Mating-Type Functions for Meiosis and Sporulation in Yeast Act through Cytoplasm

Given a nutritional regime marked by a low nitrogen level and the absence of fermentable carbon sources, conventional a/alpha diploid cells of Saccharomyces cerevisiae exhibit a complex developmental sequence that includes a round of premeiotic DNA replication, commitment to meiosis and the elaboration of mature tetrads containing viable ascospores. Ordinarily, haploid cells and diploid cells of genotype a/a and alpha/alpha fail to display these reactions under comparable conditions. Here, we describe a simple technique for sporulation of alpha/alpha and a/a cells. Cells of genotype alpha/alpha are mated to haploid a cells carrying the kar1 (karyogamy defective) mutation to yield heterokaryons containing the corresponding diploid and haploid nuclei. The kar1 strains mate normally, but nuclei in the resultant zygotes do not fuse. When heterokaryotic cells are inoculated into sporulation media, they produce asci with six spores. Four spores carry genotypes derived from the diploid nucleus and the other two possess the markers originating from the haploid nucleus, i.e., the diploid nucleus divides meiotically while the haploid nucleus apparently divides mitotically. Similarly, the a/a genome is "helped" to sporulate as a consequence of mating with alpha kar1 strains. The results allow us to conclude that the mating-type functions essential for meiosis and sporulation are communicated and act through the cytoplasm and that sporulation can be dissociated from typical meiosis. This procedure will facilitate the genetic analysis of strains that are otherwise unable to sporulate.