Monday, September 19, 2016

The Evolution and Domestication of Yeast

Anyone who knows me is aware that I’m a beer guy. While I’m a cell biologist by training one of my passions for quite some time is beer; whether it be tasting or brewing the beverage. In a recent article in the journal Cell, a group led by geneticist Kevin Verstrepen at the University of Leuven sequenced the genomes from 157 strains of Saccharomyces cerevisiea used to make ale, wine, sake, and bread [1]. When beer and science come together only good things can happen.

Yeast has been used to make fermented beverages for thousands of years. A 5,000-year-old Sumerian tablet describes an ancient party where ingredients to make fermented beverages known as beer today were used. Probably the first record of a kegger party! Since that time strains of S. cerevisiea have been developed to meet different needs.

Domestication of industrial yeasts
Figure 1.  Representation of the history and domestication of yeast used form making beer and other types of alcohol as revealed through genotypic and phenotypic analysis. Credit: Gallone and Steensels et al Cell 2016

The researchers dated the earliest cultivated yeast strains to the 1500s, which is likely a consequence of beer production in Europe moving from pubs into monasteries (Fig 1). As these early brewers fine-tuned their recipes, they also selected for favorable yeast strains. Domesticated yeasts have a greater capacity to metabolize sugar, fewer distasteful byproducts, and weaker reproductive abilities, compared to their wild-type cousins.

What is interesting is that the industrial yeast used today came from only a few ancestral strains. Five large groups separated out genetically, with strains mainly clustered together according to their industrial purpose. Geographic boundaries further divided each category: in one grouping of beer yeast, for example, the strains from Belgium and Germany were closely related, but separate from those in the UK and US [2].

As a brewer I know that the flavor of the beer depends greatly on the yeast. While many different beers can be made with the same strain using different grain mixtures, some beers that have very specific traits, such as the smoky clove and banana flavoring of German Hefeweizen, require specific strains. Hefeweizen requires the production of the compound 4-vinyl guaiacol (4-VG) in order to impart these unique flavors. These same flavors are considered flaws in other beer types. The genomes of the strains used to make Hefeweizen contain stretches of DNA, including the genes that make 4-VG, that seem to originate from wine yeast. It has been speculated that these strains emerged when an ale strain hybridized with a wine-making yeast, regaining the capacity to make the clove-smelling chemical.

Another point is that wine yeasts, which share their origins with beer yeast, show fewer signs of domestication. "This is probably because wine yeasts are only used to ferment grape juice once a year, and survive in and around the winery for the rest of the year, where they may interbreed with feral yeasts," Brigida Gallone and Jan Steensels of the University of Leuven told Scientific American.  "In that sense, beer yeasts are like dogs, completely 'tamed' and adapted to their relation with humans, whereas wine yeasts resemble the wilder character of cats." [3].

Me, I’m a beer guy who likes cats...

References
  1. B. Gallone and J. Steensels et.al (2016) Domestication and Divergence of Saccharomyces cerevisiea Beer Yeasts, Cell, 166(6):1397-1410.
  2. http://phys.org/news/2016-09-beer-yeasts-dogs-wine-cats.html#jCp
  3. http://www.scientificamerican.com/article/ale-genomics-how-humans-tamed-beer-yeast/

By: BioTek Instruments, Paul Held, PhD., Laboratory Manager

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