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Transgenic DNA in Mexican Corn Landraces

    News stories of transgenic corn growing in remote parts of southern Mexico set off alarms when first reported in September 2001 (Dalton, 2001), and when the corresponding scientific study was published in November (Quist and Chapela, 2001).

    These reports raised concerns because southern Mexico is a center of genetic diversity for maize (corn), and the effects of transgenes on a species' genetic diversity are unknown. If significant reduction in genetic diversity were to occur in a crop species, the ability of plant breeders to find genes to respond to future conditions, such as changes in temperature, drought, or pest infestations, could be compromised.


    Some of the genetic diversity for ear and kernel traits found in Mexican maize. Photo: CIMMYT

    To protect its rich genetic resources of landraces (native varieties) and wild relatives of maize, Mexico established a moratorium on the growing of transgenic corn in 1998. Although Mexico still imports a large quantity of corn products annually from the USA, of which a large percentage is transgenic, the planting of genetically engineered (GE) seed is forbidden.

    In this synopsis, we will review

    (1) what was reported by University of California-Berkeley researchers David Quist and Ignacio Chapela, along with results from subsequent surveys of the area

    (2) the criticisms of their study

    (3) the implications of transgene flow to native crop varieties.

    See additional background information on Mexican maize.

    What did Quist and Chapela report?
    Quist and Chapela's research focused on native maize ears collected from remote fields in the state of Oaxaca. Six samples were taken from two locations, and another sample was taken from local stores of the government subsidized food agency. For comparison, two samples known to be GE-free and two samples known to contain transgenes were also included in the study. The authors extracted DNA from all samples, then used polymerase chain reaction (PCR)-based approaches to detect and amplify DNA fragments associated with transgenes, but not present in native maize. (The PCR technique is described on the Access Excellence web site, http://www.accessexcellence.org/AB/IE/PCR_Xeroxing_DNA.html).

    Area of Mexico where researchers collected native varieties of corn.
    Map data from Ezcurra et al., 2002, and Ortiz-García et al., 2005.

    Quist and Chapela reported that four of their six field samples tested positive for the presence of transgenic DNA. Several agencies of the Mexican government conducted additional surveys of four Mexican states. In 2001 they sampled 19 field locations in the state of Oaxaca and two field locations in the neighboring state of Puebla. Samples from both field locations in Puebla and from 18 of the 19 field locations in Oaxaca tested positive for the presence of transgenic DNA. The locations of the samples taken in 2001 in Oaxaca and Puebla are marked with lavendar dots on the map above.

    Samples collected from 17 field locations in the state of Guerrero in 2002 (red dots on the map above) and from 20 field locations in the state of Michoacán in 2003 (green dots on the map above) tested negative for transgenic DNA. In 2003 and 2004, scientists returned to Oaxaca and collected samples from the same area where transgenic maize had been found in 2001. Sixteen field locations were tested each year; all tested negative for transgenic DNA. The locations of the samples taken in 2003 and 2004 are marked with blue dots on the map above. Many of the same localities were sampled in 2001 and in 2003-2004, resulting in overlap of dots on the map.

    There were two major conclusions of Quist and Chapela's article: that transgenic DNA was detected in Mexican maize landraces, and that the transgene had moved around the maize genome and reinserted itself in different chromosome locations. These are explained in the following sections.

    (A) Detection of transgenic DNA
    The authors looked for three specific fragments of transgenic DNA in their PCR analyses:

  • the Cauliflower Mosaic Virus 35S promoter (CaMV 35S), which acts as an on-off switch for transgenes in most GE varieties.
  • the cryIAb gene, which encodes an insecticidal protein found in many Bt varieties.
  • the nos termination sequence used to mark the end of a transgene.
  • (See How Do You Make Transgenic Plants? for details of transgene construction).

    The CaMV 35S sequence was detected in four of the six landrace samples, the nos sequence was found in two samples, and the cryIAb sequence was detected in one sample. According to the authors these sequences were also observed in the known GE samples, but were not found in the known GE-free samples.

    (B) Evidence for transgene movement in the genome
    Quist and Chapela performed inverse PCR (iPCR) to examine the regions on each side of the CaMV 35S DNA, and thus to determine where in the genome the transgenic DNA had integrated. Based on the discovery that several different DNA sequences were obtained in the tests, the authors concluded that the transgenic construct was capable of integrating into the genome at multiple sites, either during the original transformation event or during the genetic recombination that occurs each time a seed is produced. This assertion is significant because if the CaMV 35S promoter were to wander through the genome it could turn on genes that are not normally active, and turn off genes that are necessary for normal growth and development.

    See a more detailed explanation of Quist and Chapela's methods and results.

    Response to Quist and Chapela's article
    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 (http://www.foodfirst.org/progs/global/ge/jointstatement2002.html), and Mexican groups and Greenpeace have requested a halt to exports of GE corn to Mexico (http://www.greenpeaceusa.org/media/press_releases/2002/04242002.htm). 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.

    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.

    3) Confirmation tests of the PCR results with other molecular techniques were not carried out.

    See more detailed information on these criticisms.

    In a rebuttal to these criticisms, 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 reassorts in the genome. They included new data to support the presence of the CaMV 35S promoter in their samples, but a referee for Nature refused to call this new evidence conclusive. This led to an unusual editorial note in Nature's April 11 edition. 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.

