Science in Society Archive

Mexican Corn Contaminated by Horizontal Gene Transfer?

Landraces of indigenous corn growing in remote regions in Mexico are found to be contaminated by transgenic corn. Dr. Mae-Wan Ho says evidence suggests contamination by horizontal gene transfer, a process far more insidious and uncontrollable than cross-pollination.

Researchers in University of California Berkeley reported in the journal Nature (November 2001) that landraces of indigenous corn growing in remote regions of Mexico have become contaminated by transgenic corn.

This raised general alarm for two reasons. Mexico is the centre of diversity for corn, and transgenic contamination could easily wipe out the landraces. The fact that landraces in remote regions are contaminated means that contamination of other crops could be far worse. It could destroy both organic and non-GM corn crops, which are much in demand across the world, as consumers are overwhelmingly rejecting GM products.

The mystery remains as to how the landraces could have become contaminated. There has been a moratorium on commercial planting of transgenic corn in Mexico since 1998, though transgenic corn has been shipped to Mexico and elsewhere in the developing world as ‘food aid’. Could the contamination have arisen in the usual way by cross-pollination? Government-approved plantings of transgenic corn before the moratorium were at least 60 miles away.

Corn pollen is heavy, so it does not travel far by air, and is short-lived. The researchers suspect imported transgenic corn was handed out by a government agency as food, and may have been planted by the recipients near the traditional crops.

Dr. Ignacio Chapela, one of the co-authors of the Nature report tells me that a campaign to discredit their research has already begun.

The January issue of the journal Nature biotechnology carries a report on what the critics are saying. Scientist Tim Reeves at the International Maize and Wheat Improvement Centre in El Batan, Mexico, suggests that the research finding was an artefact, and claimed to have found no contamination of indigenous landraces in their own study, yet to be published. On the other hand, scientists working for the biotech industry’s public relations are saying the finding was not surprising at all. Vivian Moses of Cropgen, a UK group funded by industry, was reported to have said, "The paper shows, in essence, that genes move around in nature, and this is hardly news." And Val Giddings of the US industry group BIO: " Should we be shocked to discover gambling in a casino?" The critics are contradicting each other in their haste to discredit the research.

Another criticism from industry and proponents is that the molecular evidence failed to show that the complete transgenic insert was transferred, but only isolated fragments. So, cross-pollination could not have been involved.

The Nature report actually offers evidence of something much more serious and insidious than cross-pollination. The contamination could have been due to horizontal gene transfer, a process that cannot be prevented or controlled, once the transgenic plants are released into the environment (see Box).

The researchers collected 3 corn-cobs of native, ‘criollo’ landraces from fields in each of two 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. They also obtained a bulk grain sample, Diconsa, 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 a piece of genetic material, the cauliflower mosaic virus (CaMV) 35S promoter, which is in all transgenic crops planted or sold commercially.

Four of the six samples of criollo landraces tested positive, whereas cob samples from blue corn 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 corn and Bt-corn from Monsanto, containing the Bt gene coding for a biopesticide from the soil bacterium Bacillus thuringiensis. This confirms that unwanted transgenic food is being dumped as ‘food aid’ in many countries.

The Mexican government independently found transgenic contamination of land races in Oaxaca as well as in another Mexican 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 with the CaMV 35S promoter also tested positive for another piece of genetic material, the terminator (T-nos) from the soil bacterium, Agrobacterium tumefaciens, as did the Diconsa sample. In a third that tested positive for CaMV 35S promoter, the Bt gene (see above) was present.

The researchers then analysed the sequences at the site of insertion of the transgenic DNA, next to the CaMV 35S promoter. Each sample yielded at least 1 to 4 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 corn currently on the market, such as Novartis Bt11. Other sequences represented the criollo corn genome, whereas others showed no similarity to any GenBank sequence. (GenBank is a public database of all the genes that have been sequenced in some 50 000 genomes.)

It is true that simple cross-pollination cannot explain the fragmentary, diverse nature of the transgene contamination, as the critics have pointed out. Instead, that is a sign of horizontal gene transfer and recombination. It is significant that all the contaminated samples had acquired the CaMV 35S promoter, with the rest of the transgenic insert either missing or recombined.

This finding is consistent with our warning in 1999 that CaMV 35S promoter has a ‘recombination hotspot’, where it tends to fragment and join up with other genetic material, and is hence expected to enhance horizontal gene transfer and recombination. (Visit ISIS website www.i-sis.org.uk and search for ‘CaMV 35S promoter’.)

One possible scenario for horizontal gene transfer is if insects were to visit transgenic corn and native corn in succession. They could feed on the transgenic corn, take up and carry fragmented transgenic material to the native landraces. The transgenic material then becomes incorporated randomly into the plant cells, some of which subsequently develop into corn kernels.

We have demanded all transgenic crops with CaMV 35S promoter to be withdrawn immediately back in 1999, not only because it compromises the performance of transgenic crops, but because it is a safety issue. Horizontal gene transfer and recombination can create new viruses and bacteria that cause diseases. It can also cause cancer by the CaMV 35S promoter jumping into the genome or animal cells and triggering inappropriate expression of genes and destabilising the genome.

Two years later, the researchers who have discovered the CaMV 35S recombination hotspot recommended that the promoter should no longer be used, but fell short of calling for existing crops containing it to be withdrawn. They are still denying it is a safety issue (see ISIS News 9/10 www.i-sis.org.uk).

What is horizontal gene transfer?

A cell can pick up pieces of genetic material directly from its environment, and instead of digesting it as food, ends up inserting the genetic material into its own genome. The genetic material picked up could belong to the same species or to unrelated species. This ‘illicit’ gene trafficking is called horizontal gene transfer, to distinguish it from the vertical transfer that takes place in reproduction when transfer is from parent to offspring.

Horizontal gene transfer across species barriers is a rare event in nature, especially in multi-cellular organisms. Foreign genetic material is largely broken down or otherwise put out of action. And even after it has become inserted into the genome, it can still be thrown out.

Genetic engineering consists to a large extent, of artificial horizontal gene transfer. New combinations of genetic material from different species are made (recombined) in the laboratory. The artificial constructs are designed to cross all species barriers and to jump into genomes. They are also structurally unstable, consisting of many weak links, and tend to break and rejoin incorrectly, or to join up with genetic material from other genomes. In other words, the process of genetic engineering has greatly enhanced the potential for uncontrolled horizontal gene transfer.

I-SIS News 13 index

Article first published February 2002



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