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How to Disgust a Fruit Fly

12/07/2012
Jim Kling

Want to gross out a fruit fly? All you need is one compound and one pathway, according to a new study. Learn more...


If you have any leftovers from Thanksgiving still in your refrigerator, pop open that container and inhale deeply. Revolting, isn’t it? That aversion isn’t unique to humans, and researchers have now traced that disgust reaction in fruit flies back to a single molecule called geosmin.

Researchers have traced that disgust reaction in fruit flies back to a single molecule called geosmin. Source: Rickard Ignell





Fruit flies naturally gravitate toward vinegar and alcohol, the products of fruit fermentation, to feed and lay eggs. But when that fruit begins to rot, Streptomyces bacteria and Penicillium mold fungi become a poisonous threat. In addition, these bacteria and molds produce an unusually large amount of geosmin. It’s the chemical that imparts the earthy odor of a freshly plowed field, and the odor of some forms of cork taint in wine.

Curious about what causes fruit flies to avoid rotten food, researchers tested the compound and were surprised by the intensity of the results, which they published in Cell on December 6 (1).

“It was a hunch,” said study author Bill Hansson, director of the department of evolutionary neuroethology at the Max Planck Institute for Chemical Ecology. “It put a full stop to any kind of attraction. We became really interested because it’s not often that a single compound has such a profound effect.”

Using electrophysiology techniques, the researchers pinpointed the effect to antennae neurons that express the olfactory receptor Or56a. These receptors connect to neurons that override all olfactory input signals. They immediately put the brakes on attractant signals, feeding behavior, and egg-laying.

The receptor is surprisingly specific. The team tested 103 known odorant molecules, and only geosmin activated it. The system amounts to a neural circuit that is designed specifically for disgust.

To trace the neural pathway deeper into the fruit fly brain, the team used calcium imaging and patch clamping. As a result, they found that the pathway was just as specific, which is rather unusual for neural pathways. “There is usually a widening of the message,” said Hansson.

The researchers found that all but one Drosophila species that they studied expressed the Or56a receptor. The exception was a species that lives its entire life inside a flower. “It’s never exposed to [Streptomyces or Penicillium],” said Hansson.

The team used a recently developed computerized wind tunnel that blows puffs of odorant and tracks how the fruit flies respond. “It allows us to quantify behavior in a way that we never could before,” said Hansson.

In the end, Hansson expects to find dedicated disgust mechanisms in other insects as well. Similar pathways in mosquitoes, pine beetles, or the European corn borer would have implications for pest control, but Hansson’s not particularly interested in that development. Instead, he plans to delve further into the fruit fly pathways to identify pathways that alter behaviors like stopping wing beat.

References

1. Stensmyr, M. C., H. K. M. Dweck, A. Farhan, I. Ibba, A. Strutz, L. Mukunda, J. Linz, V. Grabe, K. Steck, S. Lavista-Llanos, D. Wicher, S. Sachse, M. Knaden, P. G. Becher, Y. Seki, and B. S. Hansson. 2012. A conserved dedicated olfactory circuit for detecting harmful microbes in drosophila. Cell 151(6):1345-1357.