    Conclusions and Implications
    Quist and Chapela's first conclusion, that transgenic DNA is present in native landraces of Oaxacan corn, is certainly plausible, given the large-scale importation of U.S. corn. Whether or not Quist and Chapela conclusively proved that point, many scientists agree that it is not only possible but probable that transgenic corn is growing in Mexico. The Mexican moratorium applies only to the planting of GE corn, not the selling or eating of it. Thus, Mexican farmers can buy maize kernels for food use and plant them instead; this is assumed to have been done by some farmers (Mann, 2002b). Also, kernels that fall from truckloads of imported corn frequently sprout along the roadside. Although this U.S.-adapted maize would not be well suited to southern Mexico growing conditions, at least a portion of it would likely survive until pollen shed. Thus, it is quite plausible that some Mexican cornfields contain transgenic corn. Follow-up studies by two Mexican government laboratories found evidence of the CaMV 35S promoter in 12% of plants sampled from 20 locations in Oaxaca and the adjacent state of Puebla (Mann, 2002a).

    The Commission for Environmental Cooperation, a joint body established by Canada, the United States, and Mexico, recommended in 2004 that Mexico should take steps to prevent the planting of GE corn, for example, by milling imported grain at the point of entry so that it cannot be planted, or by a public education campaign warning farmers that grain distributed by subsidized food outlets such as Diconsa is likely to contain transgenic materials and should be used only for food, not for seed (Commission for Environmental Cooperation, 2004). The commission's report, Maize and Biodiversity: The Effects of Transgenic Maize in Mexico, is available at http://www.cec.org/files/pdf//Maize-and-Biodiversity_en.pdf .

    Quist and Chapela's second conclusion, that the inserted transgene is capable of moving around the genome, either intact or in fragments, is more controversial. Many within the scientific community agree that the claim of transgene reassortment in the genome is unsupported by evidence at this point.

    Especially striking about this current GE crop controversy is the bitter nature of the debate (Mann, 2002a). According to a Joint Statement posted by Food First, http://www.foodfirst.org/progs/global/ge/jointstatement2002.html, Quist and Chapela are the targets of a smear campaign designed to discredit dissident scientists. The statement contends that the "intimidatory" techniques directed against Quist and Chapela are pushed by the biotech industry and pro-industry academics. On the other hand, critics of Quist and Chapela's work have accused the authors and their supporters of being anti-GE zealots who do not rely on sound scientific methods (Kaufman, 2002).

    Almost lost in the debate has been a discussion of the effects transgenic DNA may have on genetic diversity, if it is in fact present in Mexican landraces. Some points to keep in mind with regard to this issue:

  • Far from being pristine, static genetic reservoirs, maize landraces thrive on the incorporation of genes from neighboring plots, thus creating a continuously changing mix of genetic variation. Farmers select from their landrace fields the characters they want to preserve (such as ear shape, kernel color and texture, plant maturity and stature) (Louette, 1997), but selected ears usually retain some of the genes newly acquired through cross-pollination.
  • Seeds of Mexican maize landraces have been extensively collected and stored in national and international germplasm banks, a practice known as "ex situ" conservation. However, this is not a perfect or complete strategy, because not all of the genetic variation of a landrace is captured in a finite collection. Therefore, "in situ" conservation (encouraging the continued growing of landraces in their places of origin) is considered a useful, though challenging, backup strategy.
  • Although the presence of transgenic DNA in Mexican landraces is a concern, it is not clear whether or how this will affect genetic diversity, because it is uncertain whether the Bt gene will confer a competitive advantage in Mexican environments.
  • In addition to introgression in the field, another serious concern is whether transgenic DNA is present in international seed banks. As of May 3, 2002, scientists at the International Maize and Wheat Improvement Center (CIMMYT) had screened 152 Mexican landraces, either from its germplasm bank or recently collected by its staff. They failed to detect the presence of the CaMV 35S sequence, the most commonly used promoter in transgenic maize varieties, and thus these stocks are assumed to be GE-free (http://www.cimmyt.cgiar.org/).
  • Final thoughts
    Reduction of genetic diversity of crop genetic resources is a serious issue, whatever the cause. The potential effect of transgenes on landraces in their centers of diversity is a valid issue that requires research and monitoring. Whether or not parts of Quist and Chapela's study were technically flawed, they focused attention on an important concern deserving careful analysis and evaluation.

    Additional links

    The International Maize and Wheat Improvement Center (CIMMYT), http://www.cimmyt.cgiar.org/, has issued a series of statements and analyses of the reported presence of transgenic DNA in Mexican landraces.

    Information Systems for Biotechnology New Reports (http://www.isb.vt.edu/news/prev_issues.cfm#2002). Stories on this topic appear in the March and May, 2002 issues.

    The Agrichemical and Environmental News at Washington State University has published an analysis of this subject in its May 2002 edition, http://aenews.wsu.edu.

    The International Development Research Center has a story on the efforts being made to preserve the diversity of maize in Mexico.
    http://www.idrc.ca/reports/read_article_english.cfm?article_num=661

    A story by the Australian Broadcasting Corporation on the controversy surrounding this subject is available at http://www.abc.net.au/science/slab/mexicanmaize/

    Transgenes in Mexican maize landraces, an analysis of data and potential impact.
    http://www.biotech-monitor.nl/4910.htm

    Casey Boczon from the Department of Biology at Colorado State University contributed to this report.

Page last updated : January 9, 2006

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