Guinness stout and Bud Lite differ in, to be conservative, several ways, but one is that they’re brewed with very different types of yeast. Lager isn’t just a beer style, it’s a yeast lifestyle. Humans have been brewing with ale yeast—Saccharomyces cerevisiae—for thousands of years. But it was less than 600 years ago that European brewers stumbled on lager yeast, which behaves very differently and produces that distinctive lager flavor.

Lager yeast is a cross of ale yeast with another species, but it took until 2011 for that other species to finally be identified in Patagonian forests. A new study led by University of Wisconsin-Madison researchers EmilyClare Baker and Bing Wang presents the genome of this recently discovered parent, Saccharomyces eubayanus.

By comparing the genome with the two strains of lager yeast around today, the researchers may have settled a dispute about the biological origins of lager yeast. Looking at the two strains, there are many more differences between the ale yeast portions of their genomes than have accumulated in the Saccharomyces eubayanus portions. This points to independent hybridization events starting with different ale yeast parents rather than a single hybrid that has since split into two strains.

So lager yeast was born at least twice through meetings of the two species, which means we can justifiably celebrate this style's birthday twice each year—perhaps with a nice lager beer.

Molecular Biology and Evolution, 2015. DOI: 10.1093/molbev/msv168 (About DOIs).