NEW YORK (GenomeWeb News) – New genomic research suggests that the yeast strains behind crisp, clear lager-type beers arose at least twice — probably from hybridizations involving ale yeasts.
Stanford University geneticists Barbara Dunn and Gavin Sherlock used customized DNA microarrays and sequencing to characterize 17 different Saccharomyces pastorianus strains from European and North American breweries. Their genomic analyses, scheduled to appear online today in the journal Genome Research, suggest that two groups of lager yeast resulted from independent hybridizations between baker’s yeast, Saccharomyces cerevisiae, and a cold-tolerant yeast called Saccharomyces bayanus.
Over time, these location- and brewery-specific lager yeast lineages have undergone distinct genetic changes that provide insights into the history — and perhaps the future — of beer production, revealing how brewing shifted from cold-sensitive ale production to cold-tolerant lager production.
“It is likely that each of these groups derived the S. cerevisiae portions of their genomes from distinct but related ale yeast, and that these natural hybrids were then selected by brewers due to their abilities to ferment at cold temperatures,” Sherlock said in a statement. “The fact that lager yeasts isolated from different breweries each seem to have a unique genomic make-up may indicate that the yeasts are adapting to the conditions specific to each brewery.”
Lager beers, the most widely sold beers in the world, originated in central Europe in the Middle Ages. They’re significantly younger than ales — fruitier, full-bodied beers fermented from S. cerevisiae — which some believe have been around for thousands of years. One of the key differences: lagers are fermented at low temperatures, owing to a yeast species called S. pastorianus that can ferment efficiently at low temperatures.
Although budding yeast are capable of both asexual and sexual reproduction, S. pastorianus appears to have arisen via hybridization or fusion between yeast cells of different species. Research in the 1980s demonstrated that S. pastorianus is a hybrid species between S. cerevisiae and different varieties of the more cold-tolerant yeast, including S. bayanus.
“It’s possible that the ale strain provides a certain flavor profile, while the second strain conferred the ability to ferment at cooler temperatures,” Dunn said. “Mixing them together is a nice way for the yeast to double its genetic options.”
For the latest research, Dunn and Sherlock focused on lager yeast strains resulting from hybridizations between S. cerevisiae and S. bayanus, looking at the genomic structure of 17 lager strains originally collected at European and North American breweries in between 1883 and 1976.
Using array-CGH and DNA sequencing of selected genes from lager and ale yeast strains, they classified the S. pastorianus strains, which fell into two distinct groups correlating with geographical location and brewery groups. From there, the duo started unraveling the genomic architecture and ancestry patterns of each yeast group.
The first group — found in Saaz beers originating in what is now the Czech Republic and in Carlsberg brewery strains from Denmark — contained almost all of the S. bayanus genome but had lost large sections of S. cerevisiae genome. In contrast, the second group — collected at breweries in the Netherlands, Denmark, and North America — contained both genomes.
Despite their differences, the researchers were able to identify breakpoints and ploidy changes that were shared in both lager yeast groups and clustered in certain parts of the genome.
Using their new understanding of the strains’ genomic architecture, Dunn and Sherlock also began reconstructing the ancestral S. pastorianus genomes. By comparing sequences from the lager strains, three existing ale yeast strains, and 37 other S. cerevisiae strains, the researchers determined that the two S. pastorianus groups probably arose separately from ale yeast strains — conceivably at breweries where ale beers were being produced.
“These long-ago brewers were practicing genetics without even knowing it,” Sherlock said in a statement. “They’ve given us a very interesting opportunity to look at a relatively young, rapidly changing species, as well as some very good beer.”