Saccharomyces pastorianus

Saccharomyces pastorianus is a yeast, used industrially for the production of lager beer. 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.

Genomics
As S. pastorianus is a hybrid of Saccharomyces bayanus and Saccharomyces cerevisiae it is not surprising that there is 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. However there is growing evidence that S. pastorianus has inherited most of its genetic material from S. bayanus. Indeed the mitochondrial DNA and ribosomal DNA of S. pastorianus appear to be derived from S. bayanus rather than S. cerevisiae.

The genomic difference between S. pastorianus and S. cerevisiae is responsible for a number of phenotypic traits which S. pastorianus share 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, whereas S. cerevisiae will grow at 37°C. S. pastorianus exhibits a higher growth rate than S. cerevisiae at 6 to 12°C.

It has been suggested that S. pastorianus may be a hybrid of S. cerevisiae and Saccharomyces 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 that 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 (Casey, 1996). It is thought that the lager strains in use are derived from only one or two primary strains; Tuborg and Carlsberg (Casey, 1990; Casey, 1996).

Saccharomyces uvarum
Saccharomyces uvarum is a species of yeast that is believed to originate 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 that top-fermenting yeast does.

Further research was done by Martinus Willem Beijerinck in 1898, it was synonymized with S. bayanus but revived by Nguyen & Gaillardin in 2005.