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The horizontal transfer of transgenic DNA
- Unregulated Hazards ‘Naked’ and ‘Free’ Nucleic Acids

Many geneticists may accept that naked nucleic acids are transferred horizontally, especially to microorganisms, but dispute the transfer of transgenic DNA, which they regard to be no different from the host cell DNA.

There is evidence of secondary horizontal transfer of transgenic DNA to soil bacteria and fungi in the laboratory. In the case of fungi, the transfer was obtained simply by co-cultivation [52]. Successful transfers of a kanamycin resistance marker gene to the soil bacterium Acinetobacter were obtained using DNA extracted from homogenized plant leaf from a range of transgenic plants: Solanum tuberosum (potato), Nicotiana tabacum (tobacco), Beta vulgaris (sugar beet), Brassica napus (oil-seed rape) and Lycopersicon esculentum (tomato) [53]. It is estimated that about 2500 copies of the kanamycin resistance genes (from the same number of plant cells) is sufficient to successfully transform one bacterium, despite the fact that there is six million-fold excess of plant DNA present. A single plant with say, 2.5 trillion cells, would be sufficient to transform one billion bacteria.

Schluter et al [54] investigated horizontal gene transfer under a variety of conditions, some of which gave positive results. For example, a high gene transfer frequency of 5.8 x 10-2 per recipent bacterium was demonstrated for ampicillin resistance transgene -- re-isolated from the DNA of transgenic potato -- to Erwinia chrysanthem, a bacterial pathogen. This was achieved by 105 copies of the ampicillin resistance gene per potato genome, introduced into 6.4 x 108 bacteria by electroporation. When reduced to one copy of ampicillin resistance gene per potato genome, the gene transfer frequency was still significant at 4 x 10-6. The total genomic DNA from the transgenic potato, estimated to carry two copies of ampicillin resistance gene per potato genome, likewise gave a transfer frequency of 9 x 10-6. With only transgenic potato tissue, it was less than 8.7 x 10-9, effectively nil, according to the limit of sensitivity of the protocol. The same result was obtained by co-cultivation of the transgenic tuber with bacteria for 6 weeks. The negative results were not surprising, given the limited access of the bacteria to plant DNA under those conditions. The authors then ‘calculated’ an extremely low frequency of gene transfer at 2.0 x 10-17 under extrapolated "natural conditions", assuming the different factors acted independently. The natural conditions are unknown and by the authors’ own admission, synergistic effects cannot be ruled out.

Free transgenic DNA will be readily available in the rhizosphere around the plant roots, which is an ‘environmental hotspot’ for gene transfer [55]. Gebbard and Smalla [56] have also found evidence of horizontal transfer of kanamycin resistance from transgenic DNA to Acinetobactor, and positive results were obtained using just 100ml of plant-leaf homogenate. Many other factors, such as the density of bacteria, temperature, availability of nutrients, heavy metals and pH, can greatly influence the frequency of horizontal gene transfer in nature [57]. Moreover, less than one percent of all bacteria in the environment can be isolated [58] and monitored for horizontal gene transfer, so negative results in the field must be interpreted with due caution. There is no ground to assume that horizontal transfer of transgenic DNA will not take place under natural conditions.

There are also reasons to suspect that transgenic DNA may be more likely to take part in horizontal gene transfer than the organism’s own genes (see Box 4) [59].


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