Industrially Applicable <i>De Novo</i> Lager Yeast Hybrids with a Unique Genomic Architecture: Creation and Characterization

Abstract

Lager beer is produced by <i>Saccharomyces pastorianus</i>, which is a natural allopolyploid hybrid between <i>Saccharomyces cerevisiae</i> and <i>Saccharomyces eubayanus</i> Lager strains are classified into two major groups based largely on genomic composition: group I and group II. Group I strains are allotriploid, whereas group II strains are allotetraploid. A lack of phenotypic diversity in commercial lager strains has led to substantial interest in the reconstitution of <i>de novo</i> allotetraploid lager strains by hybridization of <i>S. cerevisiae</i> and <i>S. eubayanus</i> strains. Such strategies rely on the hybridization of wild <i>S. eubayanus</i> isolates, which carry unacceptable traits for commercial lager beer such as phenolic off flavors and incomplete utilization of carbohydrates. Using an alternative breeding strategy, we have created <i>de novo</i> lager hybrids containing the domesticated <i>S. eubayanus</i> subgenome from an industrial <i>S. pastorianus</i> strain by hybridizing diploid meiotic segregants of this strain to a variety of <i>S. cerevisiae</i> ale strains. Five <i>de novo</i> hybrids were isolated which had fermentation characteristics similar to those of prototypical commercial lager strains but with unique phenotypic variation due to the contributions of the <i>S. cerevisiae</i> parents. Genomic analysis of these <i>de novo</i> lager hybrids identified novel allotetraploid genomes carrying three copies of the <i>S. cerevisiae</i> genome and one copy of the <i>S. eubayanus</i> genome. Most importantly, these hybrids do not possess the negative traits which result from breeding wild <i>S. eubayanus</i> The <i>de novo</i> lager strains produced using industrial <i>S. pastorianus</i> in this study are immediately suitable for industrial lager beer production.<b>IMPORTANCE</b> All lager beer is produced using two related lager yeast types: group I and group II, which are highly similar, resulting in a lack of strain diversity for lager beer production. To date, approaches for generating new lager yeasts have generated strains possessing undesirable brewing characteristics which render them commercially inviable. We have used an alternative approach that circumvents this issue and created new lager strains that are directly suitable for lager beer production. These novel lager strains also possess a unique genomic architecture, which may lead to a better understanding of industrial yeast hybrids. We propose that strains created using our approach be classified as a third group of lager strains (group III). We anticipate that these novel lager strains will be of great industrial relevance and that this technique will be applicable to the creation of additional novel lager strains that will help broaden the diversity in commercial lager beer strains.