We may have to reevaluate whether Saccharomyces cerevisiae alone is the species used to brew beer. A paper from Gonzalez et al describes results from PCR–RFLP comparison of 24 brewing strains identifies evidence for S. cerevisiae x S. kudriavzevii hybrids. Although this hybridization is not unprecedented, most seem to be related to cultivated brewing or fermentation strains. It seems that the hybrids are better able to cope with the stress associated with fermentation process.
It seems these would also be a great test system for more whole genome sequencing or at least more polymorphism comparisons to try and determine the proportion of the genome that comes from different parents and estimate timing and frequency of hybridization. It seems possible that the hybridizations are occurring multiple times in nature so are the same regions from each parental genome kept in the hybrid offspring that are selected for fitness under fermentation stress?
Gonzalez, S.S., Barrio, E., Querol, A. (2008). Molecular Characterization of New Natural Hybrids of Saccharomyces cerevisiae and S. kudriavzevii in Brewing . Applied and Environmental Microbiology, 74(8), 2314-2320. DOI: 10.1128/AEM.01867-07