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Wednesday, 1 December, 1999, 19:04 GMT 
'Trojan gene' could wipe out fish 
 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

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

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
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

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.    Search BBC News
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See also: 
 29 Jul 99 |  Sci/Tech  
 GM salmon prompts safety pledge  
 26 Nov 99 |  Sci/Tech  
 New Zealand approves GM cows  
 08 Nov 99 |  Sci/Tech  
 Scientists discuss GM threat to butterflies  
 07 Oct 99 |  Sci/Tech  
 Call for tougher GM tests  
 18 Oct 99 |  Sci/Tech  
 GM controversy intensifies  
 26 Aug 99 |  Sci/Tech  
 'GM crop can help environment'  
 21 Oct 99 |  Sci/Tech  
 GM crops have 'significant snags'  

Internet links: 
 Purdue University Animal Sciences  
 Stirling University Aquaculture  
 New Scientist  

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Links to other Sci/Tech stories are at the foot of the page. 


Robert Rice
All Men are equal until the point of exertion