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Genetically Modified Organisms as Invasive Species? 299
17.4 Transgene Spread
Gene flow creates the opportunities for transgene spread but the rate at which
this will occur depends upon the relative fitness of transgenic versus wild
type genotypes.
17.4.1 Transgene Spread in Bacterial Populations
There is only one experiment which has directly compared the fitness of
transgenic and non-transgenic bacteria in ‘semi-field’ conditions, when the
transgene has been located on plasmids. The transgene in this case was a
marker. This was conducted in an ecotron (http://www.cpb.bio.ic.ac.uk/
ecotron/ecotron.html), a facility which houses replicated microcosms containing
simplified ecological communities. The aim of the experiment was to
compare the population dynamics of Pseudomonas fluorescens SBW25R (the
control) with the same bacterium carrying a gene cassette which had been
introduced into the genome in three different ways: directly inserted into the
bacterial chromosome (SBW25R::KX), as a similar insertion but including a
lysogenic phage (SBW25::KX-F101), and inserted into a conjugative plasmid
(SBW25R pQBR11::KX). Following seed dressing, all bacterial strains successfully
colonized the phytosphere of chickweed (Stellaria media) and
became established. However, densities of the introduced strain in the lysogenic-phage
treatment were consistently lower than in the other treatments,
at times approaching the limits of detection. The lower density of this strain
was attributed to cell deaths caused by lysis following phage induction,
which was expected in the phytosphere (Ashelford et al. 1999). In the plasmid
treatment, the densities of P. fluorescens SBW25R (pQBR11::KX) on day
53 were significantly lower on roots and leaves than was the case for the
plasmid-free control, with plasmid carriage resulting in a marked reduction
in colonizing fitness. However, from day 53 onwards, in each of the four
chambers the population density of the plasmid-carrying strain increased
significantly on roots (25-fold) and leaves (13-fold) until, by day 95, these
densities either matched or slightly exceeded those of the control. These
results can be explained if carriage of the plasmid were typically associated
with a cost to the host (as has frequently been demonstrated; Lilley and Bailey
1997) but that, periodically, an unidentified plasmid gene (or genes)
improves the fitness of the bacteria, perhaps by conferring the ability to
utilise or tolerate certain substrates exuded by the plants as they mature.
Variation in the fitness of bacteria carrying plasmids is correlated with plant
growth stage in the field.
Because of the rates of horizontal gene flow expected, there has yet to be a
field release of bacteria containing transgenic plasmids. However, there have