Good vibrations: Fly mating gets all shook up

The courtship of common fruit flies (Drosophila melanogaster), involves the male making a number of specific moves on the female.If successful, this causes her to stop in her tracks. Her immobility signifies that she’s ready to mate.

This is a surprisingly heavily researched area, and a few years ago it was found that two genes that control the differences between male and female flies, called fruitless and doublesex, also work together to control these behaviours. However, exactly what causes female flies to freeze before mating was still unknown.

Male fruit flies have a few instinctive actions for wooing females: they flutter their wings to produce a kind of mating song, and release pheromones to get their chosen female in the mood. Caroline Fabre at the University of Oxford and colleagues from the University of Cambridge, recently found that male flies also shake their abdomens during courtship, a new addition to the mating choreography. The team showed that this abdominal shaking was strongly linked with the females becoming immobile before copulation. But a question remained: how does a female fly sense the male’s vibrations? Does she see or hear him shaking? Or is it more to do with the pheromones he releases at the same time?

Measuring flies’ behaviours is not the easiest task. Drosophila melanogaster are only around 3mm long, and they’re pretty energetic; plus they can fly, which makes it tricky to track what they’re up to. Ultra sensitive equipment and rigorous experiments are essential for getting accurate results.

First, the team wanted to test if the female Drosophila could see the vibrations. This would seem the most obvious answer but, of course, obvious isn’t always right. The team turned off all the lights and observed whether the female Drosophila still responded to the male’s movements. Using special infrared cameras, they were still able to see the flies, but the flies couldn’t see each other. The females still stopped moving as the male vibrated, proving vision wasn’t the most important factor in sensing the vibrations.

They next tested whether the male vibrations were associated with the release of specific pheromones. Testing this was a little harder than just switching off the lights. The team had to genetically modify female flies to produce ‘masculinised-female’ flies that exhibited male courtship behaviours but did not have the male pheromones. When the GM fly vibrated its abdomen, the females still became immobile, despite the absence of pheromones. So pheromones were not required for this aspect of courtship.

Caroline and her colleagues next decided to test whether the vibrations might cause a faint sound that female flies can detect. The researchers clipped the male flies’ wings to stop the song produced by their fluttering so that it wouldn’t interfere with any sound produced by the abdominal shaking. They then used ultra sensitive microphones to test whether the abdominal shakes made a sound: they heard nothing, indicating that the signal is not communicated through sound.

Their last test was to see whether the females could feel the vibrations through the surface they were on. The courting pair were placed on a super-sensitive membrane, which could detect any slight vibrations caused by the abdominal shaking. Success! Caroline and her team showed that vibrations on the membrane coincided with male abdominal shaking, meaning the female flies could physically sense the vibrations through the surface.

Passing vibrations between two animals across the surface they are on is an ancient form of communication but has not been seen before in the mating rituals of Drosophila. In this case, the vibrations are very subtle, so it was not obvious that this was a possibility. The team’s findings open the door to studying what goes on in the brain to trigger these signals and how the nervous system can detect them. This research will give further insight into how genes can control behaviour not only in Drosophila, but in higher organisms too.

Reference

Fabre, C., Hedwig, B., Conduit, G., Lawrence, P., Goodwin, S., & Casal, J. (2012). Substrate-Borne Vibratory Communication during Courtship in Drosophila melanogaster Current Biology, 22 (22), 2180-2185 DOI: 10.1016/j.cub.2012.09.042