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R. Hails and T. Timms-Wilson
hood of exposure. Risk (R) may then be defined as the product of the probability
of toxicity (P t ) and the probability of exposure (P e ), i.e.
R = P e ¥ P t
This has been successfully applied to the case of the Monarch butterfly and
its risk from Bt corn (containing insecticidal transgenes from the soil bacterium
Bacillus thuringiensis) through ingestion of pollen expressing cry proteins,
giving an estimate of 1 in 10,000 larval mortality if 20 % of corn grown
was Bt, rising to 1 in 2,000 if the legal maximum uptake led to 80 % of corn
being transgenic (Sears et al. 2001). It was concluded that Bt corn posed a negligible
risk to the Monarch, and the real threats to this species lay elsewhere,
for example, in habitat destruction. This quantitative approach to risk assessment
has also been adapted for GM plants (Poppy 2004; Raybould 2004) and
transgenic fish (Muir 2004). We use a similar framework here to facilitate our
discussion of the invasive potential of transgenic organisms. We break down
the risk of transgenes causing invasiveness through escape into new recipient
species into three steps:
– P(Transgene escape) ¥ P(Transgene spread/Escape) ¥ P(Harm/Exposure)
This can be read as the probability of transgene escape, multiplied by the
probability of transgene spread – given that escape has occurred, multiplied
by the probability of ecosystem harm – given exposure to the transgene.
Escape of transgenes would be through hybridisation (plants and animals) or
transformation/conjugation/transduction (bacteria), moving organisms
beyond the genetic context in which they were originally produced. The rate
at which transgenes spread through a recipient population will then depend
upon the fitness consequences of carrying those transgenes. Finally, organisms
are usually classed as invasive only if their spread causes economic or
ecological harm, and so we discuss possible measures of harm. These three
steps will now be reviewed for three taxa of GMOs – bacteria, plants and animals
– drawing particularly on soil- and plant-associated bacteria, oilseed
rape and salmon as case studies.
The genes which have been introduced into bacteria are hugely varied,
many of them acting as marker genes, but the majority of these have been created
for laboratory purposes. One potential application of genetic modification
(GM) is to use transgenes to alter existing metabolic pathways, for example,
to degrade pollutants. Few naturally occurring microorganisms possess
the pathways required to mineralise the more recalcitrant xenobiotic compounds
such as pentachlorophenols (PCPs and PCBs; Johri et al. 1999). GM
technology has the potential to improve existing catabolic pathways or to
extend such pathways to include additional target compounds which may
otherwise not be degraded (Timmis et al. 1994; Brazil et al. 1995), and may