(Excerpt from "Transgenic
Transgression of Species Integrity and Species Boundaries
- Implications for Biosafety". Mae-Wan Ho and Beatrix Tappeser)
Unintended transboundary movements of LMOs, as everyone knows, can occur by cross-pollination between transgenic crop-plants and their wild relatives (see Meister and Mayer, 1994). Field trials have shown that cross-hybridization has occurred between transgenic Brassica napa and its wild relatives: B. campestris (Jorgensen and Anderson, 1994; Mikkelsen et al, 1996), Hirschfeldia incana (Eber et al, 1994; Darmency 1994) and Raphanus raphanistrum (Eber et al, 1994). Rissler and Mellon (1993) have predicted those problems arising from the introduction of exotic species, whether genetically engineered or not. A much more insidious, uncontrollable way for the transgenes (and associated marker genes) to spread, which is peculiar to LMOs, is by horizontal gene transfer (see below). This process recognizes no species barriers, and is inherent to many current transgenic technologies. It is, to a large extent, why transgenic organisms are different from those obtained by conventional breeding methods. The vectors for gene transfer are the means whereby the original species barriers are transgressed. They have the potential to infect and transgress further species boundaries in the process of horizontal gene transfer.
Horizontal gene transfer is the transfer of gene by infection, between species that do not interbreed. It has been known to occur among bacteria and viruses for at least 20 years.
There are three different ways for genes to be transferred. Conjugation, the mating process, requires cell to cell contact. Transduction is transfer with the help of viruses, while transformation is the direct uptake of DNA by the bacteria. As mentioned earlier, there are three kinds of genetic parasites - viruses, plasmids and mobile genetic elements. Mosaic recombinations of all classes are made and currently used by genetic engineers to multiply genes or to transfer genes. Viruses are probably the most infectious as they do not require cell to cell contact for infection and can persist in the environment indefinitely. Plasmids and mobile genetic elements are generally exchanged by cell to cell contact during conjugation or when one cell ingests (or phagocytoses) another.
It must be stressed that horizontal gene transfer has mostly been documented with specially constructed plasmids in studies carried out in microcosms (Mazodier and Davies, 1991), but the spread of antibiotic resistance markers throughout bacterial communities (see below) shows that it can happen without intentional design. The observed correlation between the presence of antibiotics and up to 100-fold enhanced gene transfer activities led to the speculation that low concentrations of antibiotics act like pheromones to enhance gene transfer (Davies, 1994). That has particular implications for the secondary mobility of transgenes carried in association with antibiotic resistance marker genes, if the profligate use of antibioitics is allowed to continue.
Like all other species, bacteria possess different 'restriction systems' which degrade or silence foreign DNA. However, stressful conditions appear to reduce the effectiveness of these systems and to encourage recombination (Saunders and Saunders, 1993; Sch fer et al, 1994). Starving bacteria are also more competent in taking up isolated DNA (Atlas et al, 1992). Transgenic plasmids, as mentioned earlier, are designed to overcome these restriction systems as well as to cross species barriers. So they are potentially much more effective in horizontral gene transfer, despite the 'crippling'.
For a long time, it was supposed that horizontal gene transfers did not involve higher organisms, and certainly not organisms like ourselves, because there are genetic barriers between species and genetic parasites are species-specific. Within the past two to three years, however, the full scope of horizontal gene transfer is slowly coming to light. A search of the isi database conducted under "horizontal gene transfer" came up with 75 references published in mainstream journals between 1993 and 1996, all but two giving direct or indirect evidence of horizontal gene transfers.
Transfers occur between very different bacteria, between fungi, between bacteria and protozoa, between bacteria and higher plants and animals, between fungi and plants, between insects. The current state of our understanding is presented in Fig. 1, where the arrows indicate transfers for which direct or circumstantial evidence already exists. If you follow those arrows, you will realize how a gene transferred to any species in a vector can eventually reach every other species on earth, the microbial/viral pool providing the main genetic thoroughfare and reservoir.
Earlier this year, a mobile genetic element, called mariner, first discovered in Drosophila, was found to have jumped into the genomes of primates including humans, where it causes a neurological wasting disease (P. Cohen, 1996). Geneticists suspect the Drosophila gene might have got into a virus which infected the primates. Although horizontal gene transfers have occurred in our evolutionary past, they were relatively rare events among multicellular plants and animals (and some geneticists have disputed the involvement of horizontal gene transfer in favour of convergent evolution).
However, the scope of horizontal gene transfer may have, or will be, increased because the vectors constructed for genetic engineering are chimaeras of many different vectors designed to transgress species integrity and species barriers, and therefore capable of infecting many species. In the process, these vectors will recombine with a wide range of natural pathogens. That they have been 'crippled' should not lull us into a false sense of security, because it is well-known, as part of the transgenic technology (see Kendrew, 1994) that they can be helped by other viruses and mobile genetic elements to jump in and out of genomes.
"Genetically Engineered Food - Safety Problems"
Published by PSRAST