Saccharomyces cerevisiae nuclear fusion requires prior activation by alpha factor

We have developed a protocol for efficient fusion of spheroplasts of the same mating type. Nuclear fusion in this whole-cell system is also efficient and closely parallels nuclear fusion in heterosexual mating of intact cells. In the spheroplast fusion system, nuclear fusion is dependent on both the KAR1 gene and prior exposure to alpha factor. The major products of nuclear fusion in the spheroplast fusion assay were true diploids that were homozygous at the mating-type locus. An additional 10% of the products were cells of ploidy greater than diploid. The dependence of nuclear fusion on alpha factor treatment could not be replaced by synchronization in G1 by mutations in CDC28 and CDC35 or by prior arrest in stationary phase. These data suggest that nuclear fusion is not a constitutive function of the nucleus, but rather is specifically induced by mating hormone.