GENETICS OF SEX Sex-Determination System in the Diploid Yeast Zygosaccharomyces sapae

Sexual reproduction and breeding systems are driving forces for genetic diversity. The matingtype (MAT) locus represents a mutation and chromosome rearrangement hotspot in yeasts. Zygosaccharomyces rouxii complex yeasts are naturally faced with hostile low water activity (aw) environments and are characterized by gene copy number variation, genome instability, and aneuploidy/allodiploidy. Here, we investigated sex-determination system in Zygosaccharomyces sapae diploid strain ABT301T, a member of the Z. rouxii complex. We cloned three divergent mating type-like (MTL) a-idiomorph sequences and designated them as ZsMTLa copies 1, 2, and 3. They encode homologs of Z. rouxii CBS 732T MATa2 (amino acid sequence identities spanning from 67.0 to 99.5%) and MATa1 (identity range 81.5–99.5%). ABT301T possesses two divergent HO genes encoding distinct endonucleases 100% and 92.3% identical to Z. rouxii HO. Cloning of MATa-idiomorph resulted in a single ZsMTLa locus encoding two Z. rouxii-like proteins MATa1 and MATa2. To assign the cloned ZsMTLa and ZsMTLa idiomorphs as MAT, HML, and HMR cassettes, we analyzed their flanking regions. Three ZsMTLa loci exhibited the DIC1-MAT-SLA2 gene order canonical for MAT expression loci. Furthermore, four putative HML cassettes were identified, two containing the ZsMTLa copy 1 and the remaining harboring ZsMTLa copies 2 and 3. Finally, the ZsMTLa locus was 39-flanked by SLA2, suggesting the status of MAT expression locus. In conclusion, Z. sapae ABT301T displays an aaaa genotype missing of the HMR silent cassette. Our results demonstrated that mating-type switching is a hypermutagenic process in Z. rouxii complex that generates genetic diversity de novo. This error-prone mechanism could be suitable to generate progenies more rapidly adaptable to hostile environments.