Researchers at the University of California, San Francisco (UCSF), have discovered a chemoreceptor called Gr32a on the forelegs of male fruit flies that sense whether female flies are of the same of different species. According to a paper published online Thursday (June 27) in Cell, male fruit flies (Drosophila melanogaster) approach prospective mates and repeatedly tap them on the side with their forelegs, attempting to detect unpleasant-tasting waxy chemicals on the females’ skin.

“In nature, this sensory system would prevent the creation of hybrids that may not survive or cannot propagate, thereby helping the species preserve its identity,” said UCSF anatomist and senior author Nirao Shah in a statement.

“If the prospective mate is not of the same species, and Gr32a is activated, the mating ritual stops right there,” added co-author Devanand Manoli. “Even if the male has never encountered a female of another species...

Males lacking Gr32a receptors, or males with their forelegs surgically amputated, attempted to mate with fruit flies of other species, even if those flies were 2-3 times larger. Though the researchers found that female D. melanogaster have neurons bearing Gr32a receptors, they did not use them to reject mates of other species. Shah told Popular Science that there “is evidence suggesting that females may use their sense of hearing for this behavior.”

When the scientists removed the forelegs of males of distantly related fly species, such as D. virillis, the inhibition of interspecies courtship was not observed. “It’s possible that D. virilis is using a different mechanism to distinguish other species—we don’t know yet,” said Manoli. 

Shah’s team also discovered that the Gr32a neurons in the male’s legs connect to neurons in the brain that express the male version of the fruitless gene, a master regulator of courtship behavior that is known to suppress interspecies mating. Discovering the connection to fruitless-expressing neurons allows the researchers to trace the neural circuitry of the species-recognition system back to central components in the brain.

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