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Yeah, but how much seltzer can a fly drink?
The old joke takes on new meaning as researchers discover a taste for carbon dioxide among discerning fruit flies

| 29 August 2007

Fruit flies like a little seltzer in their drinks, according to Berkeley researchers who have identified perhaps the first animal taste sensation not also hard-wired in humans - that is, one other than the five well-known (to us) sensations of sweet, sour, bitter, salty, and umami (savory).

While you may not catch a fly sipping Perrier, the insect has specialized taste cells for carbonated water that probably encourage it to binge on food with growing microorganisms. Yeast and bacteria both produce carbon dioxide (CO2) when they feast, and CO2 dissolves readily in water to produce seltzer or soda water.

"This was unexpected, because fruit flies also smell CO2 and they avoid it," says neurobiologist Kristin Scott, assistant professor of molecular and cell biology. "One way 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; if too rotten, it gives off a lot of CO2 and is bad tasting. They seek a balance."

Mammals have five known types of taste receptors, though there may be more to discover, Scott says. Flies may have five distinct receptors also, but not the same ones mammals have. While Scott has shown that fruit flies can detect sweet and bitter compounds, and now carbonation, she has discounted their ability to taste umami and said that their ability to taste sour compounds is questionable. She and her lab continue to investigate other unknown taste modalities in fruit flies, which could be any of a number of tastes, such as salt or alcohol.

The discovery came when grad student Walter Fischler (one of Scott's co-authors), frustrated that he could not find a chemical that stimulated an unknown type of fruit-fly taste cell he had isolated, tested the cells' reaction to a drop of Samuel Adams beer. Surprised by a positive response, he tried to narrow down the taste preference to one of the many chemicals in beer. Flat beer and dry yeast, for example, did not work. That's when he discovered the leftover bottle of Calistoga mineral water.

As he was searching for beer components to test, he says, "I opened the refrigerator and looked in, and a light bulb went on in my head: Calistoga would be a great way of testing CO2."

The rest is history. Dry ice - frozen CO2 - produced a strong response in the taste cells, while high levels of gaseous CO2 produced a weak response. Sodium bicarbonate in a basic solution that does not contain CO2 bubbles did not work; bicarbonate in a solution with CO2bubbles did. The liquid in which yeast grow, though not the yeast themselves, also elicited a response from the taste cell. These and a few other genetic tests narrowed the taste trigger down to dissolved carbon dioxide.

The preference for carbonation is weak compared with that for sweetness, Scott notes, implying that seltzer enhances taste or makes other tastes more acceptable. This makes sense because CO2 has no nutritional value but is a byproduct of organisms - yeast and bacteria - that do provide nutrients, she says.

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 carbonated-water taste cells are clustered around the margins of the sponge-like tip of the proboscis, at the base of taste bristles.

Scott investigates taste cells, which are a type of nerve cell, and is characterizing the cells and genes associated with different tastes. So far, she and her laboratory colleagues have identified the sweet and bitter cells and some of the gustatory receptor fruit-fly genes that detect sweet and bitter compounds.

Fischler now is trying to isolate the actual receptor in the CO2-sensing nerve cell that grabs the CO2 molecule and sends a signal to the fly brain that there is carbonation in the food. It will then be possible to see if others, including humans, also have carbonation receptors on taste cells.

"There may be many more taste modalities in humans" than the five known today, says Scott. Even if CO2 is a taste unique to fruit flies, its discovery suggests that other animals may have taste receptors tuned to important chemicals in their environment, she says, either to avoid them, as is the case with bitter chemicals, or to seek them out, as is the case with sugars and CO2.

"Thus, taste modalities may differ according to nutritional needs," she and her colleagues wrote. "Alternatively, CO2 may be an unappreciated taste modality in many organisms."

Scott and her Berkeley colleagues in the Department of Molecular and Cell Biology and the Helen Wills Neuroscience Institute reported their discovery in the Aug. 30 issue of Nature.