BioEd Online

Fruitflies resistant to DDT have an advantage even when they are not being sprayed with pesticides.

The finding will force a re-think of how to manage insects that have evolved resistance to chemicals. It could also help to explain the persistence of pesticide-resistant mosquitoes that carry malaria.

Scientists think the mutations that make insects resistant to pesticides come with some sort of cost to their health. In the absence of the pesticide, flies that do not pay this cost should out-compete resistant flies, perhaps driving genes for resistance extinct.

To test this, Caroline McCart of the University of Bath, UK, and her colleagues pitted DDT resistant flies against normal ones in the lab.

Over egged

The resistant fruitflies carried a gene called Cyp6g1. This produces a protein that is thought to break DDT down into less toxic products, allowing flies to survive higher doses, explains McCart. This gene was the only difference between fly populations.

Surprisingly, resistant flies produced roughly three times as many eggs as non-resistant ones, the authors report in Current Biology¹. In the early stages of egg and larval development, susceptible flies were only 70% as viable as resistant counterparts that had inherited the gene from their mothers.

"We have shown that it is possible for resistance to carry no cost," says McCart. The team speculates that the same may be true for other organisms, including bacteria that have become resistant to antibiotics. This would imply that lowering antibiotic use wouldn't be enough to get rid of resistant bugs.

Mother care

The team doesn't know how the resistant flies get their edge, or why it is better for them to inherit the gene from their mothers rather than their fathers. The researchers think the enzyme that breaks down pesticide is somehow involved.

The fruitfly result might help to answer questions about mosquito populations in Asia, says Christopher Curtis of the London School of Hygiene & Tropical Medicine. DDT resistance became a problem in India decades ago, he says, but when the spraying stopped the resistant bugs didn't die out. "It may explain this unfortunate fact," he says.

But Curtis holds to the traditional view that any advantage of resistance is probably not very large in the wild. "If it was that great, one would have expected it to be in most of the insects," he says.