Sporulation in was disturbed by transferring cells to water after 4 to 8 h incubation in potassium acetate medium, giving rise to almost exclusively two-spored asci at a high frequency. The (+ -) asci for a given auxotrophic marker (+ and - indicate the phenotypes due to the dominant and recessive alleles of the marker, respectively) predominated over the (+ +) and (- -) asci, and the (+ +) asci predominated over the (- -) asci. The frequencies of these three ascus types were related to the distance of the marker from the centromere. Most of the segregants showed a mating reaction with either or α haploids, while a few were non- or omni-maters. These observations, along with statistical data on observed and expected frequencies of various ascus types, indicate that the two-spored asci are not generated from normal meiotic products by random abortion of two spores. An alternative mechanism is proposed in which the first meiotic division including crossing over at the four-chromatid stage occurs normally, but the second meiotic division is interrupted, possibly at centromere splitting and/or formation of the spindle apparatus. On this basis, equations describing theoretical frequencies of various ascus types were derived. Expected frequencies of the different ascus types for nine auxotrophic markers having various centromere distances and for mating types were calculated from the equations and compared with the data observed in 188 dyads from three different diploids. A uniformly good fit was found between expected and observed frequencies.


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