Induction and Characterization of Artificial Diploids from the Haploid Yeast Torulaspora delbrueckii

The yeast Torulaspora delbrueckii, which propagates as a haploid, was made into a diploid by treatment with dimethyl sulfoxide (DMSO) on the regeneration of protoplasts. The diploid state was stably inherited; the cell volume was three times that of the parent strain and the cellular DNA content was two times that of the parental strain. No essential difference was found between diploids induced by DMSO and those formed through intraspecific protoplast fusion. The diploid strains sporulated fairly well, with their cells converting directly into asci. Random spore analysis revealed that diploids induced through protoplast fusion gave rise to auxotrophic segregants (haploids) with the parental genetic marker or to segregants formed by recombination, while diploids induced by DMSO from a doubly auxotrophic parent gave rise to no recombinant, indicating that it was chromosomally homoallelic in nature. The magnesium level in the protoplast regeneration medium was found to be an important factor for inducing diploid formation. At 0.2 mM magnesium diploids appeared even in the absence of DMSO, while at 2 mM magnesium diploids never appeared unless DMSO was added to the regeneration medium. Evidence is provided that the diploids induced by DMSO or a low magnesium level are due to direct diploidization but not protoplast fusion. UV light irradiation of intact cells (without protoplasts), 10% of which survived, also produced diploids among this surviving population. From these results we conclude that the perturbation of protoplast regeneration or of cell division by the treatments mentioned above somehow induced direct diploidization of T. delbrueckii.