Chromosomal D N A patterns using the transverse alternating field electrophoresis technique and mitochondrial D N A restriction profiles have been achieved for 22 enological strains of Saccharomyces cerevisiae. Both methods have evidenced a marked polymorphism o f these strains. Twenty different karyotypes and 17 mitochondrial D N A banding patterns have been observed. Only three strains originating from the same vineyard could not be differentiated by either of the two methods. The polymorphism observed at the chromosomal and mitochondrial levels makes the techniques investigated powerful tools for identification and control of industrial strains.

Electrophoretic karyotypes of two strains of Saccharomyces cerevisiae, a haploid laboratory strain and a wild strain known to be at least diploid, have been checked during vegetative growth. The karyotype of the haploid strain was very stable; however, the diploid strain underwent frequent modifications. In most cases the number of bands was reduced, but occasionally we observed one band splitting into two. In one case, chromosomal rearrangements took place between differently sized copies of chromosomes I and VI. We concluded that the chromosome length polymorphism observed among wild strains of S. cerevisiae could be explained partly by chromosomal structure reorganization occurring during mitosis. Chromosome length polymorphism among strains of Saccharomyces cerevisiae has been demonstrated by pulsedfield gel electrophoresis. The phenomenon was observed among baker's yeast strains (3, 12) and even more among enological strains (15, 16). The last case is interesting as the yeast strains used in wine making have recently been isolated from wild flora and may therefore indicate features of wild S. cerevisiae. The evolutionary mechanisms which result in polymorphism of chromosomal structures have not been elucidated. Pedersen (10), Nillson-Tillgren et al. (9), and Petersen et al. (11), studying brewer's yeast, have hypothesized interspecies hybridization, while Rank et al. (12), working with baker's yeast, concluded that intraspecific cryptic mechanisms were responsible. This work aims to contribute to the understanding of the origin of chromosomal polymorphism among wild strains of S. cerevisiae. As natural strains multiply by vegetative growth (mitotic cell cycle), we tested the hypothesis that asexual reproduction allows chromosomal rearrangements.

Exocellular and cell-wall mannoproteins of two Saccharomyces cerevisiae strains, one flocculating and the other not, have been compared. Mannoproteins (mannose 78, glucose 2, protein 20%) are the majorcomponents of yeast exocellular polysaccharides, and their amounts and structure are similar whether the strains are flocculating or not. Cell-wall mannoproteins have structural features in common with exocellular mannoproteins; however, differences in molecular weight and composition (mannose 63, glucose 1 , protein 36%) were observed. The wall of the flocculating strain seemed to be richer in polysaccharides than the wall of the non-flocculating strain, but the mannoprotein structure was similar.The flocculating character does not involve modification of solubilised polysaccharides, which suggests that the introduction of flocculating yeast in champagne vinification could be considered without change in sensory properties due to polysaccharides.

A yeast flocculation gene was isolated from a genomic library of an FLO5 strain of S. cerevisiae on the basis of its ability to trigger flocculation in a non-flocculent strain. Characterization of the cloned gene by restriction mapping, Southern analysis, and chromosome mapping have shown that it corresponds to a FLO5 gene previously located on chromosome I and that this gene is related to the already described FLO1 gene. A study of gene expression in different yeast strains has indicated that, while this gene is dominant, its expression can be suppressed in some genetic backgrounds. A Northern-blot analysis has demonstrated that the same 5000-nt transcript was present in an FLO5 and an FLO1 strain. A gene disruption experiment has led to the conclusion that another flocculation gene is present and can be active in the FLO5 strain we used.

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