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 . 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) . 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  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
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 . Gebbard and Smalla  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 .
Moreover, less than one percent of all bacteria in the environment can be
isolated  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) .