A single genetically-modified fish is enough to invade and wipe out local populations of fish species. William Muir and Richard Howard of Purdue University in West Lafayette, Indiana, USA, injected the human growth hormone gene, hGH , in embryos of fish called the Japanese medaka ( Oryzias latipes ). They found that modified individuals attained sexual maturity faster than normal fish and produced more eggs. Muir predicts that bigger, engineered fish would also attract a larger number of mates. Experiments using non-modified fish showed that larger males attracted four times as many mates as their smaller rivals, hence the modified fish would also enjoy the same advantages. So the hGH gene would quickly spread through a fish population. 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. They found that the population would become extinct in just 40 generations. Even a single transgenic animal could have the same effect, although extinction would take longer, the researchers say. "This resembles the Trojan horse," says Muir. "It gets into the population looking like something good and it ends up destroying the population" ( New Scientist , Vol 164, No 2215).
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