Saccharomyces Hybrids: Generation And Analysis

Hybrids of Saccharomyces yeasts are found in many fermentative industries; without intentional selection they have been identified as key strains in fermentations of beer, wine, and cider. Hybrids appear to confer a selection advantage as they can inherit beneficial traits from both parent species. S. pastorianus is responsible for the fermentation of lager. In the UK; a country famed for its ale, lager accounts for two thirds of beer sold. Globally this is much higher. With the rise of craft brewing and pressure upon large breweries to increase efficiency, there is a growing demand for new and improved yeast strains. This pressure is a challenge, because most hybrids are sterile and cannot be improved through selective breeding. The sterility of hybrids also means they have not previously been the subject of quantitative genetic analysis. A key requirement of this analysis is a large and diverse segregating population created through mating. This project opens the door for quantitative genetics on hybrids for the first time. Utilising higher levels of ploidy, hybrid sterility has been overcome to create a large set of new hybrids. For performing genetic analysis, huge genetically diverse populations were generated through multiple rounds of interbreeding. Phenotypes analysed here are industrially relevant, and we have begun to reveal the loci responsible for these traits. S. pastorianus strains have been used in lager production for hundreds of years due to their strong fermentation performance at low temperatures, however diversity is lacking across the species. Through introgressing genetic material from S. cerevisiae isolates, it has been possible to increase the phenotypic diversity of S. pastorianus, as well as introduce fertility to these hybrids. Using a high throughput phenotypic screen, we have been able to identify individuals with desirable brewing traits and assess their fermentation performance. Most candidates displayed an enhanced performance relative to the hybrid parent strains. This demonstrated that through the introduction of fertility it is possible to create new and improved industrial hybrids.