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R. Hails and T. Timms-Wilson
of offspring will cause local extinction of populations (Muir and Howard
1999). Thus, a transgene invasion could have the ultimately undesirable outcome
of population extinction. The mating advantage of transgenics is based
on the idea that mature transgenic fish would be larger than non-transgenics.
However, faster growth does not necessarily lead to larger size at maturity,
and the genotype-by-environment interactions discussed above should also
be taken into account. If food abundance is low, then the lower survival of the
risk-prone transgenic juveniles may prevent transgene spread in the population.Whether
the Trojan gene is a real threat remains an open question, albeit
a theoretical possibility (Reichhardt 2000).
17.6 Conclusions
Much of the current disquiet about the creation of GMOs is that their introduction
into the natural environment could be irreversible, causing a perturbation
to the ecological community which will propagate and possibly grow
in magnitude as it ripples through the ecosystem. The pathways by which
transgenes could invade ecosystems have been considered for microbes,
plants and fish (Figs. 17.1 to 17.3). In bacteria, if genes are introduced into
chromosomes, then horizontal gene flow will be low (hard to detect in ecological
time) but dispersal of bacteria per se is very high. The traditional view is
that the dispersal of bacteria is unhindered by geographic boundaries – the
environment selects (Finlay 2002; Whitfield 2005). This view of ecological
panmixis is peculiar to the microbiological world, and from this arise two
possible consequences. Firstly, the phenotype of the transgenic bacteria is all
important – it will determine the conditions under which selection will occur.
Secondly, panmixis leads to the functional redundancy discussed above. So,
perhaps counter-intuitively, greater mixing means less impact of altered phenotypes
on ecosystem services.
Studies of natural systems can be used to illustrate the same principles of
horizontal gene flow and selection which we have been discussing with
GMMs, the introduction of Lotus corniculatus to New Zealand being one
example.At first sowing, the seeds needed to be inoculated with a natural isolate,
Mesorhizobium loti, a nitrogen-fixing symbiont which passes ammonium
to the plant, in exchange for nutrients. Subsequent inoculation was not necessary.
However, this did not correlate with the establishment of the original
inoculated bacterium, as this strain could no longer be detected in the field.
The symbiotic genes carried on a conjugative element had been acquired by
indigenous Mesorhizobium species, better adapted to the prevailing soil conditions
but now able to maximise use of a new niche – the roots of L. corniculatus
(Sullivan and Ronson 1998). This dataset serves two purposes: it demonstrates
the resilience of indigenous bacteria to invasion by an alien strain but