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Genetically Modified Organisms as Invasive Species? 297
Transgenes placed on the chromosome will have much lower rates of escape
than those placed on plasmids within the bacteria, even though gene flow
between chromosomes of unrelated bacteria does occur in evolutionary time,
via conjugation or, more commonly, transduction.
17.3.1.4 Evidence for Gene Transfer from GMMs
Most, if not all studies of the field and microcosm release of GMMs have
included a component to specifically study the possibility of gene transfer to
or from the transgenic bacteria, allowing us to test the prediction posed
above. To date, no investigation has revealed an outcome which had not been
predicted and no evidence has been provided demonstrating the invasion of
transgenes from genetically stable constructs. Therefore, from existing evidence,
it is apparent that the likelihood of gene transfer directly equates to the
method of genetic construction. When introduced genes were located on
mobile genetic elements such as conjugative plasmids, transfer has been
observed in laboratory investigations. However, as this outcome was entirely
predictable, field releases have involved bacteria genetically modified to carry
transgenes on their chromosomes. Insertions have been mediated by the use
of transposons, disarmed transposons or by site-directed homologous
recombination. The order in which these three approaches are listed represents
their relative genetic stability and, therefore, the likelihood of being
transferred (Bailey et al. 1995; Troxler et al. 1997). In all the reports of field
testing of bacteria modified to carry transgenes on their chromosomes, none
have found either the transfer or loss of the transgenes. Even in the most
appropriately designed laboratory investigations, transfer frequencies were
negligible or effectively zero (Bailey et al. 1995; Troxler et al. 1997).
17.3.2 Gene Escape in Plant Communities
Gene escape from a plant species involves two steps: hybridisation with a wild
relative, and the survival and reproduction of resulting hybrids. Successful
hybridisation between crop plants and wild relatives was earlier thought to be
infrequent (Ellstrand et al. 1999). This is probably because gene flow was not
perceived to be of significant concern – people had simply not looked.A comprehensive
review of the world’s most important food crops has found that
most spontaneously hybridise somewhere in their range, although rates vary
considerably (Ellstrand 2003a). Oilseed rape can hybridise with close relatives
(reviewed in Hails and Morley 2005), principally wild turnip, Brassica rapa,
with national estimates of 32,000±26,000 hybrids being formed annually in
the UK (Wilkinson et al. 2003). Although this represents a hybridisation rate
of only 0.019 (i.e. 19 hybrids for every 1,000 B. rapa), the sheer number of