The possible importance of horizontal gene transfer in eukaryotes is controversial (Cummings, 1994; Capy et al., 1994); the most compelling argument for horizontal gene flow in eukaryotes is the ubiquity of transposable elements and endogenous retroviruses in genomic DNA, with no known means for their distribution other than by horizontal gene transfer.

It should be noted that any groups using transposable elements for genetic engineering could express the transposase or hopase in the trans configuration and then delete the gene for these enzymes from the transgene constructs, so that once inserted into the host’s chromosome, the element is immobilized. Were this a requirement applied to transposable element vector systems for genetic engineering of animals, the hazards at issue could be minimized or eliminated, so long as active elements capable of mobilizing the introduced sequences were not already present in the host animal.

Potential for Creation of New Pathogens

In addition to their potential for mobilization by interaction with related viruses, transgene sequences also can contribute elements to infecting agents that might modify their ability to cause disease. The donation of drug-resistance genes to bacteria as a consequence of their widespread presence in transgenic livestock is one theoretical example, although the resistance gene would have to be one not found in the environment for the risk of such an event to be significantly enhanced over the natural background. Another example is the possible generation of new retroviruses following recombination between endogenous or exogenous viruses and ones used as vectors for transgenes. This recombination event could result in the provision of new genes or regulatory elements (such as LTR’s capable of more efficient expression) that could adversely modify the pathogenic potential of the infecting virus. A recent natural example is the generation, through recombination between an infectious avian retrovirus and a distantly related endogenous element, of a highly virulent virus, called HPRS-103, or subgroup J avian leukemia virus (ALV) (Payne et al., 1992; Benson et al., 1998). This virus apparently arose as the result of a single, very rare event, but subsequently has been spread worldwide and has become a source of considerable economic loss to poultry breeders (Venugopal, 1999).


The generation of animals using nuclear transfer from somatic cells has received a great deal of attention recently, and it is clear that this technology is

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