MEIOTIC DIPLOID PROGENY AND MEIOTIC NONDISJUNCTION IN SACCHAROMYCES CEREVISIAE

Silvio Sora ¹, Giovanna Lucchini ¹, and Giovanni E. Magni ¹

¹ Istituto di Genetica, Universitá di Milano, Via Celoria 10, 20133 Milano, Italy

Abnormalities in chromosome number that occurred during meiosis were evaluated with a specially-constructed diploid strain of Saccharomyces cerevisiae. The strain is heterozygous for six markers of the right arm of chromosome V and heterozygous for cyh2 (resistance to cycloheximide) on chromosome VII.—Selection of meiotic spores on a medium containing cycloheximide and required nutrilites—except those for the markers of the right arm of chromosome V—allows the growth of aberrant clones belonging only to two classes: a) diploid clones, caused by failure of the second meiotic division, with a frequency of 0.54 x 10^-4 per viable spore; and b) diplo V, aneuploids derived from nondisjunctions in meiosis I or meiosis II, with a total spontaneous frequency of 0.95 x 10^-4 per viable spore. About two-thirds of the aneuploids originated during meiosis I, the rest during meiosis II. An investigation of these events in control meioses and after treatment with MMS, Benomyl and Amphotericin B suggests that this assay system is suitable for screening environmental mutagens for their effects on meiotic segregation.

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