Selecting, Inbreeding, Recombining, and Hybridizing Commercial Yeasts

Abstract

Saccharomyces cerevisiae contains a single pair of alleles controlling copulation for which all legitimate diploids are heterozygous. Continued inbreeding does not result in degeneration, provided the strain is kept heterozygous for these alleles. Naturally occurring yeasts are extremely heterozygous for other genes controlling vigor, fermentative ability, color, etc. They are unsuited for use in the breeding program until the inferior genes have been bred out. This can be accomplished by selection of the legitimately diploid progeny produced by isolating a single intact ascus and allowing cross-copulation to occur. The zygote is tested for heterozygosis of dysgenic genes by growing the spores from the inbred asci singly and testing the cultures thus produced.