Laboratory Saccharomyces yeast has a sexual life cycle like other higher organisms such as plants and animals. Haploid cells harbour one set of chromosomes (n = 16) and are either of mating type a or mating type a. Such haploid cells cannot sporulate. When mixed, a-cells can mate with a-cells, forming diploid (2n = 32) zygotes containing a double set of chromosomes. Both haploid and diploid cells can multiply asexually by budding. Under certain starvation conditions, diploid yeast cells can undergo sporulation, resulting in the formation of asci containing four spores. These spores contain the haploid (n = 16) number of chromosomes and can germinate giving rise to two a-, and two a-cell cultures. Lager brewing yeast strains are genetically more complicated, being species hybrids carrying the tetrapoid (4n = 64) number of chromosomes. Furthermore they are heterozygous (carrying more than one type of a certain gene). Sporulation and subsequent inter-crossing of the spore clones, may form new combinations of genes, resulting in yeast strains with altered characteristics, some of which may be attractive to the brewer.
Life Cycle of Saccharomyces Yeast