August 31, 2007

UC Scientists Find You Can Catch Flies With Beer, Too

By William Brand

BERKELEY -- Flies love Samuel Adams Winter Lager. That may not seem too surprising -- after all, it's great beer. But for a team of University of California, Berkeley researchers, the news was astounding and has led to a serious scientific breakthrough.

It wasn't exactly the beer, it was the carbon dioxide in the beer that drives fruit flies wild. The flies, the Berkeley scientists have discovered, have special taste receptors that are sensitive to carbon dioxide -- the stuff putting the fizz in beer.

It's one, or perhaps the only taste receptor, discovered in addition to the five that we humans and other mammals have. We have receptors for sweet, sour, bitter, salty and umami or savory, says researcher Kristin Scott, a neurobiologist and assistant professor of molecular and cell biology at UC Berkeley.

She speculates that perhaps humans also have special taste receptors for CO2, but that remains unproven.

In flies, Scott and her colleagues believe, the CO2 tasting ability leads the insects to good food.

It's a fruit fly survival mechanism, Scott believes. Yeast and bacteria at work on fruit release CO2 and alcohol as byproducts -- the same process creates the alcohol and fizz in naturally made beer like Samuel Adams.

Scott said fruit flies can also smell CO2 in strong doses and tend to avoid it.

"One way that we like to think of it is that flies seek the right amount of rottenness -- if fruit is only half rotten, producing a little CO2, it's good," Scott said in a campus news release. "If too rotten, it gives off a lot of CO2 and is bad tasting. They seek a balance."

Scott and her UC Berkeley colleagues, graduate student Walter Fischler, technician Priscilla Kong and postdoctoral fellow Sunanda Marella -- all in the Department of Molecular and Cell Biology and the Helen Wills Neuroscience Institute -- reported their discovery in the Aug. 30 issue of Nature.

Scott explains that the DNA genome of the fruit fly has been decoded, so the tiny insects are perfect for research. Their whole genome is surprisingly closely related to humans, she said.

Her lab investigates taste cells and identifies cells and genes linked to different tastes. They've found receptors on the fly that detect sweet and bitter compounds.

They also found a taste cell structure on the fly's tongue, but couldn't find its function.

Fischler tried many substances trying to make the cell react. Finally, he wasted a bit of Samuel Adams. The fruit fly taste cell lit up, so to speak.

Beer is a chemical soup, so Fischler tried many components, including flat beer and dry yeast. Nothing worked. Then he discovered Calistoga mineral water.

"I opened the refrigerator and looked in, when a light bulb went on in my head: Calistoga would be a great way of testing CO2," Fischler was quoted as saying in a campus news release.

Once the research was narrowed to CO2, lab workers tried carbon dioxide in many substances. The receptors loved dry ice, which is frozen CO2. But straight CO2 gas produced a weak response.

And yeast at work also got a reaction. Researchers discovered it wasn't the yeast, but the liquid that the yeast was working in, that attracted interest. Quite naturally, the fly was interested in the edible fruit, not the CO2, which has no food value.

However, it's a good indicator of food quality.

The newly discovered taste sensors for carbonation reside on their own structures, called taste pegs, on the tongue of the fly. While a fruit fly's four other taste cells are perched on the tip of bristles that cover the entire body, the CO2 taste cells are clustered around the margins of the sponge-like tip of the proboscis, at the base of taste bristles.

Last question: What was Samuel Adams Winter Lager doing in a UC Berkeley neurobiology lab? For research purposes, of course.

Contact staff writer William Brand at [email protected] Read his beer column, What's on Tap, every Wednesday in Bay Area Living.

Read his beer blog at http://www.beernewsletter.com/blog