Beer made with Saccharomyces
The Beerologist

Saccharomyces Yeast Biology and Diversity in the Beer Brewery

Saccharomyces. When we say “homebrewing” or craft beer, most people immediately think about yeast. Not all yeast strains, however, are created equal. Even when looking at the diversity and distribution of yeast strains commonly used in homebrewing and craft beer production, there is a wide variety out there, massive untapped potential!

Therefore, an interesting question is how yeast biology and genome evolution fits beer diversity. In a very recent accepted manuscript (made available here after peer review but before publication), Bendixsen and authors examined the current literature describing Saccharomyces genomes, (inter)fertility and crucially, used the underlying datasets to build a comprehensive picture of yeast kinship. The analyses surveyed genome sequence, and fertility data for eight recognized Saccharomyces yeast species: S. arboricola, S. cerevisiae, S. eubayanus, S. jurei, S. kudriavzevii, S. mikatae, S. paradoxus, and S. uvarum. While some of these species are closely related, others are geographically and genetically distinct.

Investigating the evolutionary relationships between Saccharomyces species.

Geographical separation, specialization and speciation have led to distinct species, each of which has its evolutionary history. One can safely assume that all Saccharomyces species originate from a common ancestor that lived long ago. The divergence between isolated strains increases over time because yeast genomes experience equal mutation rates (a safe assumption) that accumulate over time. Therefore, comparing sequence divergence between strains can illuminate the relationships of species within the Saccharomyces genus.

The authors established these relationships using orthologous genes (functionally conserved genes in all the Saccharomyces species involved) and their sequences to estimate kinship within the genus. Their phylogenetic analyses showed that S. cerevisiae and S. paradoxus are sister species (closely related). The authors also revealed that S. eubayanus and S. uvarum, and S. mikatae and S. jurei, respectively, are sister species.

Example of phylogenetic tree used to describe evolutionary relationships between 8 Saccharomyces species
Example of a Phylogenetic tree, obtained from Baum (Fig 8, 2008). This tree illustrates the evolutionary relationship between species (denoted as A, B, C, D and E). Species that cluster together (e.g. A and B) are more closely related to each other and could be considered sister species, whereas A and D are more divergent. Gene or genome sequences can help establish levels of divergence (and thus relationships) by classifying organisms on the basis sequence variation.

A biological or environmental basis of divergence in Yeast?

Given the wide range and variation in divergence between species, a sensible question one could ask is whether species have emerged as a consequence of geographical isolation or selection and adaptation (in a particular environment). Another interesting question is to what extent hybridization between different species occurs. The authors aimed to answer these questions by surveying the literature and available datasets, describing Saccharomyces yeast genomes. From these analyses, the following stood out.

Saccharomyces synopsis

  • A large fraction of the yeast isolates surveyed turns out to be hybrid. Many isolates come from the wild, so yeast hybridization is common in the natural environment and may help adaptation to extreme conditions. One could even speculate that by looking for yeasts in challenging environments, you may increase the likelihood of finding hybrids.
  • Reproductive isolation can have a geographical and ecological component. When looking at spore viability as a measure of compatibility, in some species, geographically isolated members of a species feature reduced viability rates when crossed. In others, the ecological niche appears to play a role.
  • The fact that hybrid species are ubiquitous suggests a significant degree of inter-species fertility. The authors investigated that possibility by scouring the literature for fertility data. As expected, species within Saccharomyces could hybridize and reproduce to some extent. Importantly, fertility rates for each given species combination correlated with levels of divergence. In other words, closely related species could hybridize and create viable offspring, whereas viability dropped substantially when involving distinct species. 
  • There is an inverse correlation between fertility and viability. Strains that are more diverged tend to have lower hybridization potential though the authors did identify some interesting exceptions. Despite their close relationship, S. mikatae x S. jurei derived spores had very low viability. On the other hand, S. cerevisiae x S. kudriavzevii hybrids had unexpectedly high spore viability (though it was highly variable). Why these particular combinations seem to stray from these correlations, remain unknown. A better understanding of yeast biology may help illuminate this.

Why is this research important?

Firstly, the observation that both domesticated and wild yeast strains are hybrids illustrates the vast brew potential that exists naturally. We have only started to explore a little of the immense genetic and metabolic space within Saccharomyces. Sequencing the genomes of an increasing number of species and their hybrid relatives whilst characterizing their brew potential will undoubtedly herald a new era of beer making.

We hope that you enjoyed this read from The Beerologist. If you did, why not bookmark BrewingBrowser and pay us a regular visit?

We would love to see you again!

The Brewing Browser team.

P.S. We apologise for not including the original figures in this Article. Copyright restrictions prevented us from using materials from this unpublished but accepted manuscript.

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