In order to impress a virgin female and persuade her to mate, a male fruit fly sings a courtship song. The amorous insect creates its serenade by extending one wing and beating it to generate a sequence of sound pulses. Different species have evolved their own characteristic version of the song, and this system of acoustic communication has long been a favourite subject of researchers investigating the evolution of behaviour.
Now scientists at the UCL Ear Institute, the London Centre for Nanotechnology and CoMPLEX have discovered that the ears of female fruit flies have evolved to be species specific, too. The fly's hearing organ is the antenna, which pivots due to the movement of the air in sound waves and excites mechano-sensory neurons located at its base. Using laser Doppler interferometry, the researchers measured the antenna's motion and found that it vibrated spontaneously, driven by a micro-mechanical system that is unidentified, as yet, but is most likely associated with the transduction apparatus of the sensory neurons. The vibration frequency was species specific and matched the highest frequency in the pulses of the conspecific male's song. Furthermore, the researchers showed that the miniscule vibration makes the hearing organ especially sensitive to sounds at the corresponding frequency. Thus the system appears to be designed so that a female fruit fly can listen out for the right mate, while turning a deaf ear to the advances of inappropriate suitors.
The next step is to determine the molecular basis of this evolutionary trait and the team is planning a series of experiments combining genetic, physical and neurophysiological methods. Since many of the proteins involved in the fly's auditory system are related to those that are found in our own inner ear, we may learn something surprising about human hearing too.
The work is published in Current Biology
O. Riabinina, M. Dai, T. Duke & J. T. Albert, Active process mediates species-specific tuning of Drosophila ears, Current Biology (2011) doi:10.1016/j.cub.2011.03.001
Figure: A male fruit fly (right) beats a wing to create a love song consisting of pulses whose waveforms and corresponding frequency spectra are characteristic of the species. The antenna of a female (left) vibrates spontaneously at a frequency corresponding to the highest frequency in the conspecific male's song, even though the passive mechanical properties of the antenna are alike for all species.