Yeast Hybrids and Polyploids as Models in Evolutionary Studies

Closely-related species of budding yeast mate readily and interspecific hybrid and polyploid yeasts are widespread in nature or in domesticated strains. This together with all the advantages of being fast-growing unicellular organisms with a fully sequenced genome and strong genetics tools make yeasts ideally suited for contributing to the understanding of evolution in hybrids and polyploids. We produced an hybrid between a S. cerevisiae strain with its close relative S. paradoxus. Growth vigor was obtained in the hybrid under numerous growth conditions. We addressed the basis of heterosis in this hybrid through a series of analyses including phenotyping, gene expression studies and new forward genetic approaches. Fluorescent time-lapse microscopy analysis of cell cycle progression enabled the dissection of the cell cycle into three phases and showed that hybrid increased growth rate is mostly due to a shorter G1 phase of newly made cells. This finding, which implicates cell cycle regulation involvement in heterosis was supported by a genetic analysis showing that deletion of the S. cerevisiae allele of a cell cycle regulator gene in the hybrid reduced the degree of heterosis while the expression of this allele in S. paradoxus increased growth rate. Allele-specific genome-wide expression studies showed an enrichment of respiration-related genes among overexpressed genes. Both pharmacological experiments and genetic screens confirmed the involvement of respiration in the heterotic phenotype.