by Javier Varela.

You might be wondering what kind of strange relationship would involve yeast and an insect – well, a really interesting for sure.

In nature, microorganisms need different strategies to ensure survival when environmental conditions are not optimal. For example, fungi are able to form spores that can be disseminated through the air, whereas some bacteria have the capability to physically move to a more suitable environment.

Although yeasts form spores, neither yeast nor spores are able to mobilize in search for a more suitable environment. However, these microorganisms have developed other tactics to accomplish this same task. Yeasts are commonly found on rotten grapes where they compete with other microorganisms for available resources. When reserves become scarce, dispersal into new environments is vital. Surprisingly, it has been shown that yeasts are capable of attracting flies, which act as vectors dispersing yeasts into new environments. But why would a fly be interested in a yeast? Maybe a rotten fruit is not the most romantic place, but that’s exactly where the magic happens.

Lately, it has become clear that some volatile compounds produced by yeasts are able to attract flies. These key compounds are volatile acetate esters, namely ethyl acetate which elicits pear aroma, isoamyl acetate which is associated with banana aroma and phenylethyl acetate associated with rose aroma.

These volatiles are central in providing the desirable flavors we like in fermented beverages such as wine and beer, yet researchers are not quite sure about their role in yeast physiology.

Volatile acetate ester compounds and the way they smell

In their Cell article Christiaens et al (2014) found that the production of volatile esters by yeasts is fundamental in attracting flies to promote yeast dispersal. To demonstrate this relationship the authors first studied the olfactory preference of flies towards the odor from two different yeast cultures: one from a wild type strain, naturally able to produce volatile esters and another from a mutant strain that is unable to produce these compounds. Air coming from both cultures was pumped in opposite directions of a special arena and then the behavior of the flies was recorded by camera. Interestingly, after 4 minutes of exposure, most of the flies were detected in the quadrant of the arena where the ‘wild-type odor’ was provided. This suggested that flies preferred the odor coming from the wild-type culture, which contained volatile esters. However, to confirm this observation, the same experiment was repeated using volatile esters instead of odor from cultures. As expected, the same results were obtained, indicating that these compounds are responsible for the attraction of flies.

The authors then analyzed the brain response of the flies to the presence of odors from the wild type and mutant yeast cultures. Remarkably, these odors are perceived differently and the brain’s response towards the wild-type odor is similar to the one elicited by volatile esters.

Nevertheless, perhaps the most interesting finding is that wild type yeasts are more likely to be dispersed by flies than mutant yeasts. Flies were allowed to fly freely around a petri dish containing the 2 yeast strains in opposite spots. Afterwards, the presence of both strains was traced showing that the wild-type strain was broadly disseminated across the plate in contrast to the mutant strain. This result allowed the researchers to draw conclusion about the ecological importance of volatile acetate esters production in yeasts. In nature, when yeasts produce aroma molecules, flies would be attracted to the fruit. Then, some yeast cells would stick to the fly’s body being dispersed into a new environment.

Yeast strains unable to produce acetate esters (atf^–) do not attract flies, whereas wild-type yeasts (WT) are able to, promoting their dispersal.

For yeasts it sounds like a free ride but… What do flies get out of this relationship? Well, flies get a good part of it too; they eat yeasts, which are a rich source of proteins. Thus, while some yeasts get dispersed into new niches, some others get eaten.
This complex interaction has made researchers believe that the overall outcome of the fly-yeast relationship involves benefit for both species, formally known as mutualism.

Another exciting angle about this story involves humans. As mentioned above, acetate esters provide several of the characters we like in wine and beer. Thus, over the years, humans have ‘chosen’ yeasts that can produce fruity flavors in the fermented beverages we enjoy, all thanks to the mutualism between yeasts and flies. So next time you find yourself smelling a bunch of roses or enjoying some wine for Valentine’s Day keep in mind that you could also be attracted by yeasts.

References

Christiaens JF, Franco LM, Cools TL, De Meester L, Michiels J, Wenseleers T, Hassan BA, Yaksi E, Verstrepen KJ. (2014). The fungal aroma gene ATF1 promotes dispersal of yeast cells through insect vector. Cell (2):425-32.

Goddard MR, Greig D. (2015). Saccharomyces cerevisiae: a nomadic yeast with no niche? FEMS Yeast Research pii: fov009. [Epub ahead of print]