A SINGLE genetic mutation that protects fruit flies from the lethal effects of the insecticide DDT has spread around the world like wildfire. Insects can spread genetic traits very rapidly, but no one has spotted a global takeover before.
The discovery echoes similar research on the mosquito that carries the malaria parasite. Scientists recently found that a DDT resistance gene in Anopheles gambiae had spread right up and down the west coast of tropical Africa.
Such rapid spread suggests that resistance to new insecticides is inevitable, particularly for insects that travel around the world with people or food. 鈥淚t would be virtually impossible to stop it,鈥� says Richard ffrench-Constant from Bath University.
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Widespread use of DDT to control mosquitoes and crop pests began in 1945. The pesticide also killed off fruit flies (Drosophila melanogaster), so the tiny insects had to adapt or die.
And adapt they did. Ffrench-Constant and his team analysed 75 lab populations of fruit flies originally collected around the world in the 1960s. Of those, 28 were DDT-resistant, including flies from every continent except Antarctica.
Surprisingly, every resistant fly had precisely the same genetic change 鈥� the addition of a 鈥渏umping gene鈥�, a short sequence of DNA that can insert itself into new locations in the genome. The researchers found this same extra bit of DNA very near a gene coding for an enzyme that munches up harmful chemicals. 鈥淪imilar enzymes in your liver deal with nasty compounds that you pick up from the beer and curry on a Saturday night,鈥� says ffrench-Constant. The mutation somehow prompts this gene to create 100 times more of this enzyme than normal (Science, vol 297, p 2253).
The researchers cannot pinpoint exactly when or where the mutation occurred, but they could not find it in any fly strains collected in the 1930s, prior to DDT use. This suggests that it spread rapidly and recently.
It鈥檚 very surprising that a single version of the gene is responsible for all DDT resistance, says Charles Godfray, an entomologist at Imperial College in London. He is also struck by the fact that the mutation has stuck around even in lab strains that never encounter DDT. This means that it could be very difficult to eliminate resistance to an insecticide even if you stop using it. 鈥淥nce we select these things in the wild we鈥檙e stuck with them,鈥� he says.