WIDESPREAD plans to use genetically modified mosquitoes to wipe out malaria have just cleared a major stumbling block. As long as they are not too inbred, the modified insects will not die out quickly in the wild, as scientists had feared.
One of the most promising ways to control malaria is to modify mosquitoes so that they cannot carry or transmit the parasite that causes the disease. The altered bugs would then mate and spread their genes throughout wild populations. The trouble is that no one knew if GM mosquitoes would be as good at finding food, attracting mates and reproducing as their natural cousins. If not, they would die out quickly, along with their vital genes.
To find out whether genetic modification does affect mosquito survival, Andrea Crisanti鈥檚 team at Imperial College in London studied four strains that had been given an extra gene for fluorescence. While all four strains seemed perfectly healthy, when they were mixed with an equal number of wild mosquitoes, the number of bugs carrying the fluorescence gene all died out within 5 to 16 generations (Science vol 299, p 1227).
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The team had a hunch that part of the problem might be due to inbreeding rather than genetic modification. Each population of modified mosquitoes originated from a single pair of mosquitoes and so may have inherited poor traits. To check, they mixed the GM mosquitoes with a batch of inbred wild mosquitoes. This time, they found the modified mosquitoes fared much better, suggesting genetic modification only played a minor role in denting their reproductive success. That could be due to the mosquito using valuable energy transcribing the added gene into fluorescent protein.
鈥淚t would have been a disaster if we hadn鈥檛 spotted this problem earlier,鈥� says Crisanti. The inbreeding problem can easily be addressed by simply creating more diverse strains of GM mosquitoes, he says. 鈥淭he good news is that GM itself is not associated with a big cost.鈥�
Even the minor burden of carrying an extra gene might not be so much of a problem in the wild. There is some evidence that the malaria parasite actually harms mosquitoes, so genetically tweaking them so they don鈥檛 carry it might actually make them as fit or slightly fitter than their wild cousins.