Yeast is pretty amazing. It's responsible for producing a number of vital products, most importantly beer. And while we’ve been brewing the stuff for centuries, little is known about yeast biodiversity, particularly in naturally-occurring populations. Researchers set out to sketch out the family tree of the different yeast species used to create one of the greatest inventions of the 15^th century – lager.

“People have been unwittingly using yeast; mixing, matching and recombining different species without even understanding that microbes were involved in the process,” coauthor Chris Todd Hittinger, from the University of Wisconsin-Madison, tells IFLScience.

Lager yeasts are hybrid strains made of two very different yeast species: Saccharromyces cerevisiae and Saccharromyces eubayanus. The first likes Mediterranean room temperature and the other is a cold-loving species. And while S. cerevisiae has been used for millennia to make ales, wine and bread, S. eubayanus was only discovered in 2011.

“While the species [S. eubayanus] was new to science, it turns out it’s the same species that can hybridize with conventional S. cerevisiae to form a hybrid strain that is used to brew lager,” Hittinger explains.

He describes the invention of lager as “an accident of history.” It was during the 15th century that Bavarians first noticed that the beer they were storing in caves during the winter continued to ferment. The product was a lighter and smoother beer that went on to dominate beer tastes throughout the 19^th and 20^th century, particularly in America. There are two main types of lager yeast recognized by brewers: the Saaz and Frohberg lineages. But, what are the origins of these different hybrid lineages? Herein lies a bone of contention for many lager beer makers.

^{Fermenting cellar in a brewery. Image credit: Martin D. Vonka/Shutterstock.} 

“The controversy we try to resolve here is whether there was a single origin of lager brewing yeast or multiple origins,” Hittinger says.

Using the genome sequence that they generated, researchers looked at the number of changes that had been accumulated in these different lineages. By comparing domesticated hybrids with naturally occurring populations, researchers were able to study the complete genomes of both parental yeast species used to make lager.

If only one hybridization event occurred before these two lineages began to diverge, then the nature of mutations would be largely random at a molecular level. As a result, the Saaz and Frohberg lineages would have accumulated neutral mutations – those that are neither beneficial nor detrimental – at exactly the same rate.

“Instead what we saw is that there were more than ten times as many neutral mutations in the S. cerevisiae genome than the S. eubayanus genome. What that suggests is that prior to hybridization, the S. cerevisiae sub genome has already diverged,” Hittinger says.

In short, lager yeasts didn’t just originate once, but at least twice. Saaz and Frohberg lineages were likely to have been created by at least two distinct hybridization events between the more diverse strains of S. cerevisiae and nearly identical strains of S. eubayanus. Researchers also found that both lineages of lager-brewing yeast also experienced dramatic elevation in their rate of evolution, which has been seen in domesticated animals and plants. They detail their findings in the journal Molecular Biology and Evolution.

Hittinger says that investigating and discovering new lineages of yeasts involved in brewing beer opens up an exciting area for designer strains. The brewing industry could isolate more strains from the wild and hybridize them with strains well adapted to brewing conditions.

“It could be used to create novel flavors. We’re already seeing a similar approach in the biofuel industry, where we take biofuel strains that have been developed that already have good properties for biofuel production and cross them with other lineages that are maybe more tolerant to particular industrial conditions that are needed,” Hittinger says.