Ed Yong in Not Exactly Rocket Science:
Imagine taking a course of antibiotics and suddenly finding that your sexual preferences have changed. Individuals who you once found attractive no longer have that special allure. That may sound far-fetched, but some fruit flies at Tel Aviv University have just gone through that very experience. They’re part of some fascinating experiments by Gil Sharon, who has shown that the bacteria inside the flies’ guts can actually shape their sexual choices.
The guts of all kinds of animals, from flies to humans, are laden with bacteria and other microscopic passengers. This ‘microbiome’ acts as a hidden organ. It includes trillions of genes that outnumber those of their hosts by hundreds of times. They affect our health, influencing the risk of obesity and chronic diseases. They affect our digestion, by breaking down chemicals in our food that we wouldn’t normally be able to process. And, at least in flies, they can alter sexual preferences, perhaps even contributing to the rise of new species.
Sharon was inspired by experiments by Diane Dodd, who raised two strains of fruit flies on different diets, and found that after 25 generations, their menus had affected their sex lives. Those reared on a menu of starch preferred to mate with other ‘starch flies’, while those reared on maltose had a bias towards ‘maltose flies’. These results were odd. Dodd had set up an artificial evolutionary pressure for diet but somehow, the flies’ mating habits had changed as well.
To work out why, Sharon repeated Dodd’s experiment with the fly Drosophila melanogaster, and raised two strains on either molasses or starch. After just two generations, she found the same effect that Dodd did: the flies were more attracted to individuals reared on the same diets. Something in their food was changing their behaviour.