Instability of Saccharomyces cerevisiae heterokaryons

We have constructed heterokaryons of Saccharomyces cerevisiae by crossing kar1 - mutants incapable of nuclear fusion. Approximately 1% of the total zygotes from kar1 - crosses can form heterokaryotic clones. These are very small as compared to diploid colonies, and are composed mainly of a mixture of both types of heteroplasmons (cells which contain the cytoplasmic components of both parents, but the nuclear genotype of only one of them). This fact indicates that heterokaryons are unstable.This instability is already observed by pedigree analysis in the first zygotic divisions. We suggest that missegregation is the main factor in heterokaryon instability and results from an unequal nuclear transmission, which occurs when one of the mother nuclei divides and, although viable, does not migrate to the daughter bud. However, the proportion of inviable zygotes and buds found in the pedigree analysis, as well as the recovery of only one type of heteroplasmon, indicates the complete loss of one of the parental nuclei. Consequently nuclear inactivation is suggested as the second reason for heterokaryon instability.