Last summer (2018), we published a paper on the ’Muri’ hybrid. This strain was isolated from a yeast culture that Bjarne Muri had produced when attempting to revive his grandfather’s old kveik culture. In the paper we did some genetic and phenotypic characterization of the strain (a single cell isolate from Richard Preiss). The strain turned out to be a Saccharomyces cerevisiae × Saccharomyces uvarum hybrid, with substantial contributions from Saccharomyces eubayanus as well. In addition to the characterization, we also attempted to reconstruct the hybrid through hybridization of closely related parent strains.

Interspecies hybrids have been found multiple times from beer; the most famous hybrid of course being lager yeast (S. cerevisiae × S. eubayanus). However, S. cerevisiae × S. uvarum from brewing environments have not really been reported (they have been found from wine and cider though). So this was already an interesting finding. The ‘Muri’ strain behaves very differently from other kveik strains, reaching very high attenuations (thanks to being diastatic) and producing phenolic off-flavours. Genome sequencing also revealed that the hybrid is not related to the other kveik isolates. So the question was, had this strain really been a part of the Muri family kveik culture or was this some contaminant that had unintentionally been propagated during Bjarne’s revival attempts?

As Lars mentioned in his recent article about the strain, it looked like we would probably never get the answer to this question. However, by chance, I stumbled upon an interesting finding when I was going through the recently uploaded sequence data linked to this pre-print on S. eubayanus and its hybrids from the Hittinger lab that had been deposited to NCBI-SRA. The sequence data of one strain, WLP351 Bavarian Weizen, was deposited under ‘Saccharomyces cerevisiae × Saccharomyces eubayanus × Saccharomyces uvarum’. This immediately caught my interest, and I downloaded the data. After trimming, aligning to a concatenated reference genome of S. cerevisiae, S. eubayanus and S. uvarum, and variant calling (see methodology in our 2018 paper), it became more and more evident that WLP351 might actually be Muri (or rather Muri was WLP351 or a similar strain).

First of all, based on the read coverage across the reference genomes, it appears as if the S. uvarum sub–genome in both strains have the same S. eubayanus introgressions (Muri left, WLP351 right in the second image below). These are quite distinct to what has been reported in other studies for S. uvarum. If we perform phylogenetic analysis (together with other ‘Beer 2’/’Mosaic Beer’ strains) based on the SNPs present in the S. cerevisiae sub-genomes, we see that they are very similar in Muri and WLP351. Compared to the reference genomes, Muri and WLP351 share 86534 SNPs, and differ only at 470 sites.

So the evidence unfortunately points towards Muri actually being a contaminant and not a part of the original family kveik culture. Even with these new results, I still think the hybrid is very interesting, and the methods and analysis that we have performed in the paper are still relevant and valid.