The DNA content of fusion hybrids of Pichia guilliermondii was not further increased by renewed fusions. Occasionally, the stabilized hybrids only contained approximately thc DNA content of the corresponding wild-type strain. Investigation of the early events after fusion revealed spontaneous mitotic segregation with a high frequency. The benomyl-induced mitotic segregation pattern of the hybrid clones was not uniform. In some clones certain markers did not segregate any longer. This probably indicates the loss of the respective chromosome. The inability of P . guilliermondii fusion products to form stable hybrids with the expected high DNA content may be attributed to either the aneuploidy of tho fusion partners or a specific mitotic apparatus of this yeast.Polyploidy is a common phenomenon in higher plants and animals and important both to evolution and to agriculture. Knowledge on polyploidy in lower eukaryotes such as fungi is limited (MANIOTIS 1980). In yeasts it is mainly known from Saccharomyces cerevisiae (MORTIMEB and HAWTHORNE 1969), although there is evidence for polyploidy in other yeasts too, e.g. in Schizosaecharmnyces p m b e (GUTZ et al. 1974 We applied parasexual hybridization by protoplast fusion to the fodder yeast Y . guillierrnmdii to construct polyploids and investigate the consequences of polyploidy 011 biomass production. After fusion between two auxotrophic mutants of the same wild-type strain we received mitotically stable hybrids containing in many cases nearly twice a8 much DNA/cell than the corresponding wild-type strain (KLINNER and BOTTCHER 1984b). Similar hybrids which were auxotrophic due to the homozygous presence of appropriate mutations, were fused again with another auxotrophic mutant of the same strain to construct triploids.
Materials and methodsThree wild strains of P . guilliermondii were included in the investigations: 799 mat-, H 17 matand NRRLY 2075 mat+ (KLINNER and BOTTCHER 1984a). The cultivation media, the induction of the genetic markers, the hybridization by protoplast fusion, the characterization of the hybrids by spontaneous and mitotic segregation or by determination of the DNA contentlcell have already been described in the preceding papers (KLINNER and BOTTCHER 1984a, b). Sexual crosses of 1'. guilliermondii were performed according to SIB~RNY et al. (1977) and random spore analysis was conducted by means of nystatin (SAMSONOVA and BOTTCHER 1980).Microphotographs were taken to determine the cell volume. Thoy were projected and both the long and the short axis or the diameter of the cells was measured. From the data of about 100 cells the volume was calculated assuming that they were either ellipsoids or globes.Dedicated to Prof. F. MACH on the occasion of his 60th birthday.