The enjoyment of a pint of your favourite beverage starts with the smell. As you inhale, the fruity odours wafting from the beer into your nose are caused by acetate esters formed while the brewer's yeast Saccharomyces cerevisiae ferments sugars into alcohol. Although those smells are certainly appreciated by humans, scientists have long wondered why S. cerevisiae produce compounds like acetate esters that don't appear to have any direct benefit to the yeast.
Noting that many insects are attracted to fermentation, Joaquin Christiaens and his colleagues at the Katholieke Universiteit Leuven in Belgium decided to investigate how acetate ester smells influence the behaviour of the laboratory fruit fly Drosophila melanogaster. They began by knocking out the ATF1 gene (which is responsible for producing acetate esters during fermentation) in a strain of S. cerevisiae, which they called atf1−. After confirming that this reduced the production of acetate esters, the authors compared how attractive smells from each of these yeasts were to D. melanogaster. They found that fruit flies were strongly attracted to the wild-type S. cerevisiae strains, but not the atf1− mutant strain.
Because it was possible that knocking out ATF1 could have caused other changes to the yeast, the researchers then supplemented the smells from the atf1− mutant strains with the major acetate esters produced by S. cerevisiae – phenylethyl acetate, isoamyl acetate and ethyl acetate. They found that adding ethyl acetate to the atf1− yeast smells caused fruit flies to be equally attracted to the wild-type and atf1− mutant yeast smells. This showed that the reduced production of ethyl acetate in particular when ATF1 was knocked out, was what made fruit flies turn up their noses at atf1− mutants.
To confirm that it was actually the fruit fly's sense of smell that caused the wild-type yeast to be so attractive, the researchers turned to in vivo calcium imaging of the antennal lobes of fruit flies as the insects sniffed the different smells. They found that the smell from the wild-type yeast strain caused a large brain response, whereas that from the atf1− mutant did not, and adding ethyl acetate to the atf1− mutant smell caused the fruit fly antennal lobes to respond more like they did to the wild-type yeast strain.
Finally, while the authors were convinced that the production of ethyl acetate in particular was important for the attraction of fruit flies to yeast, it remained unclear why the yeast might want to attract fruit flies. The authors hypothesized that having more fruit flies attracted to their smell might help the yeast disperse. Setting up a test arena where there were two competing yeast colonies – the nice-smelling wild-type strain, which they engineered to fluoresce green, and the less-attractive atf1− mutant, which fluoresced pink – the team then left a fruit fly in the arena overnight to see which yeasts the insect distributed around the arena. Counting the number of green and pink fluorescent colonies that had grown overnight, the authors found that the flies had dispersed the wild-type yeast much more than the atf1− mutant.
Yeast may seem like simple organisms, but by producing ethyl acetate they are able to manipulate the much more complicated fruit fly into acting like an airline – picking up yeast passengers from one area and carrying them to another. This is most advantageous for yeast wanting to swap genes, but it is especially lucky for beer-loving members of the human race.
