Rotten fruit signals sexy time for male fruit flies


Drosophila on a lemon wedge. Image via Flickr.

It’s a love story for the ages. Boy meets girl, boy smells the sweet scent of phenylacetaldehyde emanating from the nearby rotting fruit, boy and girl live happily together…for about 20 minutes. Hey, flies don’t bother with small talk; they just get right down to it.

But back to the smelly fruit: scientists have discovered the key to getting male fruit flies (Drosophila melanogaster) in the mood. A chemical found in rotting fruit and vegetables signals to the male fly brain that it’s time for lovin’. Seems like it would make a good article in Drosophila Cosmo: how to drive your fly-guy wild.

But why would anyone want to study this in the first place? Drosophila is a very important model organism. The fruit fly has helped us understand things like Alzheimer’s Disease, heart disease, Autism and mental retardation, and of course fertility. And one of the most complicated things to study in humans, the brain, has been extensively studied in flies.

Fly brains are a lot more simple than ours, but they’re still pretty complex. But scientists in the fly brain community are now very close to having a complete map of at least one circuit in the brain: the male courtship pathway.

Courtship in flies is a dream for behavioral biologists: males always do the same things every time they meet a female. They tap her abdomen and lick her genitals, he then puts out a wing and vibrates it (to serenade the lady), and finally, if she gives the all-clear, he will mount. And all of these behaviors are controlled by a set of neurons known as “fruitless” neurons.

One of the missing pieces of the puzzle was this: what turns on the fruitless neurons in the first place? Was it just the presence of a female? Was it something about how she smells?

Mating fruit flies

Melons, the perfect place for love. Photo via Flickr.

The researchers found, sort of by accident, that a protein called Ir84a was found in fruitless neurons in the olfactory center, the part of the brain that responds to smells. When they deleted the Ir84a gene from males, they stopped being interested in courting females. They still did it sometimes, but not as enthusiastically as before.

So they wanted to know, what did the Ir84a protein do, exactly? They knew it was an olfactory receptor, so it must respond to some kind of smell. They tested 163 different odors and found that only phenylacetaldehyde and two related odors, all from rotting fruits and veggies, got a response from Ir84a.

Then came the test: could these odors “turn on” courtship in male flies? They tested courtship in chambers that either had the rotten fruit smell or not with regular males and dead females. Why dead females? Males will still court them normally, but not very much. When the rotten fruit smell was put in with her, though, the males became much more interested. This required the Ir84a gene, because males that didn’t have it wouldn’t court the dead females, smells be damned.

This result was pretty shocking, because everyone assumed that males were responding to something specific about the female when they started courtship. Sure, the female needs to be there, and later parts of the fruitless pathway probably do respond specifically to the female. But the on-switch is flipped by something else: the presence of food.

In retrospect, this makes a lot of sense. Why waste precious energy (and sperm) if there’s nowhere for females to lay eggs nearby?

Besides learning something new about male fruit flies’ dating strategies, we’ve also helped to flesh out a neural pathway that’s already pretty well understood. This will help scientists understand how brains work in general, by letting them tinker with the brains of flies.

By the way, funny story about fruitless. Fruitless is actually the name of a gene (and it’s protein product) that was named after what happens when you get rid of it. Males without fruitless will start to court other males instead of females. In fact, they form long chains of courting males (see the video!). Thus, because they don’t have a chance of reproducing with those males, they’re “fruitless”. Fruit fly biologists have a sense of humor.

Grosjean, Y., Rytz, R., Farine, J., Abuin, L., Cortot, J., Jefferis, G., & Benton, R. (2011). An olfactory receptor for food-derived odours promotes male courtship in Drosophila Nature DOI: 10.1038/nature10428

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