Flies, humans, and yeast: bizarre love triangle

Scientists have officially demonstrated that humans are not the only species attracted to that bready, malty, sometimes-fruity-sometimes-flowery smell of beer, and more crucially, not the only species to incorporate beer as a finished product into its reproduction strategy.

The project was seeded about 15 years ago, when a messy graduate student returned to lab after neglecting his experiments for a weekend to find that escaped fruit flies from a neighboring lab had invaded a flask accidentally left on a counter that contained a wild yeast culture, but ignored a different flask that contained an altered yeast strain.  Years later, these same Belgian researchers have discovered the molecular mechanisms underlying the fruit flies’ flask preference.  These mechanisms create an aroma-based communication and mutualistic symbiosis between the fruit fly Drosophila melanogaster and the brewer’s yeast Saccharomyces cerevisiaeThe work involved four main experiments, using a combination of molecular, behavioral, and neurobiological techniques.

The scientists’ first step was to manipulate the yeast genes to create different types of yeast for comparison in the planned experiments.  It is commonly known that yeast is responsible for many of the aromas and flavors of beer through its production of acetate esters such as ethyl acetate (pear), isoamyl acetate (banana), and phenylethyl acetate (flowery).  These acetate esters are formed in a reaction that is catalyzed chiefly by an enzyme called ATF1Using genetic engineering techniques, the Belgians were able to create yeast “mutants” that lacked the ATF1 gene, rendering them unable to produce those acetate esters with such “fragrant” fly-enticing aromas.

The next step applied behavioral techniques to detect a preference in the fruit flies for either the un-modified yeast (“wild-type”) or the yeast lacking ATF1 (“mutant”).  The scientists set up a computer-controlled chamber wherein aromas from different yeast fermentations could be released from opposing corners.  The flies remained randomly dispersed in the chamber while odorless air was released, but once airflow contained aromas from fermentations, they significantly preferred the chamber quadrant with the “fragrant” wild-type aroma (with acetate esters) over the quadrant with “bland” mutant aroma (without acetate esters).

To probe the neuronal mechanisms underlying this behavioral preference, the researchers used calcium imaging in the antennal (olfactory) lobe of live flies.  When they compared neural activity in response to mutant yeast compared to wild-type, they found that the response of projection neurons – which receive input directly from olfactory (smell) sensory neurons – was clearly altered.  The fly brains thus represented the “bland” mutant aroma differently from the “fragrant” wild-type aroma.

While it was clear to the scientists that the yeast provided the flies’ meals, they pondered the advantage for the yeast in employing such scent-related strategies to attract the flies.  Using fluorescent labeling techniques, they demonstrated that “fragrant” wild-type yeast strains were 4 times more likely to be dispersed by a fruit fly than their “bland” mutant peers.  Dispersion provides clear evolutionary advantages because it can make yeast more viable and more likely to reproduce.  In this way, wild-type yeast benefits from being more attractive to flies.

Thus, these Belgian scientists showed that acetate esters produced by yeasts change neural activity in fruit flies, which increases the flies’ attraction to the yeast, and thus increases the potential for advantageous dispersion of the yeast.

Humans demonstrate similar attraction to the aroma of beer in addition to altered neural activity when consuming it, and indeed may use it (perhaps less directly) in the reproduction process.  What’s in it for the yeast?  The answer may be the same: increased reproduction, in the form of purposeful cultivation.  The yeast that is the most successful at creating aromas and flavors that are attractive to humans is the yeast that is isolated and cultivated for future use.

Source: Verstrepen KJ, Yaksi E, Hassan BA, Wenseleers T, Michiels J, Meester LD, Cools TL, Franco LM, and Chstiaens JF. The Fungal Aroma Gene ATF1 Promotes Dispersal of Yeast Cells through Insect Vectors. Cell Reports. 2014.

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