Complementation of auxotrophic requirements in a diploid strain of Saccharomyces cerevisiae by protoplast fusion with protoplasts of Hansenula wingei

The original concept of protoplast fusion assumed that the genome of the hybrid consisted of equal contributions from both parental strains. This theory is generally upheld with results obtained from intraspecific fusions, but not with results from interspecific and intergeneric fusions. In these latter fusions, one nucleus becomes dominant and most chromosomes from the other are lost, although some genes are retained. Intergeneric hybrids were obtained by fusing protoplasts of Hansenula wingei (wild type) with petite diploid strains of Saccharomyces cerevisiae homozygous for one or more auxotrophic requirements and heterozygous for others, carried on four different chromosomes. Dissection of asci from the hybrids yielded tetrads with ratios of prototrophs: auxotrophs which showed that some were complemented on both chromosomes of a pair (4:0) others on only one (2:2) with a few tetrads showing 1:3 and 4:0 ratios.