The traditional mass-mating protocol (i.e. mixing pools of spores; Fig. 5.3D) was employed by Wang and Hou (2010) using a diploid ale strain that spor- ulated well and produced viable spores. The starting diploid strain was subjected to both chemical and UV irradiation mutagenesis, then sporulated, and the haploid spores allowed to mass-mate in a random manner. The resulting mated diploid cells were then subjected to growth under increased sugar concentrations and ethanol concentrations, and the best-growing strains were chosen to repeat the process; a total of 3 rounds were performed. This resulted in a strain that showed increased tolerance to high sugar and ethanol concentrations, faster fermentation times in high sugar and ethanol, higher ethanol yields, and better flavour profiles. However, this method has seen limited use for brewing yeasts due to the frequent difficulty in sporulating and mating most such strains.
Mass mating and genome shuffling in asexual strains via transient HO induction
Traditional mass mating can be performed only with sexually competent yeast strains. However, it is possible that forced hybridization of asexual yeasts, via transient HO induction as discussed above (Fig. 5.4C), could yield sexually competent cells (e.g. by doubling an aneuploid or hybrid chromosome complement to make a functionally diploid cell) that are able to proceed through sporulation and produce meiotically recombined offspring. Performing several rounds of this regime could serve as a potent genome shuffling technique for genetically intractable strains, and allow screening or selection of strains with novel combinations of beneficial traits.