Induction of Normal Ascosporogenesis in Two-Spored Saccharomyces cerevisiae by Glucose, Acetate, and Zinc

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In an apomictic strain of Saccharomyces cerevisiae that normally produces two-spored asci, production of three and four-spored asci was restored by adjusting zinc and carbon source levels in presporulation and sporulation media. Sporogenic capability was compared after growth in defined and undefined presporulation environments. Cultivation in defined presporulation medium containing 6% glucose followed by sporulation in 3.5% acetate yielded 16 ± 3% three- and four-spored asci. Inclusion of 25 μg of zinc sulfate per ml in presporulation medium or 25 μg of zinc acetate per ml in sporulation medium gave a further increase in yield of three- and four-spored asci; it reached 48 ± 1% when zinc was added to both media. Cells grown in undefined presporulation medium were incapable of three- and four-spored ascus development in acetate sporulation medium unless zinc was included in the latter medium. Cultivation in defined presporulation medium amended with yeast extract, peptone, or casein hydrolysate greatly reduced three- and four-spored ascus yields. Inclusion of a meiotic inhibitor (glucose) in sporulation medium inhibited three- and four-spored ascus production but not two-spored ascus production. Nuclear staining demonstrated two classes of asci: binucleate (one and two spored) and tetranucleate (three and four spored). Hence, some control over apomictic versus normal ascus development has been gained by nutritional manipulation.

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