HYBRIDIZATION STUDIES INVOLVING SACCHAROMYCES LACTIS AND ZYGOSACCHAROMYCES ASHBYI

Abstract

The spp. S. lactis, S. fragilis, S. marxianus, Z. ashbyi and Z. dobzhanskii belong to a group which is different from the type represented by S. cerevisiae. Hybridization is readily accomplished in the former spp. simply by mixing compatible strains of two spp. on an appropriate medium in the form of an agar slant and transferring the mixture four times at 7 to 10-day intervals. Some strains of most spp. of this group produce small amounts of a red pigment closely related to pulcherrimin. The hybrids generally produce more pigment than their parents and can be easily isolated from them by this property. S. lactis is a heterothallic haploid, and Z. ashbyi is a homothallic, predominantly diploid. When strain Y-1140 of the former is hybridized with Y-1598 of the latter, red colonies of the hybrid are produced. The hybrids are stable, large-celled, and vigorous, but practically nonascosporogenous. When strain Y-1205 of S. lactis is hybridized with Y-1598, some of the hybrids sporulate profusely and the ascospores are germinable, some are heterothallic and react scantily with the opposite sex, and some are not sexually reactive and produce no spores. The ascospores are isolated by killing the vegetative cells with heat, followed by streaking the heated culture on agar medium. S. lactis assimilates the alpha-glucosides maltose, melezitose, alpha-methylglucoside, and trehalose, but Z. ashbyi has no action whatever on the first three and extremely weak action on trehalose. The hybrids assimilate all of the alpha-glucosides and nearly all of them remain stable in this respect. Some ascosporic isolates of the hybrids act on the alpha-glucosides and others do not. Those that do gradually lose this ability as they reproduce asexually. The ascosporic isolates range from white to red and they vary in the ratio of diploid to haploid cells which they produce, and white diploid ascosporic cultures are identical to S. fragilis which is believed to be a naturally occurring hybrid of S. lactis x Z. ashbyi.