Mating-type switching in CBS 732T derived sub-cultures unveils potential genetic and phenotypic novelties in haploid Zygosaccharomyces rouxii

Abstract

In haploid Saccharomyces cerevisiae, a complex recombination system regulates mating-type switching and requires one MAT expression locus, two donor cassettes (HML and HMR) and the HO endonuclease that catalyses gene conversion. Zygosaccharomyces rouxii is the most distant species from S. cerevisiae with a functional HO, but with a poorly understood mating-type switching. Here, we described that two sub-cultures of the type strain CBS 732^T underwent the α to a genotype switching leading to mixed MATα and MATa populations. Remarkably, during this event the donor cassette was copied into the MAT locus, except for its own 3' end, resulting in a new MATa2 gene copy different from the silenced HMRa copy. Moreover, CBS 732^T cells bypassed the cell-cycle control, which oversees HO transcription in S. cerevisiae, and expressed HO at the stationary phase. Despite HO dysregulation, mating-type switching seemed to occur rarely or belatedly during CBS 732^T colony formation in most of the tested conditions. When morphology and mating behaviour were analysed, two sub-cultures displayed distinct outcross fertility responses. Overall, our data support that mating-type switching causes genotype instability and phenotypic novelties in CBS 732^T and open the question whether this mechanism is shared by other Z. rouxii haploid homothallic strains.