Enhanced flavour profiles through radicicol induced genomic variation in S. pastorianus lager yeast

The yeasts, Saccharomyces pastorianus, are hybrids of Saccharomyces cerevisiae and Saccharomyces eubayanus and have acquired traits from the combined parental genomes such as ability to ferment a range of sugars at low temperatures and to produce aromatic flavour compounds, allowing for the production of lager beers with crisp, clean flavours. The polyploid strains are sterile and have reached an evolutionary bottleneck for genetic variation. Here we describe an accelerated evolution approach to obtain lager yeasts with enhanced flavour profiles. As the relative expression of orthologous alleles is a significant contributor to the transcriptome during fermentation, we aimed to induce genetic variation by altering the S. cerevisiae to S. eubayanus chromosome ratio. Aneuploidy was induced through the temporary inhibition of the cells stress response and strains with increased production of aromatic amino acids via the Shikimate pathway were selected by resistance to amino acid analogues. Genomic changes such as gross chromosomal rearrangements, chromosome loss and chromosome gain were detected in the characterised mutants, as were Single Nucleotide Polymorphisms in ARO4, encoding for DAHP synthase, the catalytic enzyme in the first step of the Shikimate pathway. Transcriptome analysis confirmed the upregulation of genes encoding enzymes in the Ehrlich pathway and the concomitant increase in the production of higher alcohols and esters such as 2-phenylethanol, 2-phenylethyl acetate, tryptophol, and tyrosol. We propose that the plasticity of polyploid S. pastorianus genomes is an advantageous trait supporting opportunities for genetic diversity in otherwise sterile strains. Significance StatementLager beer is the product of fermentations conducted with Saccharomyces pastorianus, which are hybrids of Saccharomyces cerevisiae and Saccharomyces eubayanus. A quintessential property of lager beers is the distinctive flavours produced during fermentation. Hybrids are sterile and have reached an evolutionary bottleneck. Finding ways to introduce genetic variation as a means of enhancing the flavour profiles is a challenge. Here, we describe an approach to introduce genetic variation by inducing aneuploidy through the temporary inhibition of the cells stress response. Strains with an enhanced flavour production were selected by resistance to amino acid analogues. We identified genomic changes and transcriptome analysis confirmed the upregulation of genes in the Ehrlich pathway which is responsible for the production of flavour compounds.