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Genetically modified (GM) mosquitoes — which scientists hope could one day be used to eradicate malaria — do not compete well against normal mosquitoes, according to work published today in the journal Science.

Researchers found that when GM mosquitoes are bred with unmodified mosquitoes, the introduced test gene disappears in subsequent generations. The findings stress the importance of ensuring the fitness of modified mosquitoes before they are released into the wild, says Andrea Crisanti of Imperial College, London, who led the study.

Malaria affects 300 million people every year and with no immediate prospect of a vaccine, scientists have been seeking alternative ways to tackle the disease. One strategy is to create and release a GM strain of mosquito that is unable to transmit the malarial parasite, which will eventually dominate wild populations.

Three years ago, Crisanti created the first GM mosquito by introducing a DNA fragment that glows under ultraviolet light. However, he and his colleagues have now shown that when these mosquitoes are allowed to breed with normal mosquitoes, the number of GM mosquitoes in the population quickly drops. In some experiments, the introduced gene disappeared within 4 to 16 generations.

“It casts a gloomy light on the idea of engineering mosquito strains unable to transmit malaria,” says Chris Curtis of the London School of Hygiene and Tropical Medicine.

Crisanti thinks the problem could be that the GM mosquitoes are inbred, originating from a single modified individual. As a result, the introduced gene may have become associated with harmful mutations that, while they do not kill the mosquitoes, cause them to lose out when competing with normal mosquitoes.

“A solution would be to make sure that the GM mosquito lines are outbred, by mating the GM mosquitoes only with normal mosquitoes,” says Crisanti. He also says that larger population studies are now needed, but is optimistic that anti-malaria GM mosquitoes could be employed within 10 years.

But even if the problem of inbreeding is overcome, Curtis is sceptical that the approach will ever work. Rather than modifying mosquitoes so that they are unable to transmit the malarial parasite, he believes a better approach would be to make sterile males.

“The screw worm fly — a pest of cattle — has been eradicated all the way from Texas to Panama by swamping the wild populations with sterile males unable to breed,” says Curtis. It’s an old idea, he says, but the advantage is that any decrease in fitness can simply be overcome by releasing larger numbers into the wild.

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