The first yeast breeding experiments aimed at combining desirable traits of brewing strains were conducted by Ojvind Winge during his tenure at the Carlsberg Laboratory in the 1930s
[37][35]. Hybrid yeast development has been carried out for over half a century since then, aimed predominantly at increasing attenuation and fermentation rates via intraspecific crosses with ale and lab strains
[38,39,40][36][37][38]. Modern fermentations benefit from many innate and acquired hybrids that have been isolated or developed
[11,27,28,41,42,43,44,45,46,47,48,49,50,51,52][11][27][28][39][40][41][42][43][44][45][46][47][48][49][50]. Early efforts in brewing science established the fundamentals necessary to explore the phylogeny, genomics, and strain development for
Saccharomyces fermentation. During typical rich nutrient propagations of yeast in a brewing environment, mother cells reproduce asexually to bud off small daughter clones (
Figure 2). Under poor nitrogen conditions, such as proline, yeast growth changes to a pseudohyphal form
[53,54][51][52]. The complete absence of a nitrogen source and the presence of a non-fermentable carbon source, such as acetate, will sporulate yeast cells
[55][53]. Sporulation transforms the cell wall into the ascus, or sack, that holds four spores termed a tetrad. Analogous to the human egg and sperm, these spores divide equally into mating types as either a or α
[56][54]. When conditions improve for yeast growth, new haploid (
1n) yeast can conjugate with the opposite mating type yeast as they form a shmoo.
Figure 2. Life and Mating of Saccharomyces Yeast. Diagram pertaining to the clonal growth typical of yeast fermentation cultures and the various known techniques employed to generate yeast hybrids. (a) Diploid yeast cells may bud and grow clonally to form a mother and daughter cell or undergo sporulation to form a tetrad. (b) Yeast hybridization may form by direct spore to spore mating. (c) Yeast hybridization may form by rare mating events in which one or both diploid parent cells gain competency by becoming hemizygous or MATa/MATa and MATα/MATα diploids. (d) Yeast hybridization may also form by fusion of two separate yeast cell protoplasts with their cell wall removed.
Interspecific hybridization is seen as a valuable tool for yeast strain development, enabling the combination and enhancement of characteristics from both parental strains or species
[57][55]. The development of hybrids is executed via three primary methodologies: spore–spore mating, rare mating, and protoplast fusion (
Figure 2). Spore to spore mating is most similar to what would be considered natural mating, as outlined in
Figure 2b. This approach bears a high success rate, high genomic stability, and can avoid the aid of selection markers such as drug resistance or autotrophies. Rare mating utilizes a described spontaneous loss of heterozygosity at the mating type locus. Normal diploid cells carry two sets of chromosomes with both the MATa and MATα genetic alleles and do not respond to sex pheromones for mating purposes. The spontaneous loss of either sex allele tolerates yeast mating to a yeast cell of complimentary sex. This results in yeast with high chromosome counts, influencing gene dosage during cellular processes and partially explains the outperformance over a diploid yeast of the same background
[57][55]. Rare mating, as the name implies, is uncommon and selection markers are needed to perform this technique. The frequency of rare mating is estimated to occur in 1 out of 10 million cells
[58][