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Bacteria Help Flies Select Mates

Fruit flies and humans have at least one thing in common: We both must choose a mate if we want to propagate the species. But the flies receive some unexpected help. A new study shows that bacteria that inhabit the flies help sway their choice of partners. The microbes might even lead to the creation of new species.

In the late 1980s, a fruit fly researcher noticed something curious: When she split the insects into two groups, reared them for many generations on different diets, and then recombined them, they mated mainly with individuals fed the same diet. The finding confounded biologists because there was no obvious explanation for why flies on one diet would prefer mates who ate the same thing. But microbiologist Eugene Rosenberg of Tel Aviv University in Israel thought he knew the answer.

Rosenberg and his wife, microbiologist Ilana Zilber-Rosenberg, had posited that natural selection acted not just on an individual organism but also on the many species of bacteria that live on and in it. "If you change the diet, you change the bacteria” that live on and in the flies, he says. “Bacteria have to do with smell, and smell has a lot to do with sex," he says, so bacteria might explain the mating preference.

To test the idea, Rosenberg's team split one population of flies in two, feeding one group a diet of molasses and cornmeal and the other a diet of starch. After many generations, the researchers repeatedly combined a male and a female raised on each diet with an equivalent pair raised on the other diet, thereby giving females the opportunity to choose a partner. Then they checked each foursome every few minutes to see who was mating with whom.

As the previous study had shown, the flies preferred to mate with those raised on similar diets. In 29 of 38 cases, the female mated with the male raised on the same food, even though a male raised on the other food source was ready and willing. But when the flies were treated with antibiotics before they were placed together, the female flies were no longer choosy, mating equally often with both kinds of males, the team reports online today in the Proceedings of the National Academy of Sciences.

The result suggests that the flies' bacteria were influencing their choice of mates. To confirm this interpretation, the researchers cultured bacteria from each group of flies, then restored the bugs to the same group of flies after treatment with antibiotics. This treatment made female flies again prefer mates raised on the same food.

Rosenberg's team also showed that molasses-raised and starch-raised flies had different levels of four volatile chemicals called pheromones that serve as sex hormones—and that antibiotic treatment reduced the difference. Because pheromones signal that flies are ready to mate, this finding suggests that the resident bacteria modify the pheromones the insects make, thereby steering their hosts' mate choice.

And that could lead to the development of new species, says Rosenberg. In so-called sympatric speciation, a species can break into two when individuals start mating selectively within their population until, over many generations, they become distinct from the others in the population. In the case of fruit flies, says Rosenberg, bacteria may be driving this process.

"It's a novel result, and it's pretty strong," says evolutionary biologist Jeff McKinnon of East Carolina University in Greenville, North Carolina. The findings "provide a whole new mechanism" by which organisms can quickly develop a strong mate preference, he says. Still, McKinnon cautions, the flies did not always choose mates with similar microbes, which means that the bugs are not all-powerful. "It's not one-step speciation, but it's a substantial move in that direction."