Isolation and characterization of haploid heterothallic beer yeasts

Area of Science:

• Yeast genetics and breeding
• Microbial fermentation
• Brewing science

Background:

• Conventional breeding of ale and lager yeasts is challenging due to reduced sexual reproduction in domesticated strains.
• Kveik ale yeasts exhibit high spore viability, suggesting potential for breeding applications.
• Lager yeasts, hybrids of Saccharomyces cerevisiae and Saccharomyces eubayanus, have evolved with limited reproductive capabilities.

Purpose of the Study:

• To investigate the potential of Kveik ale yeasts for conventional yeast breeding.
• To isolate and characterize stable mating type strains from Kveik yeasts.
• To explore the utility of these strains for developing improved lager yeast varieties.

Main Methods:

• Isolation of heterothallic colonies from germinated Kveik yeast spores.
• Confrontation assays to confirm stable mating types.
• Analysis of HO gene mutations and their effect on mating type switching.
• Fermentation analysis of maltose and maltotriose.
• Identification of mutations in PAD1 and FDC1 genes.

Main Results:

• Stable MATa and MATα haploid Kveik ale yeast strains (Odin, Thor, Freya, Vör) were successfully isolated.
• Heterothallism was linked to inactivating mutations in the HO gene, including a novel G217R amino acid change.
• One strain (Odin) exhibited mutations in PAD1 and FDC1, resulting in a POF- phenotype (no phenolic off-flavors).
• All four strains fermented maltose and maltotriose, with Freya and Vör confirmed as haploid.

Conclusions:

• Haploid Kveik ale yeasts with stable mating types are accessible for conventional breeding.
• These strains provide a powerful genetic tool for improving both ale and lager yeasts.
• The development of POF- strains opens avenues for novel lager yeast development and enhanced beer flavor profiles.