Genetic analysis of apomictic wine yeasts

The Saccharomyces cerevisiae wine yeast IFI256 was selected because of its high fermentative capacity and tolerance to ethanol. Sporulation of the IFI256 strain produced two-spore asci unable to conjugate, but able to sporulate again and the spores produced two-spore asci in all cases. That process was studied for at least five generations. The electrophoretic karyotype showed a pattern of 21 chromosomal bands, which was identical both in the parental and in all the descendants analyzed, from the first to the fifth generation. The DNA content of the parental and the descendants was of 1.7 n, which indicates that the capacity for sporulation shown by all descendants was due to apomixis rather than homothallism of the strain. Different concentrations of glucose and acetate and the addition of zinc salts to the presporulation and sporulation media increased the frequency of four-spore asci by up to 9%. However, the tetrads formed were in fact two dyads that resulted from induced endomitosis. Crosses of IFI256 with laboratory strains produced hybrids giving four-spore asci after sporulation, thus indicating the mutation to be recessive. Transformation of IFI256 with plasmids carrying either SPO12 or SPO13 functional genes and crosses with strains carrying functional or mutated SPO12 and/or SPO13 genes indicated that IFI256 carries several mutations, one of which was located to the SPO12 gene. Parasexual cycles and chromosome loss induced after crossing IFI256 with cir⁰ strains indicated that apomictic mutations were exclusively located at chromosome VIII. The high frequency of wine strains which are apomictic suggests apomixis to be an advantageous phenotype which allows the formation of stress-resistant asci but prevents the loss of favored chromosomal rearrangements.