Gut bacteria change the sexual preferences of fruit flies

View Images

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.

Bacteria in our guts change according to the food that we eat, so Sharon suspected that something similar was happening in the flies. This idea was dramatically confirmed when he gave the insects a dose of antibiotics. Immediately, their sexual bias disappeared and they were just as likely to mate with flies from either group.

As further evidence, Sharon isolated bacteria from the food that the flies had eaten and added them to vials of sterile food. When the antibiotic-treated flies ate this food, laced with a drizzle of bacteria, they regained their sexual preferences after a single generation. Those that ate food containing ‘starch bacteria’ preferred to mate with starch flies, and those that ate food containing ‘molasses bacteria’ preferred to mate with molasses flies.

Many bacteria are probably responsible for this effect but Sharon singled out Lactobacillus plantarum for special attention. It’s particularly common in starch-based food and while every molasses fly harbours around 2,600 of these bugs, starch flies contain around 23,000 apiece. After a dose of antibiotics, flies that are infected with this bacterium alone take a fancy to starch flies over molasses flies.

It’s possible that the bacteria influence the levels of sex pheromones that affect the fly’s attractiveness, either by producing those chemicals themselves or stimulating the fly to do the same. That’s not too far-fetched: bacteria can alter the smells given off by many animals, and smell certainly affects sexual behaviour. Indeed, Sharon found five different chemicals that are present at different amounts on the outer shells of starch and molasses flies. Antibiotics brought the levels of these chemicals down to similar levels. For a fruit fly, these seem to be the smells that make the differences between striking lucky and getting rejected.

These experiments could have important implications for the origin of new species. Imagine that two wild populations of the same species mate with each other less and less often, because they start to harbour different bacteria (just as the starch and molasses flies did). The stronger these preferences, the greater the odds that these populations will diverge into separate species.

Diet, of course, is just one thing that affects the membership of the bacterial club in our bodies, so it’s unlikely that a species would split in two just because two groups started eating different foods. However, Sharon speculates that diverse diets could intensify the effects of other barriers that restrict the flow of genes, such as physical obstacles.

In any case, the study suggests that you can’t understand an animal’s evolution simply by considering the evolutionary pressures that act on its genome. You also have to consider the genes of the bacteria and other passengers that live inside it, which also create variations in its behaviour and affect is chances of survival. Sharon calls this the hologenome – the combined genes of a host and all the microbes it contains.

More on the microbiome