Triploid meiosis and aneuploidy in Schizosaccharomyces pombe: an unstable aneuploid disomic for chromosome III

Summary Triploid meiosis in crosses between haploid and diploid strains of the fission yeast Schizosaccharomyces-pombe was investigated by tetrad analyses. Viability of the segregants was low. Only about 10% of the total tetrads contained four colony-forniing spores and most of these segregated into two haploids and two diploids. The remaining tetrads were rather normal in germination but were defective in colony formation. More than 50% of the total tetrads had no colony-forming spores while about 30% contained 1–3 colony-forming spores. Among the colony-forming segregants from the defective tetrads, a class of aneuploids disomic for the shortest chromosome III was obtained. In such aneuploid cells combined with a cold-sensitive ß-tubulin mutation, an additional short chromosome corresponding to chromosome III was observed by DAPI staining when the cells were incubated at a restrictive temperature. The other classes of aneuploids appeared able to be to germinate but not to enter into vegetative growth. Detailed genetical analyses indicated that recombination frequencies near the centromere were strikingly different between triploid and normal diploid meiosis.