Predatory mites are only 200 to 300 micrometers long, but to 3 millimeter-long fruit flies, these tiny arachnids pose as much of a threat as a rat-sized blood-sucking tick would to a human. Now scientists find that fruit flies have a protective reflex that helps them quickly and accurately kick off mites that land on their wings—a defensive behavior that works even when researchers remove the fruit flies’ heads. The scientists detailed their findings in the Journal of Neuroscience.
Neuroscientist Yuh-Nung Jan at the University of California, San Francisco, and his colleagues investigate fruit flies to learn more about the neural and molecular basis of mechanosensation, which underlies detection of sounds, touches, and pressure. “Lots of diseases are linked to the dysfunction of our mechanosensation system, like hearing loss, chronic pain, and high blood pressure,” says study co-lead author Jiefu Li, a neuroscientist now at Stanford University in California. “Unfortunately, we have a limited understanding of the basis of mechanosensation.”
During a hallway chat with colleagues, Jan wondered aloud: “What are the mechanosensors on the wings?” The scientists focused on recurved bristles, one of three bristle types on fruit fly wing margins. Previous research had not systematically investigated the purpose of these recurved bristles, Li says.
The scientists exposed fruit flies to predatory mites, which directly feed on fly eggs and pupae, explains study co-lead author Wei Zhang, a neuroscientist at the University of California, San Francisco. “Infestations of predatory mites can lead to deaths of fly populations, especially the less healthy flies.” They saw the flies accurately kick off and dislodge the mites. But they wanted to know more about the neural circuits governing complex responses to mechanical stimuli, which are not yet well understood.
High-speed videos revealed that fruit flies accurately kicked mites off within 200 milliseconds, or roughly the length of a blink of an eye. Similar kicks were seen when the researchers brushed recurved bristles with human eyelashes, an ideal probe to simulate the movement of mites. When the recurved bristles were bent, the scientists detected electrical activity from neurons linked to the bristles, confirming that these bristles are mechanosensitive.
“The kicking behavior is most remarkable, given its speed and spatial precision,” says neuroscientist Martin Göpfert at the University of Göttingen in Germany, who did not take part in this research. “The kick seems to be initiated within some 30 milliseconds, indicating that this behavior only involves very few synapses and neurons.”
Indeed, experiments with decapitated fruit flies showed that they were able to kick mites just as quickly and precisely. Apparently this response does not need the brain, but only the fruit fly equivalent of the spinal cord.
The scientists analyzed mutant fruit flies that lacked genes previously linked with mechanosensation. They found that fruit flies lacking the gene for the ion channel “Nanchung” kicked at mites far less than their counterparts. The researchers identified neurons along fruit fly wing margins that expressed Nanchung, observing that recurved bristles were each innervated by a single Nanchung-expressing neuron. Getting rid of Nanchung-expressing neurons eliminated the kicking response.
In principle, the roots of mechanosensation in fruit flies could apply to humans. “Many molecular mechanisms are conserved between humans and fruit flies,” Li says. “For example, the gene that we studied in the fly kicking behavior, Nanchung, is a homolog of human TRPV4 gene, which is associated with human hearing impairment.”
The neuronal circuit underlying this kicking “must be very elegant,” Göpfert adds. As such it “might potentially have applications in prosthetics—for example, in how to drive artificial hands.”