HO-induced switching and hybridization
Another method for creating hybrids that remains unexplored for brewing yeast breeding is HO- induced hybridization (Fig. 5.4C), which was originally reported as a method to dissect the genetic basis of pentose utilization in a naturally occurring sterile hybrid yeast (Schwartz et al., 2012). This technique involves transforming each parent strain with a plasmid that can transiently induce expression of the site-specific HO endonuclease, which facilitates mating type switching in higher ploidy cells (normally the HO endonuclease enzyme is active only in haploid cells as described in ‘Life cycle of Saccharomyces yeasts; genomes, ploidy, aneuploidy, and interspecific hybrids' above). Each parent strain is transformed with a plasmid containing an inducible HO gene; each plasmid has a different dominant selectable marker, enabling easy selection of successful mating events on double selective media. The HO-induced hybridization method works on a principle similar to rare mating, where normally mating-incompetent higher ploidy cells switch to mating type ‘a' or ‘a'; however, in this case the switch occurs at a higher rate due to HO enzyme induction. Once mating has occurred, the resulting hybrid yeast can be grown non-selectively for several generations. This causes the yeast to ej ect the plasmids, leaving no trace of genetic modification in the hybrid (Fig. 5.4C).
This method has been successfully used to create a tetraploid saison ale yeast hybrid (D.J. Kvitek, unpublished), formed between two parental saison yeasts with very different characteristics: parent one is a classic saison yeast with an earthy phenolic aroma profile but poor fermentation characteristics, while parent two is a very strong fermenting saison yeast with a subdued ester-dominant aroma profile. The hybrid between these yeasts produced a beer that combined the earthy aroma of parent one with the strong fermenting characteristics of parent two. The hybridization of these two strains thus eliminated the need to pitch multiple strains of yeast into the same wort to get the desired combination of aromas and fermentation behaviour; instead, the same results were obtained from a single hybrid strain, making both brewing and yeast strain maintenance easier and more streamlined. Recently, a similar HO-induced hybridization method has been reported and used to create synthetic interspecific hybrid lager-, Belgian ale- and cider-type yeasts (Alexander et al., 2016). These studies suggest that this method of generating both interspecific and intraspecfic hybrids, especially for cases where the desired parental strains do not produce viable spores, could be very useful in generating brewing yeasts with novel flavour and aroma profiles that can be further combined with desired fermentation behaviours.