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Horizontal gene transfer

Horizontal gene transfer (HGT) is the stable transfer of genetic material between organisms without reproduction (OECD, 2010). The risk assessment of recombinant genes of a GMM must therefore not exclusively be linked to the GM host but require consideration of what those genes could do in another biological and ecological context (Davison, 1999; Snow et al., 2005). The environmental persistence of a recombinant gene may totally change once it has been transferred from one organism, e.g. a bacterium optimised to grow and survive in the gut, to another bacterium, capable of growing under nutrient limitation in soils or surface waters or a surface-colonizer with resistance to sunlight. Thus, because of the spread of microbial cells and because of HGT, an environmental risk assessment of a GMM should not only look at target environments but also at relevant non-target environments.

The likelihood of HGT depends on temporal and spatial aspects, densities of donor and recipient cells and their in situ physiological status, but also on molecular characteristics of the recombinant genes and their genetic context, e.g. the presence of homologous DNA stretches which may serve as sites for recombination, and whether the genes are located on mobile genetic elements and what the host range of such elements would be (Brigulla and Wackernagel, 2010; Thomas and Nielsen, 2005). Even though these factors have a dramatic influence on the likelihood of HGT in the range of ten or more orders of magnitude, the crucial question to be answered in risk assessing a GMM is what a hazard of the recombinant gene could be in any imaginable host, including potential pathogens, thus following a worst-case scenario approach. It should be noted that the HGT event itself has, in principle, no immediate consequences as it normally would occur between single cells within a background of billions. To become detectable and environmentally significant, growth in competition with indigenous micro-organisms would be required. Thus, the consideration of whether a selective advantage would be provided by the genetic modification is a crucial component in considering the environmental risks of horizontal gene transfer.

 
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