylla’s team. Samples from Italian landraces of maize that had been conserved from the 1950s were examined using molecular markers.

The results were compared with those for landraces grown in Italy today and with the results for modern, non-GM hybrid varieties that have been grown in Italy since the 1950s. It was shown that genetic material from the modern hybrid varieties has ‘migrated’ into some landrace populations, but that at the same time the genetic diversity of the landraces has increased. Does it make a difference if the ‘migrant’ gene is a transgene? This question has yet to be answered.

Mexico brought in a ban on the cultivation of genetically modified maize in 1998 in order to protect its regional landraces. In March 2009, however, the law was changed to allow experimental planting. Commercial cultivation may be permitted in 2012. One reason for these decisions could be the temporary shortage of maize supply at the beginning of 2007, which led to protests by thousands of people in Mexico. At the time the Mexican farmers’ association called for the authorisation of GM maize to increase national maize production and reduce Mexico’s reliance on maize imports. The Mexican government’s decision has brought the conflict surrounding the cultivation of GM maize in Mexico to an end, at least temporarily.