Curr Genet (1993) 24:45-52 Current Genetics © Springer-Verlag 1993 Polyploidy in the haplontic yeast Schizosaccharomycespombe: construction and analysis of strains M. Molnar, M. Sipiczki Department of Genetics, Universityof Debrecen, P.O. Box 56, H-4010 Debrecen, Hungary Received: 24 January 1993 Abstraet. The fission yeast Schizosaccharomyces pombe has a haplontic life cycle in which the diplophase is con- fined to the zygote. Through the use of one- and two-step protoplast fusions we show that the ploidy can be in- creased up to pentaploid. The polyploid fusion products are rather unstable and segregate cells of lower ploidies by gradual loss of chromosomes during mitotic divisions. The polyploid cells conjugate normally but are prone to arrest at various stages of meiosis (1-, 2- and 3-spored asci, binucleate spores) and/or produce inviable, most probably aneuploid, spores. Marker segregation in the complete tetrads indicates the multiple association of ho- mologous chromosomes. In tetra- and penta-ploid meio- sis, multispored (6- to 7-spored) asci are also produced, probably by postmeiotic division of the nuclei. Key words: Polyploidy - Fission yeast - Mitotic stability - Meiosis Introduction The fission yeasts classified in the genus Schizosaccha- romyces all show a strictly haplontic life cyclic (for a review see Sipiczki 1989). Under ordinary conditions they propagate as haploid cells; but as soon as the level of the nitrogen source in the medium falls below a critical threshold, cells of opposite mating types fuse by sexual conjugation. The resulting zygote functions as a transient diploid meiocyte, as it undergoes meiosis shortly after completion of nuclear fusion and converts into a zy- mosporangium (ascus) containing four (S. pombe) or eight (S. octosporus and S. japonieus) spores. This scheme precludes the formation of diploid organisms. However, by a sudden increase of nitrogen level in the medium during a very short period between the completion of conjugation and the onset of meiosis, the sexual develop- ment pathway in S. pombe can be interrupted and the Correspondence to. M. Molnar zygote can be turned to mitotic division instead of enter- ing meiosis (Gutz et al. 1974). The diploid clones arising in this way show very low stability because they cease to grow and undergo meiosis as soon as the nitrogen con- centration falls or when the culture conditions drastically change (Egel 1989). Diploids gain a significantly higher level of stability by infrequent mitotic recombination from which mating-type homozygosity can arise (Flores da Cunha 1970). Since the introduction of the protoplast fusion technique, non-sporulating diploids can also be constructed in a single step by induced somatic hybridiza- tion of vegetative cells (Sipiczki and Ferenczy 1977). Unlike S. pombe, polyploids are quite abundant in the diplontic yeast Saccharomyces cerevisiae. The industrial strains are commonly polyploid: most brewer's yeasts are tetraploid, some are triploid or aneuploid (for a review see Kielland-Brandt et al. 1983), and tetraploid strains of baker's yeasts have also been reported (Gunge 1966). Polyploids up to hexaploids are easily obtained by cross- ing, even in laboratory strains, and viable aneuploids frequently arise from tri- and hexa-ploid meiosis or from biased diploid meiosis (Mortimer 1958; Takagi etal. 1983; Whittaker et al. 1988). As this species normally alternates between haplophase and diplophase, its mei- otic and mitotic machineries are probably more suited to handling the increased number of chromosomes than analogous mechanisms of the haplontic fission yeast. The results presented in this report demonstrate that the difficulties in polyploid construction in S. pombe, arising from the haplontic nature of the organism, can be circumvented by the application of the protoplast fusion technique (Sipiczki and Ferenczy i977). Polyploidy pre- sents an unnatural state that causes frequent uneven mi- totic divisions and irregular distribution of chromosomes in meiosis. Materials and methods Media and growth conditions. Yeast strains were grown and sporu- lated at 30°C. Yeast-extract liquid (YEL), yeast-extract agar (YEA), minimalagar (MMA) and malt-extractagar (MEA)wereas