Fruit Flies Prefer Fizzy Drinks

Fruit flies detect and are attracted to the taste of carbon dioxide dissolved in water, such as water found on rotting fruits containing yeast, concludes a study appearing in the August 30 issue of the journal Nature. Scientists at the University of California, Berkeley, who conducted the study, suggest that the ability to taste carbon dioxide may help a fruit fly scout for food that is nutritious over that which is too ripe and potentially toxic. The research is partly funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health.

"Fruit flies contain similar versions of many human genes, which is why we study them for a variety of health issues, including taste," says James F. Battey, Jr., M.D., Ph.D., director of the NIDCD. "This research raises the question of whether people also may have the ability to taste carbon dioxide and perhaps other chemicals in food. If this were found to be true, our sense of taste could be even more complex than we realize." Currently, scientists recognize five tastes in humans: sweet, salty, bitter, sour, and umami, or savory. Before today's findings, fruit flies were known to be able to taste sweet, bitter, and salty.

The researchers note that a fruit fly's attraction for the taste of carbon dioxide is on a much smaller scale than for sugar, so it may be used more as a possible flavor enhancer as opposed to a full-fledged taste. This makes sense, they say, since carbon dioxide offers no nutrition to the fly.

In humans, taste occurs by way of taste cells, sensory cells that are clustered in the taste buds of the mouth, tongue, and throat, and that express certain proteins, called receptors. These receptors are activated by specific chemicals--called tastants--found in foods and drinks. When a receptor is activated by a tastant, an electrical signal is generated, which travels to the brain.

Taste in the fruit fly, or Drosophila melanogaster, operates much the same way, except fruit flies have taste neurons instead of taste cells, and the taste neurons are found in structures called taste pegs and taste bristles instead of buds. Although taste pegs and bristles can be found all over a fruit fly's body, most are concentrated on the labellum--the equivalent of a tongue--which is housed in the proboscis, a long tubular structure originating from the fly's head.

To arrive at their findings, senior author Kristin Scott, Ph.D., and her research team made use of a powerful genetics technique that enables fruit fly researchers to tightly control which genes are expressed in a cell and which remain silent.

The team first homed in on a class of taste neurons, called E409, found on taste pegs in the fruit fly's labellum. These neurons had not been characterized before and were not already associated with known taste receptors for sweet and bitter. They then labeled the neurons with a fluorescent protein and found that their projections extended to separate parts of the taste area of the brain in comparison to the sweet and bitter neurons.

Next, the researchers tested the E409 neurons' response to an array of compounds and found that substances high in carbon dioxide, such as beer, yeast, and carbonated water, elicited heightened neuron activity as opposed to substances low in carbon dioxide.

Finally, they found that fruit flies were attracted to solutions with high carbon dioxide concentrations, while those whose E409 neurons were shut off were not.

Because fruit flies are also able to smell carbon dioxide, the team also wanted to learn if the two senses influenced one another. Under normal conditions, when fruit flies smell carbon dioxide in the air, they are repelled by it.

Scott and her team showed that fruit flies that had their E409 neurons shut off avoided high carbon dioxide concentrations in the environment; likewise, flies that were missing antennae, the structures they use to smell their surroundings, were attracted to solutions with high carbon dioxide concentrations. These results indicate that the senses of taste and smell operate independently. As a result, the team concluded that fruit flies use both senses of taste and smell separately to gauge their environment for a potential food source.

"Our model is that flies like high local concentrations of carbon dioxide," says Scott. "So if carbon dioxide is being produced by the yeast, flies taste it and they like it. But if there are increased global levels of carbon dioxide in the air--such as if a food source becomes spoiled and potentially toxic--then flies are repelled by it. So we think by having these two different carbon dioxide detectors, flies are able to compare global to local levels of carbon dioxide and then regulate their behavior accordingly."

The research is partly funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), one of the National Institutes of Health. Other funding sources include a Burroughs Wellcome Fund Career Award, a McKnight Scholar Award, and a John Merck Award.