Researchers from Caltech and the Howard Hughes Medical Institute have identified fruit fly neurons that are related to the animal’s sensing of wind conditions. Because unique neurons were identified within a section of the fly’s brain that was thought to have been used for hearing, it is believed that sensory mechanisms are activated depending on which neurons fire rather than the pattern of neuron activations.
HHMI investigator David Anderson became curious about how flies sense wind during a mini-sabbatical from his California Institute of Technology lab in 2003, when he was learning how to use a tube-like device to deliver alcohol vapors to excite Drosophila melanogaster flies. He noticed that the stream of air coming from the tube was enough to stop the flies from walking. The flies resumed walking when he stopped the stream of air.
The behavior was remarkably consistent, but no one in the lab had noticed it before because alcohol excites flies and this had masked their response to the wind. Puzzled by the observation, Anderson searched for relevant articles in the scientific literature to see if anyone else had described this phenomenon. His search turned up only a few papers published decades ago that described this type of behavior in wild Drosophila flies in Hawaii.
In a new study published March 11, 2009, in the journal Nature, Anderson and colleagues have come one step closer to determining how WISL works. Using new genetic tools, they determined that wind-sensing neurons reside in the Johnston’s organ—a hearing organ in the fly’s antenna. What’s more, the Johnston’s organ contains specific cells designated to detect wind and different cells to detect sound.
“Behavioral responses to wind are thought to have a critical role in controlling the dispersal and population genetics of wild Drosophila species, as well as their navigation in flight,” Anderson says. “But the underlying neurobiological basis of these behaviors is unknown.”
The study is the first to demonstrate that Johnston’s organ is directly involved in wind sensing. During earlier experiments, Anderson began to suspect that the organ was involved because when his graduate student Suzuko Yorozu glued the flies’ antennae to their heads or removed segments of the antennae, they stopped responding to wind.
Those observations suggested to Anderson and Yorozu that the flies were either detecting wind using the sensory hairs on their antennae, or using a specific structure in the antennae that senses movement. Yorozu next used genetic techniques to interfere with the function of cells in Johnston’s organ, which is the only known structure in the fly’s antennae. Those genetically manipulated flies could not respond to wind, suggesting that the organ itself is important for wind detection.
Since Johnston’s organ senses both sound and wind, the researchers next asked whether it could tell the two signals apart. With the help of scientists from the University of Tokyo, Yorozu developed flies with genetic alterations that allowed her to visualize the activation of specific groups of neurons using fluorescent proteins. To watch these neurons in a living fly, she cut away a tiny piece of the cuticle that encases the brain.
Looking through this “window” underneath a microscope, Yorozu was able to see which neurons lit up when she exposed a female fly to a stream of air or played the fly a love song – a chirpy mating sound. A bright glow from the neurons indicated that they were receiving a strong activation signal.
Not only did distinct groups of neurons light up for sound and wind, but separate groups of neurons lit up when the air stream came from different directions — either straight-on or at the side of the fly’s head.
Check out this amusing video of wind activated fruit flies:
Press release: Special Neurons Help Flies Sense the Wind
Research abstract: Neural Circuits for Innate, “Emotional” Behaviors in Flies and MiceAbstract in Nature…
Image: Mrgprd axons (green/yellow) represent a subset of all nerve fibers (red) in the skin.