Nguyen & Gaillardin ex. Beijerinck
Saccharomyces pastorianus is a yeast used industrially for the production of lager beer, and is named after Louis Pasteur. It is a synonym of the yeast species Saccharomyces carlsbergensis, which was originally described in 1883 by Emil Christian Hansen, who was working for the Danish brewery Carlsberg.
S. pastorianus is an interspecific hybrid between Saccharomyces eubayanus  and Saccharomyces cerevisiae, and has a degree of phenotypic and genomic similarity between the two species. The hybrid nature of S. pastorianus also explains the genome size, which is up to 60% larger than that of S. cerevisiae, as it includes large parts of the two genomes. There is growing evidence, however, that S. pastorianus has inherited most of its genetic material from an S. bayanus-like yeast. Indeed the mitochondrial DNA and ribosomal DNA of S. pastorianus appear to be derived from the S. bayanus-like parental strain rather than S. cerevisiae.
The genomic difference between S. pastorianus and S. cerevisiae is responsible for a number of phenotypic traits which S. pastorianus shares with S. bayanus, but not S. cerevisiae. The ability of S. pastorianus to break down melibiose is dependent on up to ten MEL genes, which are exclusive to strains metabolising melibose such as S. bayanus. S. pastorianus never grows above 34 °C (93 °F), whereas S. cerevisiae will grow at 37 °C (99 °F). S. pastorianus exhibits a higher growth rate than S. cerevisiae at 6 to 12°C.
S. pastorianus has been suggested as a hybrid of S. cerevisiae and S. monacensis, as the LEU2, MET2 and ACB1 genes of S. pastorianus had been reported to have a high level of similarity or be identical to the S. monacensis homologues. However, subtelomeric sequence hybridisation has suggested S. monacensis is likely to be a closely related hybrid to S. pastorianus, rather than an ancestor.
Ale strains are genetically more diverse than lager strains, as lager strains are thought to derive from a hybrid gene pool. The lager strains in use are thought to derive from only one or two primary strains, Tuborg and Carlsberg.
The yeast species Saccharomyces uvarum is believed to have originated as a hybrid of S. cerevisiae and S. monacensis, because of its allopolyploid genome. It is a bottom-fermenting yeast, so-called because it does not form the foam on top of the wort as top-fermenting yeast does.
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