'Long-Term’ Adaptation and Non-Mendelian Inheritance in Yeast

Abstract

HAPLOPHASE yeasts capable of fermenting galactose ordinarily produce gas in from sixteen to thirty-six hours at 30° C. after inoculation into nutrient medium containing galactose. Winge and Roberts1 studied clones which fermented the sugar in from three to fourteen days after they were seeded in galactose yeastants to the time required by the cells in the haploid popu extract medium. Following the adaptation period, however, the rate of fermentation was identical with that of ordinary galactose-fermenting yeasts. In their analyses, they found that two haploid segregants from asci heterozygous for galactose fermentation were the ordinary fast fermenters and the other two were ‘long-term’ adapting clones. They attributed the delay in fermentation of the ‘long-term’ adapting segreglation to build up galactozymase to a concentration sufficient for the detection of its presence. The regular segregations of fast/'long-term’ adapting phenotypes among the haploid progeny were offered as proof that delayed fermentation of galactose is controlled by a specific allele. Such originally 2: 2 fast/'long-term’ tetrads were transformed into 4: 0 fast/'long-term’ ratios by extended cultivation in galactose yeast extract medium. Winge and Roberts considered that under these conditions there was no change in the population but merely a ‘long-term’ adaptation of the entire population. In proposing this view, they denied the occurrence of mutations as a factor operative in the ‘long-term’ adaptation, and held that de-adaptation of an adapted clone, as well as its subsequent re-adaptation to galactose, were effected through a modification.