Yeasts and Flies

I can't actually read the article, which is gated, but the introductory paragraphs really say it all. In fact just the headline is enough: "Yeast smell underpins partnership with fruit flies." (Edited to add: Actually I guess you can read the paper for free.)

The moment I read this, it immediately made sense to me. In fact it is quite a coincidence, because this weekend I was wondering how yeast spread in the wild. They are not motile (though within a liquid they can use CO2 to rise to the surface), but they are found all over the place in nature, including on things like the skins of apples and grapes while they are on the tree or vine. It's obvious how yeast could get onto apples that have fallen to the ground, but it's not immediately obvious how the yeast can spread to fruit growing way up in the air. Simply drying up and blowing around with the wind seems horribly inefficient.

Hitching a ride with a fruit fly makes much more sense. The yeast ferment fallen fruit to make alcohol (to kill competing microbes) and aromatic esters (to attract fruit flies). The fruit flies, drawn by the esters, carry the yeast with them wherever they go, and of course they tend to go to fruit!

This also explains, I think, why fruit flies are so strongly attracted to vinegar. (Contrary to the saying, you attract a lot more flies with vinegar than with honey.) Acetic acid is easy to detect by smell, and wherever acetobacter is, there is probably a source of sugar. Meanwhile of course acetobacter benefits from the same dynamic as yeast, hitching a ride to new fruits. (In the case of acetobacter, the acetic acid obviously helps suppress competing microbes, so it's sort of dual-purpose, whereas I'm unaware that the esters made by yeast have any effect on other microbes.)

But so anyway I really like this symbiosis. What started as a signal to attract flying friends is now a pleasant part of the beer we drink. So if you like the flavor of your beer, thank a fruit fly.