The Yeasts That Make Lager

Around 500 years ago, a group of stowaways crossed the Atlantic to begin an illustrious career in Europe. Leaving behind a large and flourishing family in Patagonia, they met up with distant cousins in the Old World to join the growing family business. Bringing in new skills, they merged with their relatives and expanded their business. Centuries ago, two yeast strains combined by chance somewhere in Bavaria and brought lagers to the world.

Remarkably, despite the enormity of the brewing industry and the popularity of lager beers, it wasn't until 2011 that scientists identified where lager yeast comes from. Lagers result from fermentation at cold temperatures, a process which is accomplished by the hybrid yeast Saccharomyces pastorianus. Several years ago, Chris Hittinger's team at the University of Wisconsin-Madison and Diego Libkind's team at Institute for Biodiversity and Environment Research in Argentina uncovered the wild ancestor of S. pastorianus. They discovered that the Patagonian yeast S. eubayanus had paired up with S. cerevisiae, the yeast commonly used in baking, brewing, and wine-making; the product of their hybridization was S. pastorianus, a cold-adapted brewing yeast which ushered in a new era for beer drinkers.

In a new study appearing in the journal Molecular Ecology, Hittinger's team co-operated with labs in Argentina and Portugal to survey S. eubayanus populations in the wild. Yeasts are exceptionally good at so-called "reticulated evolution", which is a general term for non-vertical transfers of genetic information. This can happen through a variety of processes, from horizontal gene transfer and genetic recombination to interspecific hybridization. By studying the genetic diversity and population structure of S. eubayanus, the researchers hope to get a better understanding of how these processes occur, as well as a picture of the S. cerevisiae x S. eubayanus hybridization.

The team discovered extensive diversity in the Patagonia populations of S. eubayanus, which clearly divided into two groups. "We found two major populations that seem to be distinct," said Hittinger. "The trees they're associated with provide everything they need. They're happy there." S. eubayanus lives in association with southern beech trees in Patagonia, where the lack of pressures on the populations make successful hybridizations uncommon. By contrast, the huge populations and strong selective forces in fermentation make it more likely that hybrids will appear and also give them an edge.

"What we think is happening is that well-established, genetically diverse populations are sending migrants around the world. Generally, they're not successful, but occasionally they are," explained Hittinger. For example, in addition to the successful European offshoots, undergraduate Kayla Sylvester discvered a population in Wisconsin — the first time S. eubayanus has been found outside of Patagonia. In addition to improving our understanding of the history and biology of these organisms, characterizing the biodiversity embodied in these wild populations might reveal new alleles which could imrove fermentation processes and other biotechnologies.

Ref
Peris D, et al. Population structure and reticulate evolution of Saccharomyces eubayanus and its lager-brewing hybrids. Molecular Ecology 23(8): 2031–2045 (2014) doi: 10.1111/mec.1270

Image credits
The image of Patagonian galls is courtesy of Diego Libkind / University of Wisconsin-Madision.