Every time you drink a pint of lager, you owe a debt to a small fungus that lives in the beech forests of Patagonia. This previously undescribed species – Saccharomyces eubayanus – merged with a close relative to create a hybrid, whose fermenting abilities produce all of today’s lagers. Without it, our pints would have a much darker complexion.
Ask someone to think of a domesticated species and they’ll probably think of something like a dog, cat, cow or horse. But domesticated fungi are just as close to our hearts or, at least, our livers. The yeast, Saccharomyces cerevisiase, has been used to bake bread and ferment wine or ales for centuries. But it’s only partially involved in lagers.
Lager is fermented at a lower temperature than either ale or wine, and the fungus for the job is a cold-tolerant species called S.pastorianus. It has never been found in the wild, and its genes tell us why. It has four of each chromosome, and appears to be a fusion of two different yeast species. One of these is S.cerevisiae but the identity of the second partner has been a long-running mystery. Until now, the best guess was yet another species of cold-tolerant yeast called S.bayanus. But like S.pastorianus, S.bayanus has never been found in the wild.
Now, Argentinian scientist Diego Libkind thinks he has tracked down the real species that merged with S.cerevisiae to help us brew our lagers. And he has found it in a most unexpected place – Patagonia, the southernmost tip of South America.
Libkind has been trying to catalogue the Saccharomyces yeasts around the world. In the northern half of the world, these fungi are found on oak trees and in the southern half, beeches play the same ecological role as oaks. So Libkind’s fungal quest took him to the beech forests of Argentina’s Lanin and Nahuel Huapi National Parks. Almost all the yeasts there could tolerate cold temperatures, since the average temperatures are a chilly 8 degrees Celsius.
Libkind’s found 123 samples of such cold-tolerant yeasts, which belonged to two different species. They lived on different trees and couldn’t cross-breed successfully. By sequencing their entire genomes, Libkind found that one of them was virtually identical to S.uvarum, a species associated with wine and cider. The other, however, was a 99.5% match for the mystery half of S.pastorianus. Here, in the cold forests of Patagonia, he had found the missing wild ancestor of the lager yeast. He named it S.eubayanus.
But how did S.eubayanus make its way from Patagonian forests to European breweries? “Well, quite frankly, we don’t know!” says Jose Paulo Sampaio, who led the study. Certainly, there’s no trace of the species in Europe, whose forests have already been thoroughly searched for yeast species. “For the time being the most reasonable hypothesis is that it migrated from South America to European breweries and that that travel could have been caused by human trade some centuries ago.”
That raises a slight timing problem. Lager-brewing in its current form began in 15th century Bavaria, and would have predated any serious transatlantic trade. Sampaio says, “The first lager beers might have been made with other yeasts and that the arrival of S. eubayanus might have occurred somewhere between the 15th and 19th centuries.” Somewhere between those centuries, this species fused with S.cerevisiase, creating S.pastorianus, which thrived in the environments created by brewers.
Since that merger, the genes of S.eubayanus have changed in small but important ways. For example, they have incorporated an S.cerevisiase gene that allows the fungus to feed off maltose, one of the most common sugars found in the liquids of the brewing process.
Meanwhile, S.bayanus – the strain previously thought to be S.pastorianus’s ancestor – has a much more complicated history. Rather than being a maker of hybrids, it’s a hybrid itself. Around two-thirds of its genome comes from S.uvarum, a third comes from the newly identified S.eubayanus, and a smattering from S.cerevisiase. This complicated ancestry explains S.bayanus has never been found in the wild. Like S.pastorianus, it’s a result of early genetic engineering – an artificial fungus that only exists where humans brew booze.
Reference: Libkind, Hittingen, Valerio, Golcalves, Doven, Johnston, Golcalves & Sampaio. 2011. Microbe domestication and the identification of the wild genetic stock of lager-brewing yeast. PNAS http://dx.doi.org/10.1073/pnas.1105430108
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