Speciation mochauism of hemiascamycetans yeasts

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In order to clarify the effect of gene diversity concerned with sexual interactions on speciation mechanism, I tried to perform comparative study of sex-differentiation genes between the yeasts Saccharomyces cerevisiae and Saccharomyces naganishii. S. naganishii is a new species described by us through the molecular taxonomic reexamination of Saccharomyces yeasts, has the ability to highly sporulate, and has a perfect Ho-type homothallism. I have already isolated heterothallic strains from the original homothallic strain of S. naganishii.I constructed a host-vector system specific to S. naganishii. The vector constructed has an autonomously replicating sequence and a centromere sequence derived from S. naganishii chromosome. Then, I constructed a genomic library of S. naganishii with the above highly stable YCp-type vector. From the library, I cloned a gene coding for alpha pheromone of S. naganishii, and compare it with the one of S. cerevisiae. A precursor of alpha pheromone of S. naganishii is very close to the one of S. cerevisiae. Then, I tried to draw difference of upstream regulatory sequences (URSs) of alpha pheromone genes between the two species. The URS of alpha pheromone gene from S. naganishii was activated in alpha-type cells of both species, in terms of LacZ reporter gene expression. When a region predicted to be essential to activate the gene in S. naganishii was omitted from the URS, LacZ reporter gene was not expressed at all in S. naganishii, but was partially expressed in S. cerevisiae. These results suggested that products of mating-type genes from the two species recognize different sequences as functional URSs.