We are often cautioned that ‘we are what we eat’; eat poorly and pay with your health. In an interesting twist, new research in Proceedings of the National Academy of Sciences shows that diet can influence far more than your waistline. It can also determine mate choice, with potentially important consequences for the evolution of new species.

Scientists led by Gil Sharon in Tel Aviv established two populations of fruit flies that differed only in the diet they were provided. To their surprise, when flies from the two diets were mixed they preferred to mate with like-diet flies, while shunning those from the other diet. The most striking aspect of this shift in mate preference was the fact that it appeared within a single generation. This implied that the altered behaviour was not caused by genetic changes in the flies, but rather by something that evolved on a more rapid time scale.

Because of the known role of bacterial symbionts in manipulating insect reproduction, the team asked whether these were responsible for the shift in fly mating behaviour. Consistent with this possibility, when flies were fed antibiotics that rid them of their bacterial flora, they became less choosy. Reinfecting antibiotic-treated flies with the bacteria isolated from their food caused them to become more discriminating. Most surprisingly, they found a single bacterial species, Lactotoccus plantarum, was sufficient to cause flies to mate selectively. But how do bacteria within the gut modify a behaviour as apparently complex as fly mate choice?

Flies make mating decisions based upon sex pheromones. They smell and taste one another before deciding whether to initiate courtship and accept or reject propositions. As the final piece to this puzzle, Sharon and his collaborators found that flies fed different diets produce different pheromone cocktails and that antibiotic treatment reduces these differences. This provided the direct link between diet and mate choice. In short, diet, through its effect on gut bacteria, can cause flies to either accept or reject one another.

New species arise when reproductive barriers form between groups of individuals. This research shows that these barriers can form within a single generation and be caused by factors outside of an animal's genome. Apparently, because of the influence of bacteria on fly pheromones, the route to a fly's heart is indeed through its stomach.

J. M.
Commensal bacteria play a role in mating preference of Drosophila melanogaster
Proc. Natl. Acad. Sci. USA