The sex determination system and the role of chimeric a1/alpha2 heterodimer in the sterility of an allodiploid Zygosaccharomyces yeast

In Saccharomyces cerevisiae diploids, the a1/alpha2 protein heterodimer acts as repressor of haploid-specific genes, such as MATalpha1 and HO genes, and allows a/alpha diploid cells to undergo sporulation. The regulatory routes governing the cell type and fate have been not yet studied in the yeasts belonging to the Zygosaccharomyces rouxii complex. These yeasts are relevant in foodstuff elaboration and spoilage due to a wide repertoire of tolerances to osmotic stress. Their genetic variability favors phenotypic diversity and adaptation to hostile environments by ectopic recombination of mating-type (MTL) loci through an error-prone switching mechanism. Among them, the allodiploid strain ATCC42981 exhibits hybrid vigor and multi-stress tolerance, but it is unable to undergo sexual reproduction. Aim of this work was to characterize allodiploid strain ATCC42981 for the genetic organization and the transcriptional expression of mating type-like (MTL) loci and HO genes. Genetic dissection of ATCC42981 sex determinants revealed a MATa/MATalpha genotype with a redundant number of partial divergent HMR cassettes. MATa expression locus contained Z. rouxii-related MATa1 and MATa2 genes, whereas MATalpha expression locus contained Z. sapae-related MATalpha1 and MATalpha2 genes (termed MATalpha1 and MATalpha2 copy 2, respectively). Both MAT expression loci are linked to phylogenetically congruent HML silent cassettes on non-homologous chromosomes. Other than a “hybrid” and redundant three-cassette system, ATCC42981 possesses two divergent HO genes. Consistently with this gene organization, ATCC42981 expressed MATa1 and MATalpha2 copy 2 genes under standard conditions, whereas aand α-idiomorph genes from HMR and HML cassettes were silent. Differently from S. cerevisiae diploids, ATCC42981 did not repress either MATalpha1 or HO gene transcription. Under hypersaline stress (which should be induce meiosis in Zygosaccharomyces cells by turning on the haploid gene-specific program), ATCC42981 was not able to undergo meiosis and over-expressed HO copy 2, MATa1 and MATalpha1 copy 2. We hypothesized that the partial incompatibility between ATCC42981 Z. rouxii-like a1 and Z. sapae-like alpha2 subunits in MATa1/alpha2 heterodimer causes a defective silencing of haploid-specific genes, including meiosis inhibiting factors, leading to clonality as the only possible reproduction strategy for this allodiploid yeast