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Wednesday, 1 December, 1999, 19:04 GMT
'Trojan gene' could wipe out fish
GM salmon are currently confined to labs GM salmon are currently confined to labs

Just one genetically-modified (GM) fish could wipe out local populations of the species if released into the wild, biologists have warned.

The least fit individual in the population is getting all the matings - this is the reverse of Darwin's model
Professor William Muir
The researchers believe their results are the first evidence that GM organisms could have catastrophic consequences on their own species. They also believe that other organisms could face similar risks from GM relatives.

William Muir and Richard Howard of Purdue University, Indiana, US, have dubbed their proposal the "Trojan gene" hypothesis, which is reported in New Scientist magazine.

"This resembles the Trojan horse," said Professor Muir. "It gets into the population looking like something good and it ends up destroying the population."

Human growth hormone

The researchers studied fish carrying the human growth hormone gene hGH, which increases growth rate and final size. Biologists in the US and Britain are experimenting with salmon engineered in a similar way, although no-one has yet begun commercial production.

Muir and Howard included hGH in embryos of a fish called the Japanese medaka (Oryzias latipes), a common aquarium fish that is widely used in research. They found that modified individuals became sexually mature faster than normal fish and produced more eggs.

The model predicts a wipe-out The model predicts a wipe-out
Other experiments, using non-modified fish, showed that larger males attracted four times as many mates as their smaller rivals. This effect is also known in salmon.

Professor Muir predicts that fish made bigger by genetic engineering would enjoy the same reproductive advantages. So the hGH gene would quickly spread through a fish population.

But Muir and Howard also found that only two-thirds of engineered medaka survived to reproductive age, compared with wild medakas. So the spread of the growth hormone gene could make populations dwindle and eventually become extinct.

It would make it very difficult for anyone at the moment to approve the release of GM fish carrying growth hormone
Professor John Beringer
To quantify this, the researchers plugged their results into a computer model to find out what would happen if 60 transgenic individuals joined a wild population of 60,000 fish. The population became extinct within just 40 generations. Even a single transgenic animal could have the same effect, they found, although extinction would take longer.

"You have the very strange situation where the least fit individual in the population is getting all the matings - this is the reverse of Darwin's model," said Professor Muir. "Sexual selection drives the gene into the population and the reduced viability drives the population to extinction."

Professor David Penman, a fish geneticist at the University of Stirling, said there is evidence that some GM fish modified with growth hormone have reduced sperm production and mating success.

"If large males tend to mate with large females, this would often result in matings between GM fish," he added. This would decrease rather than increase the spread of the gene.

GM warning

But Professor John Beringer of Bristol University, a former chairman of the committee that advises the UK Government on GM organisms, says the research is a warning.

"It would make it very difficult for anyone at the moment to approve the release of GM fish carrying growth hormone," he said. "I would have to give a great deal of consideration about whether that's an intelligent route to go down."

Professor Muir says that the model may prove an invaluable tool in assessing the dangers of GM organisms. He now hopes to test its predictions in tightly controlled fish farm ponds.
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See also:
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