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).