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Commonly known as lager yeast to brewers, this yeast is a hybrid of ''S. eubayanus'' and ''S. cerevisiae'' <ref name="wikipedia_cereisiae" />. ''S. pastorianus'' is named after the first description by Max Reess in 1870 following his work with German breweries utilising bottom-fermenting lager yeast, naming it originally after Louis Pasteur.

For a long time the origins of the hybrid were unknown and was postulated to be a hybrid between ''S. cerevisiae'' and ''S.uvarum'', or ''S. cerevisiae'' and ''S.bayanus''. Recent work eg. Libkind et al 2011 proved that the hybridisation was between ''S. eubayanus'', which had been recently found in South America and ''S. cerevisiae''.  Separated into two main lineages, "Saaz" and "Frohberg". The two lineages are believed to have descended from different hybridisation events between ''S. eubayanus'' and ''S. cerevisiae''. The two lineages also have different genetic structure, with Frohberg types having two copies of each of the ''S. eubayanus'' and ''S. cerevisiae'' chromosomes (triploid), and Saaz types having one copy of the ''S. cerevisiae'' chromosomes and two copies of the ''S. eubayanus'' chromosomes (allotetraploid). (Ref: https://www.ncbi.nlm.nih.gov/pubmed/24578374 ) Emil Christian Hansen was the first in the world to isolate a single pure strain of ''S. pastorianus''while he was working at the Carlsberg brewery, leading to its first the synonymous classification name, ''S. carlbergensis''. The type that Hansen isolated was a Saaz type, and it's believed that Saaz types are also prevalent in Bohemian / Czech ''S. pastorianus'' strains while German types are typically Frohberg. ([https://onlinelibrary.wiley.com/doi/full/10.1002/yea.2960] , [https://genome.cshlp.org/content/18/10/1610.full] )

This is in addition to [https://www.whitelabs.com/yeast-bank/wlp051-california-v-ale-yeast WLP051 California V Ale] yeast is also ''S. pastorianus''. Recent gene sequencing / PCR work has led to it being re-classified as a ''S.pastorianus'' yeast, even though it has been used successfully for American-style Ale production.