The role of mutation in the adaptation of a yeast to galactose
Adaptation of a diploid yeast strain of Saccharomyces cerevisiae to a synthetic medium containing galactose as carbon source is shown to involve selective growth of a few cells that have mutated to a condition enabling this sugar to be utilized readily, rather than gradual development in the majority of cells of the ability to utilize galactose. That the first mechanism is the correct one is shown by: ( а ) the ability of 1 cell in 10 5 of the unadapted strain to utilize galactose more or less immediately and form large colonies in 2 days on a galactose-agar growth medium, under circumstances in which all the cells of the fully adapted strain can grow at once; ( b ) the identical rates of multiplication in the liquid galactose medium of the adapted strain and of the small number of variant cells, the measurement in the latter case being made during the lag phase before visible growth has started; ( c ) the length of this lag phase (3 days), which is consistent with the initial proportion of variant cells; ( d ) the hereditary difference between the adapted and unadapted diploid strains which seems to depend on a single genetic factor. Two haploid cultures from a four-spored ascus of the adapted strain grow with a short lag in the liquid galactose medium, and two grow with a long lag, being similar in this respect to the four haploids from the unadapted strain. This difference between the two types is preserved through subsequent matings. Exceptional properties are shown by spore cultures from two out of twenty-two asci both in the glucose and the galactose medium. This may be due to extra-chromosomal material being partitioned in unusual proportions at meiosis. The adapted strain does not revert during 500 generations in a medium containing glucose as carbon source. To account for the stability relationships between the adapted and unadapted strain, it is proposed that the co-ordination of cell processes is disturbed while adjustments conditioned by a mutation are taking place. Thus the growth rate may be reduced and, for this reason alone, accumulation of mutated cells in a population retarded.