When moving genetic material from one organism to another, the subtlety of the gene’s action may make it difficult and tedious to find whether it has actually been transferred. This can be overcome by ensuring that the gene of interest is linked closely with some other gene whose action is easy to spot. If the easy gene has been transferred, there will be a good chance that the interesting one has gone along with it.

Using extensive experience from microbiological genetics, favourite marker genes have been those conferring resistance to particular antibiotics. If the recipient is generally sensitive to the antibiotic – and cannot grow in its presence – transferring a resistance gene can act as a marker because those individuals receiving it will be able to grow in the presence of the antibiotic while the others will not. Anything that grows will probably also have the interesting gene.

However, this procedure has been attacked by anti-GM campaigners because of the risk they perceive of introducing antibiotic resistance from the food produced from the GM crop plant into disease-causing organisms and hence possibly compromise the use of that antibiotic for therapy. Never mind that microbiologists dismiss the risk as negligible: in samples drawn from both healthy and hospitalised people, a gene that makes bacteria resistant to the antibiotic tetracycline has been found in 80 percent of bacteria normally found in the human colon. Since less than a third of colonic bacteria collected 30 years ago contained the gene, the new finding shows how rapidly resistance genes can spread through the bacterial population of the colon (1) even without any GM crops.

In New Zealand, where no GM crops had been cultivated at the time. random sampling of the environment in 1999 generally recovered antibiotic resistant micro-organisms at frequencies much greater than that expected for gene transfer from genetically engineered plants. For example, of 390 natural bacterial isolates from Lincoln soils, 29% were ampicillin-resistant, 67% penicillin-resistant, 66% bacitracin-resistant, 7% gentamycin-resistant, 13% polymyxin-resistant, and 6% tetracycline resistant (2). Consequently, the risk of transfer of antibiotic-resistant genes from plants to microbes is insignificant compared to the rich source of resistance already available in natural microbial populations.

Nevertheless, divergent policies and their implementation in the European Union and the rest of the world have resulted in disputes with serious consequences for agricultural policy, world trade and food security. Much research effort has been directed towards the development of marker-free transformation or systems to remove selectable markers.

In a new study (3), the authors conclude that there is no scientific basis to argue against the use and presence of selectable marker genes as a class in transgenic plants. Their conclusions are supported by numerous studies; interestingly, they point out that most of the studies they looked at were actually commissioned by some of the very parties who have objected so strongly to the use of antibiotic selectable marker gene systems.

Sources:

1. Great increase seen in antibiotic-resistant bacteria in human colon. Upward Quest Health (18.4.02) (http://www.upwardquest.com/chronic.html)

2. Are antibiotic resistance genes harmful? New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, New Zealand (1999).

3. Ramessar, K., Peremarti, A., Gomez-Galera, S., Naqvi, S., Moralejo, M., Munoz, P., Capell, T. and Christou, P (26.4.07). Biosafety and risk assessment framework for selectable marker genes in transgenic crop plants: a case of the science not supporting the politics. Transgenic Research, 16(3), 261-280 (http://www.springerlink.com/content/d1g3n6888xw762x8/)

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