Figure 2.

Hypothetical routes to the origin of S. pastorianus. From genetics, it is known that S. pastorianus arose by the hybridization of S. cerevisiae and S. eubayanus but the question is where and when this occurred. One theory is that S. pastorianus emerged in Bohemia and became part of the Bavarian Stellhefen mixture (Bavarian pitching yeast mixture) that came to be used in the Hofbräuhaus in Munich (1). The weakness with this argument is that it posits a Bohemian origin to both S. eubayanus and S. pastorianus, which are bottom-fermenting yeasts, whereas top-fermentation was the norm in Bohemia. The alternative hypothesis that emerges from our historical research is that S. eubayanus was part of the normal community of bottom-fermenting yeasts used in Bavaria from the 14th century and the critical event took place in the Hofbräuhaus in Munich early in the 17th century when particular top-fermenting S. cerevisiae strains entered the brewery. The attractive feature of this hypothesis is that historical records document two such introductions at the time that phylogenetic calculations say that the hybridization happened. These introductions, from the wheat beer brewery in Schwarzach (1602) (2) and the Einbeck brewery (1612) (3) led to the unique situation of a brewery producing both top- and bottom-fermented beers at the same time. The relationship between the S. cerevisiae subgenome of S. pastorianus and the genome of modern wheat strains of S. cerevisiae may favour the Schwarzach origin, but both are plausible. Perhaps future work that will uncover more genomic information about the ancestral populations of S. eubayanus and S. cerevisiae in the different regions will ultimately resolve the issue.