Transgenic DNA in Mexican Corn Landraces


The Quist and Chapela paper received widespread media attention, and prompted calls for further restrictions on GE crops. The Institute for Food and Development Policy (Food First) issued a joint statement calling for numerous actions by international organizations (, and some Mexican groups and Greenpeace have requested a halt to exports of GE corn to Mexico ( In addition, the paper provoked intense scientific scrutiny and detailed critiques of the study's methods and interpretations (Christou, 2002; Kaplinsky et al., 2002; Metz and Futterer, 2002). The main elements of the criticism are as follows:

1) The methodology, results, and interpretation of the iPCR experiments were flawed.

  • (a) The PCR primers used to hybridize with CaMV 35S DNA were similar to DNA sequences found in conventional maize, resulting in amplification of DNA segments that were not associated with transgenes.
  • (b) None of the amplified flanking sequences contains an obvious transgene component, as would be expected. The alleged detection of adh1 sequences was an incorrect conclusion; the sequence in question appears to be a repetitive element known to occur in conventional maize, rather than part of the adh1 intron used in some transgenic maize.
  • (c) Negative controls (maize DNA from samples known to be GE-free) were not included in this part of the study.

2) The assertion of transgenes scattering throughout the genome is unprecedented, and the authors incorrectly interpreted conclusions of a previous study to support their results. According to co-author W. Pawlowski, as quoted in the letter by Metz and Futterer, the cited study (Pawlowski and Somers, 1998), claims that transgenes can insert into multiple locations during transformation, not that they move around after initial integration into the genome.

3) Confirmation of the PCR results with other molecular techniques was not carried out. Although PCR is a standard technique for DNA analysis, results are subject to error due to contamination or subtle changes in experimental conditions. Therefore, verification with another molecular technique, such as a Southern blot, should have been done.

In a rebuttal to the letters in Nature by Kaplinsky et al. and Metz and Futterer, Quist and Chapela (2002) acknowledged the possible misinterpretation of some of their iPCR results. However, they stood by their original conclusions, including the assertion that the transgene is reassorting in the genome. They included new data to support the presence of the CaMV 35S promoter, based on DNA-DNA dot-blot hybridization, using the CaMV 35S DNA as a probe. A referee for Nature refused to call this new evidence conclusive, resulting in an editorial note that accompanied all three published letters. The note states that there is insufficient evidence presented in the original paper to justify its publication, but that since the authors wish to stand by their original conclusions, the journal would let the readers judge the quality of the science for themselves.

Close Window