S42: Genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus

The economically important lager beer yeast Saccharomyces pastorianus is an interspecific hybrid, formed by the union of Saccharomyces cerevisiae and Saccharomyces bayanus-related yeasts; efforts to understand its complex genome, searching for both biological and brewing-related insights, have been underway since its hybrid nature was first discovered. It had been generally thought that a single hybridization event resulted in a unique S. pastorianus species, but it has been recently postulated that there have been two or more hybridization events. Using microarray-based Comparative Genomic Hybridization (aCGH), we show that there may have been two independent origins of S. pastorianus strains, and that each independent group—defined by characteristic genome rearrangements, copy number variations, ploidy differences, and DNA sequence polymorphisms—is correlated with specific breweries and/or geographic locations. Finally, by reconstructing common ancestral genomes via aCGH data analysis and by comparing representative DNA sequences of the S. pastorianus strains with those of many different S. cerevisiae isolates, we have determined that the most likely S. cerevisiae ancestral parent for each of the independent S. pastorianus groups was an ale yeast, with different, but closely-related ale strains contributing to each group’s parentage.