Variability of the physiological features and of the nuclear and mitochondrial genomes of Bakers' yeasts

Seven bakers' yeast strains, characterized according to properties of interest in the baking industry, showed a great heterogeneity with regards to their ability to leaven dough and to grow in laboratory minimal medium with either glucose, sucrose, maltose, raffinose or melibiose as the carbon source, and in molasses. This ability was not related to the capacity to hydrolyze the corresponding carbon source. Each strain had a characteristic karyotype when their chromosomes were individually separated in an electrophoretic CHEF system. These variations were detected both in the number and length of the bands even when strains came from the same source, thus, allowing each strain to be perfectly distinguishable from the;others. Chromosome polymorphism has been suggested to result from optimal fitness in each industrial environment. In fact, most strains grew very well in molasses; no strain grew in melibiose, had melibiase activity or possessed MEL genes as deduced from the lack of hybridization with Saccharomyces cerevisiae and S. carlsbergensis MEL probes; all strains were biotin auxotrophs and showed a ''grande'' phenotype with a frequency of ''petite'' formation less than 0.1%. All but one strain grew, although at different growth rates, at 22, 30, 37 and 42 degrees C and in 20% glucose, and their viability dramatically decreased at 50 degrees C. Surprisingly, the restriction fragment length polymorphism (RFLP) of the mitochondrial DNA (mtDNA) was the same for all the strains after incubating mtDNA with three different restriction enzymes, indicating that in bakers' yeasts mtDNA is relatively stable compared to the nuclear genome.