The ability to detect rotten food is so crucial for survival that even flies have a dedicated neural circuit to do just that, according to a study published on December 6th in the Cell Press journal Cell. The brain circuit allows flies to avoid feeding and laying eggs on fruit covered in toxic molds and bacteria and represents a unique, specialized system for detecting a repulsive（排斥的） odor. "When this compound is present in the air, even the most attractive food source becomes unattractive," says senior study author Bill Hansson of the Max Planck Institute for Chemical Ecology. "This is highly interesting, as it's seldom that single compounds have a direct behavioral effect and that they are active at extremely low concentrations as we observe here."

The vinegar fly (Drosophila melanogaster) feeds on yeast that grows on fermenting fruit, and the olfactory（嗅觉的） pathways that underlie the insect's attraction toward the smell of vinegar have been well characterized. But relatively little is known about circuits that mediate avoidance of foul odors. One potential candidate for a dedicated avoidance circuit was recently discovered by Hansson and his collaborators, who found that the chemical geosmin（土臭味素）, which is produced by harmful fungi and bacteria, is strongly repulsive to flies.

In the new study, the researchers identified a specialized neural circuit that is highly sensitive to low concentrations of geosmin and responds exclusively to this earthy odor. Activation of this circuit was necessary for the ability of flies to avoid feeding or laying eggs on substances emitting geosmin and was sufficient to repulse flies from the alluring scent of fruit. Moreover, the geosmin detection system was found across species in the genus Drosophila, suggesting that the circuit evolved to enable avoidance of toxic feeding and breeding sites in the environment.

"It is obviously highly important for all organisms to stay away from spoiled food," says lead study author Marcus Stensmyr of the Max Planck Institute for Chemical Ecology. "Detecting and avoiding food infested by bad microbes seems to be a ubiquitous（普遍存在的） phenomenon, where the nervous system has evolved to extreme degrees to make it work."