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
(3) the implications
of transgene flow to native crop varieties.
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
- 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
See a more
detailed explanation of Quist and Chapela's methods
Response to Quist and Chapela's
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
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.
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
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,
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
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
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
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/).
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.
The International Maize and Wheat Improvement Center (CIMMYT),
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
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
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.
Casey Boczon from the Department of Biology at Colorado
State University contributed to this report.