Ha! As I wrote yesterday, researchers in yeast genetics are changing the story on the history of yeast all the time, and the day I put that post up, new findings on the genetics of lager yeast came out which, as New Scientist reported, take the hybridisation narrative further down the road to a fascinating destination.
To quote New Scientist, Gavin Sherlock and Barbara Dunn of Stanford University, California, compared the genes of 17 lager and ale yeast strains across the world, with origins dating from between 1883 and 1976, and derived from breweries as diverse as Carlsberg and Labatt, Rainier and Heineken:
It has long been thought that Saccharomyces pastorianus, the yeast used in lager production, formed only once from the hybridisation of S. cerevisiae and S. bayanus. Instead, the team discovered that it happened at least twice in two separate locations in Europe, giving rise to the two different lager families … The hybrid, which makes lager instead of ale, probably evolved in Bavarian beer-brewing cellars during the 16th century.
The team also found that Saaz yeasts have a single copy of each parent yeast’s genome, whereas the Frohberg yeasts have an extra copy from S. cerevisiae. They believe this difference affects the flavour of the lager, as well as how quickly the yeasts can ferment the hops
[my emphasis, and sic, fer gawd's sake. Bloody journalists ... do they know nothing?]
Actually, it’s been suspected since 2005 that the two main types of lager yeast recognised by brewers, Saaz (which is weakly attenuating, and was found in a brewery in Saaz, or Zatec, in Bohemia by a German biochemist called Lindner around 1895) and Frohberg (strongly attenuating, found by Herr Lindner in a German brewery), might each be the result of separate hybridisation events. but anyway, the Stanford team seem to have amassed the evidence that make this look pretty definite.
There’s slightly more info from the team’s own abstract here and if you want the full paper, Reconstruction of the genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus it’s available as a pdf here.
Plucking out the plums from the full paper – and if you’re into this stuff, I really recommend you read it in full, it’s fascinating – their analysis suggests that of all the possible parents on the S cerevisiae side of things,
Our sequencing results also suggest that the S. cerevisiae parent, for both Group 1 [Saaz] and Group 2 [Fruhberg], may have been an ale strain; this would not be surprising as the progenitor hybrid lager yeasts almost certainly arose in a brewery where ale was produced.
This is not necessarily as obvious as it might seem – other strains of dear old S cerevisiae than the ale-brewing varieties are also found everywhere from the skins of fruit to the sap exuding from trees, and another possible, though less likely candidate as the S cerevisiae parent of lager yeast is a strain actually categorised as an ale spoilage yeast, rather than one to brew with.
Sherlock and Dunn say that
Either the S. pastorianus strains that we examined had two independent origins, one giving rise to the Group 1 [Saaz] strains and the other to the Group 2 [Fruhberg] strains, or there was a single origin, followed by an evolutionary bottleneck that resulted in Group 1 and Group 2 strains. We view the first hypothesis, that of multiple origins, more likely … albeit with closely related strains serving as the S. cerevisiae parent … The S. cerevisiae and S. bayanus parents [of the Fruhberg lager yeasts] would have been distinct from the parents of the Group 1 ancestor, but extremely closely related, showing only 0.3% and 0.1% sequence divergence, respectively … These … strains probably experienced their respective interspecific hybridization events on the order of mere hundreds of years ago.
In other words the hybrids most likely arose in a medieval or early modern brewing cellar, quite likely in Bavaria, where the practice of long, slow, cool brewings would have suited a yeast whose parents included S bayanus, which prefers life on the cold side. It is because they have continued to develop in a cool environment, Sherlock and Dunne suggest, that the two yeasts have lost more genes from their S Cerevisiae parent than from the bayanus one, encouraged by brewers repeatedly using the bottom-settling portions of the yeast for the next brew:
… cold temperatures may have driven the retention of the S. bayanus genome and loss of portions of the S. cerevisiae genome in Group 2; flocculation and other brewing-related selections undoubtedly have also played a role in both groups
None of the lager yeasts Sherlock and Dunn analysed were ones from current brews, but the pair say their findings suggest that
at least some, and possibly all, current production lager strains belong to the Group 2 type [Fruhberg] lager yeast class [which] have experienced the least loss of S. cerevisiae sequences from the hybrid genome … which may be of importance in obtaining successful lager fermentations under current brewery practices.
Sherlock and Dunne don’t speculate, particularly, on how we ended up with two types of lager yeast, but here’s my WAG (I like to speculate, but I try very hard not to present my speculations as in any way better than a slight possibility), based on the fact that S and D reckon Saaz yeasts seem to be best associated with the Carlsberg brewery, and also down in Bohemia, while Frohberg yeasts are predominant in Germany and the Netherlands, and yet the two types’ parents were so similar.
Hybrids between ale strains of S cerevisiae and S bayanus, which is what both the Saaz and Frohberg strains are, only really thrive when they’ve got malt sugars to eat and a cold environment where they can outperform ordinary S cerevisiae, which prefers it warm. The only place that was happening the few hundred years ago these two strains came into existence was in the cellars of Bavarian brewers. So the place where both these hybrid forms were born was almost certainly Bavaria. That explains the similarity of the parents,
They most likely both formed part of a mixed collection of cold-loving, bottom-dropping hybrid yeasts that Gabriel Sedlmayr and the other Bavarian master-brewers used to make their lagers, and when Sedlmayr started giving his Bavarian bottom-fermenting yeast away, I’d guess, both types were carried off around Europe to be used by brewers such as Heineken in the Netherlands and Jacobsen at Carlsberg in Denmark.
Then, and this is where I’m really speculating, Emil Christian Hansen, in his research into single-strain yeast, isolated the Saaz type, propagated it and gave it away, seeing it spread at least as far as Bohemia. When the Dutch and others picked up on Hansen’s ideas of pure yeast culture propagation, they chose, I’m guessing, the Frohberg strain out of the mixed colection they had received from Bavaria, rather than the Saaz one Hansen picked.
Sound plausible? All comments from yeast geneticists gratefully received ….