Abstract
Protoplasts of Saccharomyces cerevisiae were prepared from two different haploid strains both of mating type a, which carried different nuclear (ade1, ura1, his4, leu2 and thr4) and mitochondrial (ρ, ω, CR, ER and OR) markers, and were fused with the aid of polyethylene glycol. Cells of fused products (prototrophs) displayed phenotype of mating type a and were crossed to mating type α/α diploids having auxotrophic markers, e.g. trp1. On sporulation of the resulting hybrid clones, as a rule, there were three tetrad types for mating types, i.e. (I) 4non-maters, (II) 2a:2α and (III) a:α: 2non-maters. The relative frequencies of these three tetrad types were close to the ones theoretically predicted from a/a/α/α tetraploids, suggesting that the fusion products were a/a diploids. Auxotrophic markers involved in these crosses, which were located on four different chromosomes, were also segregated to yield the tetrad distributions expected from the parentages. Consequently, it was concluded that the protoplast fusion proceeded to karyogamy to produce stable diploids. A study of mitochondrial recombination demonstrated that the fusion products accepted the mitochondrial genome (the polar gene ω as well as the drug resistance genes) from one parent of ρ+, but not from another of neutral petite.
