啤酒进化史

From Austrian monks to American craft brewers, beer geeks are everywhere. But making a good beer not only depends on the best ingredients, but also the best yeast. The beer world is divided into ales and lagers. The original and highly versatile yeast, Saccharromyces cerevisiae, has been used for millennium to make ales, wine and bread. But the second great beer innovation was the origins of lager beer during the 15th century, when Bavarians first noticed that beer stored in the caves during the winter continued to ferment (from the German lagern: to store). The result was a lighter and smoother beer that, after sharing it with their neighboring Bohemians, went on to dominate 19th and 20th century beers tastes, especially in America. 

Lager yeasts are hybrid strains, made of two different yeast species, S. cerevisiae and S. eubayanus, which was discovered in 2011. Lagers now represent a whopping 94 percent of the world beer market. But the origins of different hybrid lineages has been a bone of contention for lager beer makers. 

In a new study in the journal Molecular Biology and Evolution, graduate student Emily Clare Baker, corresponding author Chris Todd Hittinger et al., attempted to solve the mystery. 

Taking advantage of a newly described wild yeast species from Patagonia, Saccharomyces eubayanus, the research team was able to complete and assemble a high-quality genome of S. eubayanus using next-generation sequencing. 

They compared it to domesticated hybrids that are used to brew lager style beers, allowing for the first time the ability of study the complete genomes of both parental yeast species contributing to lager beer. 

They show two independent origin events for S. cerevisiae and S. eubanyus hybrids that brew lager beers.

The findings show that domestication for beer making has placed yeast on similar evolutionary trajectories multiple times. In this context, these results suggest that the Saaz and Frohberg lineages (named for their area of origin) were created by at least two distinct hybridization events between nearly identical strains of S. eubayanus with relatively more diverse ale strains of S. cerevisiae.

"Lager yeasts did not just originate once. This unlikely marriage between two species, genetically as different from one another as humans and birds, happened at least twice. Although these hybrids were different from the start, they also changed in some predictable ways during their domestication," said corresponding author Chris Todd Hittinger of the University of Wisconsin-Madison.