טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKatz Ezov Tal
SubjectMolecular-Genetic Biodiversity in Natural Populations of
yeast S. cerevisiae
DepartmentDepartment of Biotechnology and Food Engineering
Supervisors Professor Yechezkel Kashi
Professor Emeritus Uri Cogan
Full Thesis textFull thesis text - English Version


Abstract

The yeast S. cerevisiae is a central model organism in eukaryotic cell studies and a major component in many food and biotechnological processes. However, the wide knowledge regarding genetics and molecular biology of S. cerevisiae is based on a narrow range of strains. Studies of natural populations of S. cerevisiae, not associated with human activities or industrial fermentation environments, are very few.

We isolated a panel of S. cerevisiae strains from a natural microsite, ‘‘Evolution Canyon’’ at Mount Carmel, Israel. Analysis of 19 microsatellite loci revealed high allelic diversity and variation in ploidy level across the panel, from diploids to tetraploids, confirmed by FACS. No significant differences were found in the level of microsatellite variation between strains derived from the major localities or microniches, whereas strains of different ploidy showed low similarity in allele content. Maximum genetic diversity was observed among diploids and minimum among triploids. Phylogenetic analysis revealed clonal, rather than sexual, structure of the polyploid subpopulations. Phenotypic analysis revealed industrially important properties such as tolerance for high osmotic stress and thermotolerance. These properties would allow the strains to be a robust platform for industrial processes.

Haploid cells of S. cerevisiae exhibit one of two mating types, a or alpha. Mating type can be stable (heterothallic yeast) or unstable (homothallic yeast). When cells of opposite mating types meet they participate in a mating process that results in the creation of an a/alpha diploid. Homothallic yeast can undergo mating type switching, a gene conversion process initiated by the HO endonuclease, which generates a site-specific double-stranded DNA break. This can be followed by mother-daughter mating. Heterothallic yeast can mate with unrelated haploids (amphimixis), or undergo mating between spores from the same tetrad (intratetrad mating, or automixis), but cannot undergo mother-daughter mating.

Our isolates were found to be heterothallic. Sequence analysis of the HO gene revealed multiple mutations including insertions, deletions and single nucleotide polymorphisms (SNPs), causing missense, nonsense or frameshift mutations. Good correspondence was found in the comparison of the population structure gained based on HO sequence and based on our SSR markers, which are spread over eight different chromosomes. Our results strongly suggest that loss of function of HO is the cause of heterothallism in the natural isolates. Furthermore, this work supports the hypothesis that clonal reproduction and intratetrad mating may predominate in natural yeast populations, while mother-daughter mating might not be as significant as was considered earlier.