A Blog by Ed Yong

Gut bacteria change the sexual preferences of fruit flies


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.

Reference: PNAS http://dx.doi.org/10.1073/pnas.1009906107

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12 thoughts on “Gut bacteria change the sexual preferences of fruit flies

  1. Another great omg! (and slightly wtf?!) article :)
    The good thing is that we have even more evidence for evolution. The bad– no, actually even better thing is that we have even more evidence that evolution is more complicated than was previously thought. But no matter how many new, unexpected evolutionary factors we find, the core of the darwinian (sensu lato) theory not only does not falter, indeed grows even stronger – you gotta love this theory!

  2. My quick armchair theory is that flies use their own smell as a basis for comparison when identifying another animal as a conspecific. Therefore, flies that smell different smell more like another species, and so the fly is less likely to treat them as a possible mate.

    But, I don’t know anything about fruit flies. Neat post!

  3. Really fantastic study, as always written up beautifully by you.
    @Mike I was actually going to ask in my comment why they might be driven to mate with flies that smell the same. Great explanation!

  4. “Gut bacteria change the sexual preferences of fruit flies”, but it appears that science writer changes sex/gender of researcher. 😉

    Correct me if I’m wrong, Ed, but in paragraph 4 I see, “After just two generations, she found the same effect that Dodd did: the flies were more attracted to individuals reared on the same diets.” And in paragraph 5 there is, “This idea was dramatically confirmed when he gave the insects a dose of antibiotics.” In both sentences the antecedent of the personal pronoun seems to be researcher Sharon.

    It’s still a fascinating piece of research that is, as usual, well reported.

  5. A question for every one:
    Am I misunderstanding evolutionary theory?
    You see I personally understand evolution says that small mutations cause the changes we see over time, but i don’t think that that works, due to the fact that biological systems try to stamp out mutations. Incest would make the most evolutonaryly advanced humans otherwise no?

    Sorry for this not relating directly to the post, but walter’s comment made me remember this question I had. Nobody thus far has been able to properly elucidate this for me :/

  6. Wow, fascinating. Loved this post, and so did my undergrad student :p My vote for the reason for the preferences would be smell as well, different bacteria will have very distinct ones, in particular thinking of poo-smell here.

    Shade: I’m kinda wary of going into this but basically some mutations are harmful, and some can be useful. Most organisms try to err on the side of having few mutations as possible, but some will occur despite various mechanisms being in place to stop them. And some of those will have useful effects and thus be retained within the organism.

    The reason incest is a bad idea is not because it increases mutations but because every human has two copies of each gene. In some cases negative mutations will happen on one gene copy but not the other. If you procreate with a relative it increases the changes that your child will get two copies of the negative mutation and thus will be a lot less healthy.

    Also sometimes in evolution you can get big mutations, like whole chunks of genome being moved, duplicated, or deleated. These often lead to severe negative outcomes, but can also lead to positive ones, such as creating two copies of a protein, which can then specialise in different tasks.

    [Ed – feel free to delete that if it’s derailing the conversation]

  7. Wow, I had never expected besides temporal, sexual, and spatial differences, now culinary habits can drive species apart! Biology’s beautiful.
    While I agree with Sharon’s comment that we should consider the hologenome to understand an organism’s evolution, I think we should take it one step further and consider all these other factors that exert pressures (ecosystems, behaviour etc.). It’s holobiology, so to speak.

  8. Re: incest. I wrote about a paper a while back suggesting that third-cousin couples have more children/grandchildren than couples who were more/less related. Will repost it at some point.

  9. Thanks for tha answers, incest was just an example of something that has a very high rate of mutation is all.
    could have been toxic waste for all it matters.

    p.s. as lab rat said, sorry for derailing.

  10. I thought that you might pick this one up. Pretty cool, eh? Makes one wonder how much the microbiome influences human mate choices. I can see headlines in 5 years: “Women who want to attract doctors from Sweden should eat sushi” etc.

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