Concealed nuclei in Saccharomyces strains

Bud selection and apoptosis-like degradation of nuclei in yeast heterokaryons: a KAR1 effect

It has been shown that defects in cell fusion during mating can trigger programmed cell death in the yeast Saccharomyces cerevisiae. We wished to test whether defects in nuclear migration during cell fusion have the same effect. A partial pedigree analysis of nine kar1 x KAR1 crosses of two different types (four alpha KAR1 x a kar1 and five alpha kar1 x a KAR1 crosses) was carried out, and quantitative estimates of the frequencies of different mother/daughter (m/d) classes were obtained. The kar1 mutation affects nuclear congression and delays nuclear fusion. In each cross tested, the nucleus that entered the first bud tended to be the one contributed by the cell that carried the wild-type allele of KAR1. If budding was delayed by nutrient limitation, the kar1 nucleus could be rescued, indicating that the primary effect of the kar1 mutation is that it slows spindle action. Many m/d classes appear as a result of the degradation of one of the nuclei in the heterokaryon. Loss of nuclei in heterokaryons was accompanied by an accumulation of reactive oxygen species (ROS), and by abnormalities in nuclear structure revealed by TUNEL (terminal transferase-mediated dUTP nick end-labeling) analysis, DAPI staining and by histone-GFP fluorescence patterns which suggested an apoptosis-like process. Often only one nucleus was degraded, and ROS accumulation was restricted to one half of the zygote. We therefore suggest that the data obtained can be explained by apoptosis-like death of a half-cell (cell body).

The impact of manipulations with cytoplasmically inherited factors on nuclear transmission and degradation in yeast heterokaryons

Heterokaryotic zygotes in yeast provide a unique possibility to study the survival and transmission of two genetically diverse nuclei in one cell. Using partial pedigree analysis, we show that various treatments used to change cytoplasmic hereditary determinants can essentially affect nuclear transmission in yeast heterokaryons. This includes choice of nucleus to enter the first bud and incidence of various classes of mother/daughter pairs demonstrating nuclear degradation patterns in heterokaryotic zygotes. These treatments include guanidine hydrochloride, a prion-curing agent, ethidium bromide, an agent causing elimination of mitochondrial DNA, and cytoplasm replacement by cytoduction, which leads to mtDNA replacement and transfer of some other cytoplasmically inherited determinants. The genetic and cytological evidence obtained favors prion involvement in nuclear transmission and suggests apoptotic features in nuclear degradation in yeast heterokaryotic zygotes.