The Genetics of Beer
Ale, fermented using the yeast Saccharomyces cerevisiae, has been brewed since ancient times, possibly as early as 6000 BC. In contrast, lager beer, with its hallmark low-temperature fermentation (5°C–14°C), is a more recently developed alcoholic beverage, arising in Bavaria near the end of the Middle Ages. Lager gained worldwide popularity from the late 1800s with the advent of refrigeration, which allowed the necessary cool fermentation temperatures year-round. The lager yeast, Saccharomyces pastorianus, is distinct from S. cerevisiae in both physiological and genetic characteristics and is thought to have arisen in response to selective pressures from cold brewing temperatures. This selection may have taken place during successive rounds of cold-temperature fermentations resulting from a 16th century Bavarian law that prohibited brewing during summer months because of the inferior quality of summer-brewed beers. S. pastorianus has been shown to be a hybrid organism, and it is likely that lager yeast arose by “instantaneous speciation” due to an interspecific hybridization event between Saccharomyces cerevisiae and Saccharomyces bayanus that occurred during these selective growth conditions.
By examining the genome of S. pastorianus, a recently-published paper shows that the hybridization between yeast species which gave rise to lager yeasts happened independently at least twice, not once as previously thought, giving rise to two broad families of lager beer, Group 1 yeasts used to brew “Saaz”-type beers such as Pilsner and Budweiser, and Group 2 yeasts used to brew “Frohberg” lagers such as Orangeboom and Heineken. Both groups contain multiple copies of genes beneficial to brewing, such as those that ferment maltose. Likewise, genes that adversely affect the process have been lost.
This work paves the way for characterization of specific genetic features of each strain that could aid in the brewing process, and could lead to new insights on how to directly control flavor and aroma in beer.
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Reconstruction of the genome origins and evolution of the hybrid lager yeast Saccharomyces pastorianus. Genome Research, September 11, 2008
Inter-specific hybridization leading to abrupt speciation is a well-known, common mechanism in angiosperm evolution; only recently, however, have similar hybridization and speciation mechanisms been documented to occur frequently among the closely related group of sensu stricto Saccharomyces yeasts. The economically important lager beer yeast Saccharomyces pastorianus is such a 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. Here, 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 array-CGH 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.
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Tags: Biology, Biotechnology, Food, Fungi, Genetics, Microbiology, Mycology, Podcast, Science
