An efficient method for isolating mating-competent cells from bottom-fermenting yeast using mating pheromone-supersensitive mutants.

Crossbreeding is an effective approach to construct novel yeast strains with preferred characteristics; however, it is difficult to crossbreed strains of brewer's yeast, especially the bottom-fermenting yeast Saccharomyces pastorianus, because of the relative inefficiency of the available methods to obtain mating-competent cells (MCCs). Here, we describe a productive method for the isolation of MCCs without artificial genetic modification. We focused on the characteristics of two mating pheromone-supersensitive mutants, Δbar1 and Δsst2, that show a growth defect in the presence of the mating pheromone. When MCCs secreting α-factor and a-factor were spotted on to a lawn of MATa Δbar1 and MATα Δsst2, a halo was observed around the respective MCCs. This plate assay was successful in identifying MCCs from bottom-fermenting yeast strains. Furthermore, by selecting for cells that caused the growth defect in pheromone-supersensitive cells on cultures plates, 40 α/α-type and six a/a-type meiotic segregants of bottom-fermenting yeast strains were successfully isolated and crossed with tester strains to verify their mating type. This method of isolation is expected to be applicable to other industrial yeast strains, including wine, sake and distiller's yeasts, and will enable MCCs without genetic modifications to be obtained. As a result, it will be a useful tool for more convenient and efficient crossbreeding of industrial yeast strains that can be applied to practical brewing. Copyright © 2017 John Wiley & Sons, Ltd.

Generation of new hybrids by crossbreeding between bottom-fermenting yeast strains.

The genetic diversity of bottom-fermenting yeast classified as Saccharomyces pastorianus is poor because strains are restricted to a few genetically distinct groups. Crossbreeding is an effective approach to construct novel yeast strains, but it is difficult because of inefficiency to obtain mating-competent cells (MCCs) of bottom-fermenting yeast. By using mating pheromone-supersensitive mutants, we previously isolated several mating-competent meiotic segregants from two bottom-fermenting yeast strains: high isoamyl acetate-producing KY1247, and low diacetyl-producing KY2645. Here, we constructed novel non-GM hybrids carrying preferable characteristics from both parents by crossbreeding these bottom-fermenting strains for the first time. Sixteen a/a-type meiotic segregants from KY2645 and 12 α/α-type meiotic segregants from KY1247 were mixed, and cells resembling zygotes were isolated via micromanipulation. In total, 149 hybrids were obtained and verified by examining known single-nucleotide polymorphisms (SNPs) between the parental strains. A sporulation test showed that some of the hybrids were able to sporulate. Moreover, fermentation tests on a test-tube and pilot-plant scale identified two hybrids with production levels of isoamyl acetate and diacetyl that were almost the same as KY1247 and KY2645, respectively. Both of these hybrids produced satisfactory beer in terms of taste, flavor, and overall quality, comparable to that produced by the parental strains. Collectively, our results suggest that crossbreeding between bottom-fermenting yeast strains has the potential to increase the diversity of yeast strains available for brewing, and our method of isolating MCCs provides a huge advance for crossbreeding of bottom-fermenting yeast without using DNA recombination techniques.