Mitochondrial DNA loss caused by ethanol in Saccharomyces flor yeasts

Saccharomyces flor yeasts proliferate at the surface of sherry wine, which contains over 15% (vol) ethanol. Since ethanol is a powerful inducer of respiration-deficient mutants, this alcohol has been proposed to be the source of the high diversity found in the mitochondrial genomes of flor yeasts and other wine yeasts. Southern blot analysis suggests that mitochondrial DNA (mtDNA) polymorphic changes are due to minor lesions in the mitochondrial genome. As determined in this work by pulsed-field gel electrophoresis, restriction analysis, and Southern blot analysis, ethanol-induced petite mutants completely lack mtDNA (rho zero). Ethanol-induced changes in the mitochondrial genome that could explain the observed mtDNA polymorphism in flor yeasts were not found. The transfer of two different mtDNA variants from flor yeasts to a laboratory strain conferred in both cases an increase in ethanol tolerance in the recipient strain, suggesting that mtDNAs are probably subjected to positive selection pressure concerning their ability to confer ethanol tolerance.