Analysis and dynamics of the chromosomal complements of wild sparkling-wine yeast strains

We isolated Saccharomyces cerevisiae yeast strains that are able to carry o ut the second fermentation of sparkling wine from spontaneously fermenting musts in El Penedes (Spain) by specifically designed selection protocols. A ll of them (26 strains) showed one of two very similar mitochondrial DNA (m tDNA) restriction patterns, whereas their karyotypes differed. These strain s showed high rates of karyotype instability, which were dependent on both the medium and the strain, during vegetative growth. In all cases, the mtDN A restriction pattern was conserved in strains kept under the same conditio ns. Analysis of different repetitive sequences in their genomes suggested t hat ribosomal DNA repeats play an important role in the changes in size obs erved in chromosome XII, whereas SUC genes or Ty elements did not show ampl ification or transposition processes that could be related to rearrangement s of the chromosomes showing these sequences. Karyotype changes also occurr ed in monosporidic diploid derivatives. We propose that these changes origi nated mainly from ectopic recombination between repeated sequences interspe rsed in the genome. None of the rearranged karyotypes provided a selective advantage strong enough to allow the strains to displace the parental strai ns. The nature and frequency of these changes suggest that they may play an important role in the establishment and maintenance of the genetic diversi ty observed in S. cerevisiae wild populations.