HYBRIDIZATION STUDIES INVOLVING SACCHAROMYCES FRAGILIS AND ZYGOSACCHAROMYCES DOBZHANSKII

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Abstract
Non-chromogenic strain Y-610, S. fragilis, a homothallic diploid, was mixed with nearly nonchromogenic strain Y-1974, Z. dobzhanskii, a homothallic haploid, and the mixture serially transferred until hybrids had been produced. Parent S. fragilis neither ferments nor assimilates alpha-glucosides, but it ferments lactose; parent Z. dobzhanskii ferments maltose, melezitose, alpha-methylglucoside and trehalose but neither assimilates nor ferments lactose. The hybrids are isolated from the parents by serial transfer through maltose medium, then through lactose medium; this causes S. fragilis to drop out of the culture first; then Z. dobzhanskii is eliminated, leaving the hybrids. Some of the hybrids are white, and some are very faintly pigmented. Some of the ascosporic isolates of the hybrids are white, but most are deeply pigmented. S. fragilis characteristically produces reniform spores, those of Z. dobzhanskii are crescent-shaped, but many of the hybrids produced predominantly spheroidal spores, the shape of the spores of grandparent S. lactis. Hybrids ferment both lactose and alpha-glucosides. Ascospore isolates from the hybrids vary by minute degrees of intensity in their action on alpha-glucosides all the way from strong fermentation to weak assimilation of maltose, with no action on the others. All hybrids and their ascosporic isolates ferment lactose strongly or moderately.
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