That Beer Smell? Designed to Attract Flies

The telltale aroma of beer first evolved, not to tempt thirsty people, but to attract fruit flies. Continue reading →

We have fruit flies to thank for beer's familiar smell, according to new research.

The most prominent odors released by beer are produced by common brewer's yeast, which evolved the aroma to attract fruit flies. The flies, in turn, benefit yeast by dispersing its cells into the environment.

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"Two seemingly unrelated species, yeasts and flies, have developed an intricate symbiosis based on smell," researcher Kevin Verstrepen of KU Leuven and VIB in Belgium said in a press release. "The flies can feed on the yeasts, and the yeasts benefit from the movement of the flies."

A paper on the unlikely duo - beer and flies - is published in the latest issue of Cell Reports.

Verstrepen had a light bulb moment 15 years ago while studying how yeast cells contribute to the flavor of both beer and wine.

He discovered that yeast cells produce several pleasant and appetizing aroma compounds similar to those produced by ripening fruits. Like fruit, the yeast uses these tempting smells to lure in beneficial others. One yeast gene in particular, alcohol acetyl transferase (ATF1), was responsible for most of the volatile chemicals.

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Verstrepen recalled, "When returning to the lab after a weekend, I found that a flask with a smelly yeast culture was infested by fruit flies that had escaped from a neighboring genetics lab, whereas another flask that contained a mutant yeast strain in which the aroma gene was deleted did not contain any flies."

The years passed, but he never forgot that moment.

For the recent study, he teamed up with fruit fly neurobiologists Emre Yaksi and Bassem Hassan. The researchers used a combination of molecular biology, neurobiology and behavioral tests to show that loss of ATF1 changes the response of the fruit fly brain to a whiff of yeast.

As predicted from the earlier work, mutant yeast cells were a turnoff to the flies. This was bad for the yeast too, since the altered yeast wasn't dispersed much by the flies.

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The research as a whole suggests that the lives of microbes and insects may be intertwined, with each evolving various traits, including smell, to benefit both parties.

The scientists also examined the flies themselves and found that the tiny insects always gravitate toward fragrant yeast, based on yeast remains found in and on the flies' bodies.

Beer and wine connoisseurs often wax poetic when characterizing the smells of these popular beverages, but the next time you get a whiff of beer, think of fruit flies and their contribution to this drink's characteristic odor.

Photo: A frosty mug full of beer. Credit: Len Rizzi, National Institutes of Health

The U.S. Geological Survey is posting photos of insects on its

Flickr page

, offering a macro look at this hidden world. First up, this Festive Tiger Beetle (

Cicindela scutellaris

) was found on top of a butte in Badlands National Park that had ancient windblown sand at its crest. Here, this sand specialist can build its long burrows.

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This Agapostemon bee species is one of the most common native bees in the eastern United States. In almost any field there can be hundreds, if not thousands, of these bees visiting a wide variety of blooming plants. One of the largest of the sweat bees, it still goes undetected if you don't get down on your knees, face close, among the flowers. This one was collected at Colorado National Monument, Mesa County, Colo.

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This wild bee (

Hoplitis fulgida

), a female from Grand Tetons National Park, was collected as part of a study of climate change. Most species in this genus are black , but a few, like this one, are as the Latin in name implies, glittering jewels.

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This unknown wasp was collected in Cecil County, Md.

This is an unknown species of Robber Fly from Charles County, Md. Robber flies, a very large and widespread type of fly, feed on many different kinds of insects, making them a key player in maintaining the insect balance in different environments.

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One species of the rarely seen leafcutting bee, this is

Megachile integrella

from the sandhills of North Carolina. Leafcutter bees are so called because they cut plant leaves to create the cells in their nests. The bees tend to build their homes in rotted wood or in the strong stems of plants.

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Phidippus clarus

is a type of jumping spider. This one was found in Beltsville, Md., but

Phidippus clarus

lives in fields and prairies across North America. It feeds on seasonal plants.

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Eggplant Tortoise Beetles like eggplants (go figure), eating holes in the plants' leaves. From the underside, the insects look quite queenly, with their ruffled collars. This one was gathered at the Patuxent Wildlife Research Center, in Anne Arundel County, Md.

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The Karner blue butterfly,

Lycaeides melissa samuelis

, is endangered. Karner blue butterflies feed on nectar from many different types of flowers, but their larvae can survive on the leaves of only one specific plant, which has been decimated by habitat loss or change.

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Deer flies like this one, despite their groovy eyes, deliver a ferocious bite. And no wonder: when the female bites (males don't bite), she lacerates the skin and when the blood flows, sponges it up with her mouth. There are over 110 species of deer fly.

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Centris bees, like this one, make their homes in holes, either in trees or in the ground.

The biggest visual difference between damselflies and dragonflies are their wing positions when resting. Dragonflies hold their wings open, while damselflies close them above their backs. This Ebony Jewelwing Damselfly (

Calopteryx maculata

) was found on a Beltsville, Md. stream.