Abstract
Lager-style beers constitute the vast majority of the beer market, and yet, the genetic origin of the yeast strains that brew them has been shrouded in mystery and controversy. Unlike ale-style beers, which are generally brewed with Saccharomyces cerevisiae, lagers are brewed at colder temperatures with allopolyploid hybrids of Saccharomyces eubayanus x S. cerevisiae. Since the discovery of S. eubayanus in 2011, additional strains have been isolated from South America, North America, Australasia, and Asia, but only interspecies hybrids have been isolated in Europe. Here, using genome sequence data, we examine the relationships of these wild S. eubayanus strains to each other and to domesticated lager strains. Our results support the existence of a relatively low-diversity (π = 0.00197) lineage of S. eubayanus whose distribution stretches across the Holarctic ecozone and includes wild isolates from Tibet, new wild isolates from North America, and the S. eubayanus parents of lager yeasts. This Holarctic lineage is closely related to a population with higher diversity (π = 0.00275) that has been found primarily in South America but includes some widely distributed isolates. A second diverse South American population (π = 0.00354) and two early-diverging Asian subspecies are more distantly related. We further show that no single wild strain from the Holarctic lineage is the sole closest relative of lager yeasts. Instead, different parts of the genome portray different phylogenetic signals and ancestry, likely due to outcrossing and incomplete lineage sorting. Indeed, standing genetic variation within this wild Holarctic lineage of S. eubayanus is responsible for genetic variation still segregating among modern lager-brewing hybrids. We conclude that the relationships among wild strains of S. eubayanus and their domesticated hybrids reflect complex biogeographical and genetic processes.
Highlights
Humans changed from living in hunter-gatherer societies to agricultural societies in part through the domestication of animals and plants [1,2]
In contrast to the process of introgression seen in domesticated strains of S. uvarum, lager yeasts were generated through allopolyploidization of S. eubayanus and S. cerevisiae
We have presented evidence that lager yeasts are derived from a relatively low-diversity lineage of S. eubayanus with a Holarctic distribution. These strains from the Holarctic lineage diversified from within one of two diverse populations found primarily in Patagonia
Summary
Humans changed from living in hunter-gatherer societies to agricultural societies in part through the domestication of animals and plants [1,2]. Humans began unwittingly domesticating microorganisms for the production of fermented beverages and foods, but the underlying source populations and genetic processes for microbial domestication are not well understood [3]. Beer is the most common fermented beverage in the world and can be classified as ale or lager, depending on the fermentation conditions and yeasts used. Ale-style beers are mainly produced by strains of S. cerevisiae [4]. 94% of the beer market is dominated by lager-style beers, which are fermented at colder temperatures by allopolyploid hybrids of S. cerevisiae x S. eubayanus
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