View - ResearchGate

Genetically Modified Organisms as Invasive Species? 301
In the first group, crop–wild relative hybrids containing the transgene were
grown under experimental conditions often close to the conditions found in
cultivation and, most particularly, with the experimental plants being
released from plant competition. The densities of those natural enemies
which were transgene targets were then manipulated, the results illustrating
that, under appropriate conditions, those hybrids containing the transgenes
have greater fitness. For example, F1 hybrids between B. napus and B. rapa
containing Bt transgenes were found to have a fecundity advantage under
high insect pressure (Vacher et al. 2004). These studies do little more than
illustrate that the transgene will behave as expected in the hybrid as well as
the crop plant, conferring a selective advantage when those plant populations
are affected by the natural enemy (herbivore or pathogen) targeted by the
transgene.
In the second group, far fewer studies have addressed the same questions
under natural field conditions, without manipulating natural enemy pressure.
One exception to this involved a Bt gene backcrossed into wild sunflower populations.
The transgenic backcrossed line had significantly higher fecundity,
compared to the backcrossed control line (Snow et al. 2003). All else being
equal, this would lead to enhanced fitness of plants carrying this transgene.
The difference between these two classes of study is crucial in determining
the frequency of those conditions under which the transgene would be
expected to confer a selective advantage and, therefore, the rate at which it
would spread. The difficulties associated with estimating the relative fitness of
transgenic hybrids may in part explain why the first class of studies is so
much more common than the second. Herbivores and pathogens occur sporadically
in space and time in natural communities: over many years and at
numerous sites, pathogen-resistance genes may provide little advantage until
that one year when a new, virulent pathogen sweeps through an area. Thus,
experiments conducted over a limited number of years and sites run the risk
of being unable to detect any fitness differences. The temptation is then to
manipulate natural enemy pressure to demonstrate the obvious – as long as
some significant result is obtained. The key question is ‘what role do natural
enemies play in regulating existing plant populations?’ So, the most informative
studies will not necessarily involve transgenic plants at all but, rather,
underpinning ecological processes in natural communities.
Another parameter in determining the relative selective advantage of
transgenic plants is the cost of carrying the transgene in the absence of the
target. Costs would contribute to the rate at which transgene frequency may
decline when the population is not under that specific selection pressure.
Again, studies of natural herbivore and pathogen resistance in non-transgenic
plants can be highly informative. A recent review revealed that, if
genetic background is controlled, then 82 % of studies demonstrated fitness
costs associated with carrying herbivore-resistance genes (Strauss et al. 2002),
either as direct costs, such as a trade-off in resources allocated to defence or