Pollution by Horizontal Gene Transfer?
Landraces of indigenous maize growing in remote
regions in Mexico have been found contaminated with transgenic
DNA. Molecular analysis suggests horizontal gene transfer mediated
by CaMV 35S promoter. Dr. Mae-Wan Ho reports.
Researchers in University of California Berkeley
collected 3 maize cobs of native, 'criollo' landraces from fields
in each of 2 locations of Sierra Norte de Oaxaca in South Mexico,
more than 20 kilometres from the main mountain crossing road. A
cob contains 150 to 400 kernels, each kernel resulting from an
individual pollination event. A bulk grain sample, Diconsa, was
obtained from local stores of the Mexican government agency that
distributes subsidised food throughout the country. These seven
samples were analysed for transgenic DNA using probes for the cauliflower
mosaic virus (CaMV) 35S promoter, as this promoter is in all transgenic
crops planted or sold commercially.
Four of the six samples of criollo landraces tested
positive for the CaMV 35S promoter, whereas cob samples from blue
maize of Cuzco Valley in Peru and seed samples from historic collection
in Sierra Norte de Oaxaca both tested negative. The bulk grain
sample Diconsa tested strongly positive, as strongly positive as
the Roundup Ready maize and Bt-maize from Monsanto, confirming
that unwanted transgenic food is being dumped as 'food aid' in
The Mexican government independently found transgenic
contamination of land races in Oaxaca as well as in another state.
Analysis of individual kernels on a single cob found 3-10% had
transgenes, similar to the level found by the Berkeley scientists.
Two of the four criollo samples that tested positive
for CaMV 35S promoter also tested positive for the terminator (T-nos)
from Agrobacterium tumefaciens, as did the Diconsa sample. In a
third that tested positive for CaMV 35S promoter, Bt gene sequence
The researchers then analysed the sequences at
the site of insertion of the transgenic DNA, next to the CaMV 35S
promoter. Each sample yielded 1 to 4 DNA fragments differing in
size. The sequences found next to the CaMV 35S promoter were diverse.
Two sequences were similar to synthetic constructs containing regions
of the adh1 gene found in transgenic maize currently on the market,
such as Novartis Bt11. Other sequences represented the criollo
maize genome, including retrotransposon regions, whereas others
showed no similarity to any GenBank sequence.
How did the landraces growing in such remote regions
become contaminated? A moratorium on planting transgenic maize
has been in place in Mexico since 1998. Is the contamination due
to "loose implementation of the moratorium"? Or to "introgression
before 1998 followed by the survival of transgenes in the population"?
However, simple cross-pollination cannot explain
the fragmentary, diverse nature of the transgene contamination,
which is a sign of horizontal gene transfer and recombination.
The researchers themselves did not raise this possibility, however.
It is significant that all the contaminated samples had acquired
the CaMV 35S promoter, with the rest of the transgenic contruct
either missing or recombined.
This observation is consistent with our warning
that CaMV 35S promoter has a recombination hotspot, where it tends
to fragment and join up with other DNA, and is hence expected to
enhance horizontal gene transfer and recombination [2-4]. We have
demanded all transgenic crops with CaMV 35S promoter to be immediately
withdrawn in 1999. Since then, the researchers who have discovered
the CaMV 35S recombination hotspot have recommended that the promoter
should no longer be used , but fell short of calling for existing
crops containing it to be withdrawn.
1. Quist D and Chapela IH. Transgenic DNA
introgressed into traditional maize landraces in Oaxaca, Mexico.
Nature 2001, 414, 541-3, 2001.
2. Ho MW, Ryan A and Cummins J. Cauliflower mosaic viral promoter
- a recipe for Disaster? Microbial Ecology in Health and Disease
1999: 11: 194-197.
3. Ho MW, Ryan A and Cummins J. Hazards of transgenic plants
with the cauliflower mosaic viral promoter. Microbial Ecology
in Health and Disease 2000: 12: 6-11.
4. Ho MW, Ryan A and Cummins J. CaMV35S promoter fragmentation
hotspot confirmed and it is active in animals. Microbial Ecology
in Health and Disease 2000: 12: 189.
5. Christou P, Kohli A, Stoger E, Twyman RM, Agrawal P, Gu X.
Xiong J, Wegel E, Keen D, Tuck H, Wright M, Abranches R and Shaw
P. Transgenic plants: a tool for fundamental genomics research.
John Innes Centre & Sainsbury Laboratory Annual Report 1999/2000,
p. 30. See "Top research centre admits GM failure" ISIS
News 7/8, February 2001, ISSN: 1474-1547 (print) ISSN: 1474-1814
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