Characterization of the transmission during cytoductant formation of the 2 μm DNA plasmid from Saccharomyces

The transmission of the yeast 2 μm DNA plasmid has been examined in heterokaryons formed between a haploid donor cell containing the plasmid and a haploid recipient cell lacking the plasmid. Strains lacking the plasmid were mated to donor strains and cytoductants, haploid exconjugants arising from heterokaryons, were selected. The cytoductants bearing the genotype of the recipient cells were then tested for the presence of 2 μm DNA. The frequency with which the recipient received plasmid copies varied between 0 and 46%. This frequency depended on the recipient strain but was not highly dependent on whether or not the donor strain carried a kar1 mutation. Exceptional cytoductants, which had acquired a chromosome from the mating partner, were examined and found to have a much greater probability of acquiring plasmid DNA than cytoductants in general. This correlation supports the contention that plasmid copies are associated with the nucleus. In one mating the donor strain contained nearly equal amounts of two physically distinct plasmid types. Of the cytoductants bearing the recipient genotype which had acquired the plasmid during mating, most contained only one of the two plasmid types present in the donor. Analysis of this result using a Poisson distribution indicates that the average number of plasmid copies transmitted between nuclei of a heterokaryon is 0.2.