Genetic hybridization and karyotypic analyses revealed the biological species Saccharomyces paradoxus and Saccharomyces cerevisiae in exudates from North American oaks for the first time. In addition, two strains collected from elm flux and from Drosophila by Phaff in 1961 and 1952 were reidentified as S. paradoxus. Each strain studied showed a unique profile of chromosomal hybridization with a probe for the retrotransposable element Ty1. The wild distribution of natural Saccharomyces sensu stricto yeasts is discussed.
Chromosomal DNAs of many monosporic strains of the biological species Saccharomyces cerevisiae, S. paradoxus and S. bayanus were analysed using contour-clamped homogeneous electric field electrophoresis. Southern blot hybridization with eight cloned S. cerevisiae genes (ADC1, CUP1, GAL4, LEU2, rDNA, SUC2, TRP1 and URA3) assigned to different chromosomes was used to study homology and chromosomal location of the genes in the three sibling species. A comparative study of Ty1, Ty2 and telomere-associated Y' sequences having multiple chromosomal location was also done. Chromosome length polymorphism was found in cultured strains of S. cerevisiae. Wild S. cerevisiae and S. paradoxus strains yielded chromosome banding patterns very similar to each other. The karyotype pattern of S. bayanus was readily distinguishable from that of S. cerevisiae and S. paradoxus. Southern blot analysis revealed a low degree of homology between the S. cerevisiae genes studied and the corresponding S. paradoxus and S. bayanus genes. The number of chromosomes appears to be 16 in all three species.
The genomic constitution of different S. bayanus strains and natural interspecific Saccharomyces hybrids has been studied by genetic and molecular methods. Unlike S. bayanus var. uvarum, some S. bayanus var. bayanus strains (the type culture CBS 380, CBS 378, CBS 425, CBS 1548) harbour a number of S. cerevisiae subtelomeric sequences: Y , pEL50, SUC, RTM and MAL. The two varieties, having 86-100% nDNA-nDNA reassociation, are partly genetically isolated from one another but completely isolated from S. cerevisiae. Genetic and molecular data support the maintaining of var. bayanus and var. uvarum strains in the species S. bayanus. Using Southern hybridization with species-specific molecular markers, RFLP of the MET2 gene and flow cytometry analysis, we showed that the non-S. cerevisiae parents are different in lager brewing yeasts and in wine hybrid strains. Our results suggest that S. pastorianus is a hybrid between S. cerevisiae and S. bayanus var. bayanus, while S. bayanus var. uvarum contributed to the formation of the wine hybrids S6U and CID1. According to the partial sequence of ACT1 gene and flow cytometry analysis, strain CID1 is a triple hybrid between S. cerevisiae, S. kudriavzevii and S. bayanus var. uvarum.
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