On the Cytoplasmic Nature of “Long-Term Adaptation” in Yeast

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Abstract
Strains of Saccharomyces chevalieri were designated "slow" or "negative" and "fast" or "positive" with respect to formation of galactozymase. Growth of the slow-adapting strain C1d on EMB (eosin-methylene blue)-galactose plates and in Durham fermentation tubes resulted in both positive and negative clones. In negative clones 10-60% of the cells could potentially give rise to positive clones, but the negative cells inhibited the expression of the positive phenotype when galactose was introduced after the third generation of growth in its absence. A variance analysis of the appearance of positives in cultures grown in the absence of galactose indicated that the changes to the positive phenotype were induced by the adaptive substrate rather than by a random mutation process. Tests of recently derived positive clones on EMB-galactose plates showed that their growth was normal until the fifth generation, when there was a sudden cessation in the increase of positives coincident with a burst in the appearance of negatives. After prolonged growth in the presence of galactose, there was great variability in reversion of positives to negatives, both in no. of generations required for the burst and in no. of individuals participating. The data indicated that "long-term adaptation" to galactose fermentation was controlled by cytoplasmic units which increased autocatalytically.