The familiar fragrance of your favorite brew is partly caused by aroma compounds produced by brewer’s yeast. Now, researchers have figured out why the yeast even makes that smell to begin with: By mimicking rotting fruit, these single-celled organisms can attract fruit flies, which help disperse the cells when things get too crowded. The work was published in Cell Reports this week.
We’ve have been using yeast for thousands of years to make bread, beer, and wine. The microbes eat sugars and convert them into carbon dioxide gas and alcohol; yeast cells also contribute to taste. About 15 years ago, Kevin Verstrepen of VIB/KU Leuven in Belgium discovered that yeast cells produce several aroma compounds similar to that of ripening fruits, and one yeast gene in particular -- called alcohol acetyl transferase, or ATF1 -- was responsible for most of those fruity, volatile chemicals. It started with an accident...
"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," he says in a news release.
Now, Verstrepen and colleagues set up an experimental compartment where they pumped ATF1 aromas into one corner, while another corner received a dose of odors from ATF1-deficient yeast, Science explains. The other two control corners emitted odorless air streams. Deleting ATF1 took away the attraction Drosophila melanogaster flies had to the yeast cells, and brain activity in flies exposed to these aroma-mutant yeast differed drastically from that in flies exposed to normal, fruity yeasts.
"Two seemingly unrelated species, yeasts and flies, have developed an intricate symbiosis based on smell," Verstrepen says. "The flies can feed on the yeasts, and the yeasts benefit from the movement of the flies." In the picture to the right, you can see fluorescent yeast cells sticking to tiny hairs on the legs of flies that have eaten from a yeast colony.
Additionally, yeast emit fruity, flowery odors more when their populations grow rapidly, suggesting that the aromas were an evolutionary adaptation to prevent overcrowding, Science reports. “When [yeast is] feeding on a piece of banana, the population is growing exponentially. So maybe it makes sense for a few of them to go somewhere else,” Verstrepen says. And when they hitch a ride to another colony, the yeast also get to breed with genetically different strains.
“It seems that the same flavors that allow us to enjoy our beer probably evolved to attract flies and to help yeast disperse into broader ecosystems," says study coauthor Emre Yaksi of VIB/KU Leuven in a statement. This curious case of aroma-based communication and mutualism between microbes and insects likely exists in other plant-associated microbes as well.