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Petri dish with fly larvae, irradiated with blue light from underneath. Unchanged larvae normally avoid areas exposed to light. The light has a pleasant odor for the modified larvae thus they move towards it. Larvae that appear to be white are discernible on the illuminated surface.

Blue light floods a petri dish holding fruit fly larvae.

Image courtesy Ruhr-Universitaet-Bochum

James Owen

for National Geographic News

Published June 1, 2010

What does the color blue smell like? Ripe bananas, if you're a genetically altered fruit fly.

According to a new study, fruit fly larvae with algae proteins inserted in their "noses" will mistake blue light for the aroma of bananas, marzipan, or glue—all odors found in the scent of rotting fruit.

Researchers modified the fruit fly grubs to explore how the brain translates chemical signals into smells. (Related: "Rats Smell in Stereo, Study Says.")

The team specifically targeted the olfactory nerve cells, or neurons, in fruit flies that are normally triggered by the smell of ripe fruit. In the gene-altered larvae, the researchers wove into the neurons a light-activated protein found in a species of marine algae.

Detecting light would be important for the algae, which convert sunlight into energy. The algae's protein is specifically sensitive to blue light, because blue wavelengths are the only ones that reach the depths where the species dwells, said study leader Klemens Störtkuhl of Ruhr University Bochum in Germany.

Since fruit fly grubs are suited to life in dark, moist, warm environments, "normally they avoid light," Störtkuhl noted.

But when a blue light was shone near the modified grubs, they wriggled toward the glow as if it were a mushy banana. (Related: "Blue Banana Picture: Glowing Spots Reveal How Cells Die.")

No Flies Were Harmed in the Making of This Study

As part of the experiment, the team developed tiny electrodes that could trace the electrical signal sent from the grubs' olfactory neurons to the animals' brains.

The new, noninvasive method will allow researchers to carry out tests on the neural networks of living animals without harming them, Störtkuhl said.

Such a technique will be key to better understanding smell, since it will help scientists watch exactly how chemical information is transformed into an electrophysiological signal and how this signal gets interpreted in the brain.

What Would Blue Smell Like to You?

The team is now planning a similar experiment on adult fruit flies, which, Störtkuhl predicts, "will fly to blue light."

It's possible that such light-activated smells could also be triggered in humans. (Related: "Better Smellers are More Sympathetic, Study Says.")

"You can imagine taking these photo-activated proteins, spray them into the nose, and see that you get stimulations with blue light," Störtkuhl said.

And as researchers learn more about how odors are detected and then processed in the brain, Störtkuhl envisions other, potentially more useful applications than light-induced food smells.

For instance, he said, the research could aid development of artificial noses, which may one day conduct round-the-clock monitoring to sniff out air and water pollution.

The findings are published in the journal Frontiers in Behavioral Neuroscience.

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