Why Do Flies Like Our Food? They Have Taste

There's a reason why a nasty fly zeroes in on the tastiest morsels in your picnic lunch. New research shows that the ordinary fruit fly, Drosophila melanogaster, is remarkably similar to humans in the way it picks out what it wants to eat, and what it wants to avoid.

First it smells it with the help of a "great olfactory system," says Kristin Scott, assistant professor of molecular and cell biology at UC Berkeley. That allows it to find that juicy banana in your lunch. Then it tastes a bit of it and decides which parts it really wants to eat.

And like humans and most other mammals, if it tastes sweet, the fly pigs out. If it tastes bitter, it looks for something sweeter. The fly knows that bitterness may well mean it is toxic, and sweetness means it is likely loaded with energy resources and good to eat.

That's very similar to how humans decide which foods to eat, and it shows once again why the ugly little fruit fly is so much like us, despite millions and millions of years following a very different evolutionary path.

"It's amazing," says Scott, who published some of her recent findings in the June 24 issue of the journal Cell. "They are such funny looking critters," and yet they are surprisingly similar to humans genetically, thus lending their services to researchers around the world.

Humans: More Fly-Like Than Worm-Like

Fruit flies, along with humans and nematodes, commonly called roundworms, are the first three species to have their genes completely sequenced, leading some scientists to wonder about whether we are more similar to flies or nematodes. Evolutionary biologists at Penn State have devoted considerable time to answering that question and determined that we are more like the fly than the worm.

That's one reason why the fly is the most widely studied insect on the planet. Very subtle changes in its genome have led to vast changes in its appearance, physiology and behavior, thus giving scientists a platform for studying how relatively minor changes in genes can have major impacts.

It also reproduces quickly, so many generations can be studied in a short period of time. And for people like Scott, it offers one other advantage.

"It has a fairly simple brain," she says.

So if scientists can figure out exactly how the brain of the fruit fly helps it distinguish good food from bad, that might help them determine how the human brain processes enormous amounts of complex data in a very short period of time.

That's why Scott and her colleagues at Berkeley, and a separate team of researchers at Duke University in Durham, N.C., are so intrigued with the fly's food selection processes.

Using Taste to Keep Flies at Bay

The Duke study, led by assistant professor of genetics Hubert Amrein, has found evidence that the fly may be even better than humans at discriminating between bitter and sweet tastes. Unlike mammals, the bitter-sensitive nerves that help flies sort through their food are quite different, and more complex, than the cells that tell the fly something is sweet.

Amrein thinks the Duke research might lead to new types of insect repellants that can invite flies to leave the picnic rather than join in.

Scientists can determine which part of the brain is activated by various stimuli, so they now know that the fly processes information about a bitter taste in a different region of the brain than it uses if the taste is sweeter. That's cutting edge stuff, but Scott and her colleagues at Berkeley just needed a little glue and some chemicals to find at least some of the answers to their questions.

Unlike humans, the fruit fly has taste neurons on its wings and its legs, as well as its proboscis, or mouth. Scott wanted to know if the fly has a "body map" in its tiny brain. If it does, when a nerve cell on the fly's leg finds something sweet, the fly should know exactly where that sweet taste is and whip around with its proboscis and take a bite.

"We're meanies," admits Scott, who says her team glued the wings together on a few fruit flies so they could see what happens if just the leg of a fly touches something that is bitter or sweet.

"If you dip a fly's leg in sugar, then the fly's proboscis extends," revealing that the fly immediately knows the location of the sugar, Scott says. "And if you dip the leg in sugar but you have some bitter substance in there, then the proboscis won't extend."

So its taste neurons also serve as locators, but Scott wanted to take it a step further. Flies have two sets of taste neurons, and the researchers wanted to show that one set is used to detect bitterness and the other for sweetness.

So they disabled specific neurons to see which ones did what. When some were disabled, the proboscis did not extend when the fly's leg was poked into sugar water, so the fly no longer knew the sugar was sweet.

In the end they determined that taste receptors among the flies are similar to those found in mammals. Most are used to detect bitterness, not sweetness, because nature provides a more abundant variety of bitter compounds. That's important for a fly to know because bitterness can also signal the presence of poisons.

Taste Has Final Say

All this tells the researchers a bit more about how the fly earns its living. Scott thinks the fly uses odor to detect the presence of food, and that's also similar to how humans do it. It can be hard to pass up the smell of a chocolate-covered donut, for both human and fly.

"Flies can travel long distances for a banana because they can smell it," she says.

But what if they get there and part of the banana is contaminated? How do they find something worth eating in a bucket full of garbage? That's where the taste neurons come in.

"The taste system is just sort of the final checkpoint," Scott says. "The taste system only works when the fly is already on the food. It helps it make the decision to eat it, or don't eat it."

Sort of like us. If it doesn't taste good, we're not likely to eat it, even if it smells like a banana.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.