Molecular Typing Demonstrates Homogeneity of Saccharomyces uvarum Strains and Reveals the Existence of Hybrids between S. uvarum and S. cerevisiae, Including the S. bayanus Type Strain CBS 380

PCR/RFLP of the NTS2 sequence of rDNA was shown to be suitable for differentiating Saccharomyces sensu stricto species. We previously showed that, within the presently accepted S. bayanus taxon, strains formerly classified as S. uvarum represented a distinct subgroup ( Nguyen and Gaillardin, 1997). In this study, we reidentified 43 more strains isolated recently from wine, cider and various fermentation habitats, and confirmed by karyotyping, hybridization and mtDNA analysis the homogeneity of strains from the S. uvarum subspecies. Molecular typing of nuclear and mitochondrial genomes of strains preserved in collections, and often originating from beer like S. pastorianus NT, revealed the existence of hybrids between S. uvarum and S. cerevisiae. Surprisingly, S. bayanus T CBS380 appeared itself to be a hybrid between S. uvarum and S. cerevisiae. This strain has a mitochondrial genome identical to that of S. uvarum, and a very similar karyotype with 13 isomorphic chromosomes, six of which at least hybridize strongly with S. uvarum chromosomes or with a S. uvarum specific sequence. However, four of the chromosome bands of S. bayanus T bear Y′ sequences indistinguishable from those of S. cerevisiae, a feature that is not observed among presently isolated S. uvarum strains. Because of the hybrid nature of S. bayanus T and of the scarcity of similar hybrids among present days isolates, we propose to reinstate S. uvarum as a proper species among the Saccharomyces sensu stricto complex.