A SINGLE genetically modified fish could turn Darwinian evolution upside down and wipe out local populations of the species if released into the wild, biologists warn. They add that other organisms could face the same risk from transgenic relatives.
William Muir and Richard Howard of Purdue University in West Lafayette, Indiana, made the discovery while modelling ecological risks associated with genetically modified organisms (GMOs). They have dubbed their idea the "Trojan gene" hypothesis. "This resembles the Trojan horse," says Muir. "It gets into the population looking like something good and it ends up destroying the population."
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.
Other experiments using non-modified fish also showed that larger males attracted four times as many mates as their smaller rivals. This effect is also known in salmon. Muir predicts that bigger, engineered fish would enjoy the same 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.
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," says Muir. "The sexual selection drives the gene into the population and the reduced viability drives the population to extinction." The researchers say their results are the first evidence that GMOs could have catastrophic consequences on their own species.
David Penman, a fish geneticist at the University of Stirling, welcomes the discovery. But he says there is evidence that some transgenic 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 transgenics," he adds. This would decrease rather than increase the spread of the gene.
But John Beringer of Bristol University, a former chairman of the British committee that advises the government on GMOs, 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 says. "I would have to give a great deal of consideration about whether that's an intelligent route to go down."
Muir says that the model may prove an invaluable tool in assessing the dangers of GMOs. He hopes to test its predictions in tightly controlled fish farm ponds.
Author: Matt Walker From issue New Scientist dated 4th December 1999 Source: Proceedings of the National Academy of Sciences (vol 96, p 13 853)
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