How Evolution Gave Us Fruity Beers

Cirencester, looking east down Market Place.

We've just arrived back home from a very pleasant Autumn mini-break in Gloucestershire and Wiltshire, staying for a couple of nights in a hotel overlooking the Cotswold Water Park at South Cerney, near Cirencester. Cirencester, as the name ...cester implies, was an old Roman fortified town used as both a center of Romano-British culture and a garrison to maintain military dominance and political control over the native Britons. 'Castra' is Latin for 'camp', which came to mean a military garrison.

The Romans knew it as Corinium, Corinium Dobunnorum (after the local Celtic tribe the Dubonnii) or Corinium Castra. The name 'Corinium' derives from the name of the local river Churn, and the river Churn may have been called that by the Dubonnii because an even earlier tribe called the Cornovii lived there. Such is the glorious evolution of language that the modern English place-name "Cirencester" means something like "the fortified camp in the town of the Dubonnii on the river of the Cornovii", derived from two or three earlier languages, none of them English or even Germanic.

But enough of this ancient history. I'll be posting a couple more blogs about the lessons from ancient history later but this one is a little illustration of how evolution can have a completely unexpected result - but one that's fascinating when you understand it.

After spending some time exploring Cirencester and having spent an hour or so in the local "Corinium Museum" we visited a local hostelry to partake of the amber nectar and to sample the local ales. I chose an IPA from one of the local independent breweries that have done much to transform English Ales in recent years. It was a little too sweet for me but it had a distinctly fruity taste which many people will find attractive. Certainly a pleasant drink, but I prefer a little more hops and more of the sugar fermented out - more of a bitter person than an IPA person really.

Now, why would the product of fermented malted barley, flavoured with hops, have a fruity taste?

Coincidentally, the answer to this came in a piece of research published the other day in Cell Reports - The Fungal Aroma Gene ATF1 Promotes Dispersal of Yeast Cells through Insect Vectors. It has nothing to do with humans and our likes and dislikes, other than that we have benefitted from it.

Highlights

• The S. cerevisiae ATF1 gene controls the production of volatile acetate esters
• Aroma of ATF1 mutants elicits different neuronal activity in the fly antennal lobe
• Flies are significantly more attracted to wild-type yeast than to atf1-null mutants
• Addition of isoamyl acetate and ethyl acetate restores attraction of Drosophila

Summary

Yeast cells produce various volatile metabolites that are key contributors to the pleasing fruity and flowery aroma of fermented beverages. Several of these fruity metabolites, including isoamyl acetate and ethyl acetate, are produced by a dedicated enzyme, the alcohol acetyl transferase Atf1. However, despite much research, the physiological role of acetate ester formation in yeast remains unknown. Using a combination of molecular biology, neurobiology, and behavioral tests, we demonstrate that deletion of ATF1 alters the olfactory response in the antennal lobe of fruit flies that feed on yeast cells. The flies are much less attracted to the mutant yeast cells, and this in turn results in reduced dispersal of the mutant yeast cells by the flies. Together, our results uncover the molecular details of an intriguing aroma-based communication and mutualism between microbes and their insect vectors. Similar mechanisms may exist in other microbes, including microbes on flowering plants and pathogens.

The Fungal Aroma Gene ATF1 Promotes Dispersal of Yeast Cells through Insect Vectors, Christiaens, Joaquin F. et al.
Cell Reports DOI: http://dx.doi.org/10.1016/j.celrep.2014.09.009^*

Basically, yeasts (Saccharomycetes) have evolved to produce fruity odours as they ferment sugars in order to attract fruitflies (Drosophila) which lay their eggs on fruit - hence their common name. The fruitflies then pick up yeast spoores and so help to disperse them.

A simple little deception by the yeasts and a change in genetic information which would be meaningless in the absence of fruitflies but which, in the presence of fruitflies means the yeasts with the right genes get dispersed more widely onto targeted media and so produce more copies of themselves.

Perhaps more importantly this illustrates how what religious people will often present something in nature from which we benefit as something which has been provided for us and so is evidence of some sort of anthropophilic designer - which naturally we are supposed to conclude is the god they are pushing - Christian, Muslim, Shinto or any of their sects, no matter how minor. This appeals particularly to people with a hyper-inflated sense of self-importance which makes them imagine that the Universe must have been made just for them.

In reality, of course, this product of evolution had nothing whatever to do with us. It's just that, because we too evolved from fruit-eating remote ancestors and still have genes which make us enjoy fruity flavours, we too have been vicariously fooled by a deception evolved by yeast to trick fruitflies. There is no fruit in fruity-tasting beers.

And this of course, illustrates the interconnectedness of living things by virtue of us all having evolved from a common ancestor, whether the 'us' in question are Homo sapiens, Saccharomycetes or Drosophilae.

^*© 2014 The Authors. Published by Elsevier Inc. User rights governed by Creative Commons Open Access licence.

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