Brewing up a storm: The genomes of lager yeasts and how they evolved.

Yeasts used in the production of lager beers belong to the species Saccharomyces pastorianus, an interspecies hybrid of Saccharomyces cerevisiae and Saccharomyces eubayanus. The hybridisation event happened approximately 500-600years ago and therefore S. pastorianus may be considered as a newly evolving species. The happenstance of the hybridisation event created a novel species, with unique genetic characteristics, ideal for the fermentation of sugars to produce flavoursome beer. Lager yeast strains retain the chromosomes of both parental species and also have sets of novel hybrid chromosomes that arose by recombination between the homeologous parental chromosomes. The lager yeasts are subdivided into two groups (I and II) based on the S. cerevisiae: S. eubayanus gene content and the types and numbers of hybrid chromosomes. Recently, whole genome sequences for several Group I and II lager yeasts and for many S. cerevisiae and S. eubayanus isolates have become available. Here we review the available genome data and discuss the likely origins of the parental species that gave rise to S. pastorianus. We review the compiled data on the composition of the lager yeast genomes and consider several evolutionary models to account for the emergence of the two distinct types of lager yeasts.