AN ANALYSIS OF THE PHENOMENON OF LONG–TERM ADAPTATION TO GALACTOSE BY SACCHAROMYCES

Abstract

IT HAS LONG been known that Saccharomyces cerevisiae ferments galactose by an adaptive enzyme produced in the cell during the course of a few hours in the absence of cell division. Winge and Roberts (1948) have studied a culture of S. Chevalieri which ferments galactose by long-term adaptation which occurs after cells have grown from 3-14 days in a medium containing galactose. After the adaptation, the fermentation itself occurs at a rapid rate. They state that long-term adaptation of S. Chevalieri to galactose does not involve a shift in the population (mutation followed by selection) but is an adaptation of the entire original population. They consider the delay an indication of the time required by the cells to build up galactozymase to a sufficiently high concentration to make it demonstrable. They made hybrids between these two species and found a regular segregation of rapidly fermenting (1-2 days) to long-term adapting (3-14 days) clones from each ascus, provided the ascospores were isolated in beer wort, a medium containing considerable amounts of maltose, but lacking galactose. If the ascospores were isolated in galactose broth, all four spores from each ascus fermented galactose immediately (1-2 days). However, they used only asci in which all four spores germinated and the possibility exists that some nonutilizers of galactose may have failed to grow in maltose. They described a series of experiments in which they deadapted and readapted galactose fermenting cultures and denied the significance of mutation as a factor in this process. Extended genetical analyses bv Lindegren (1949) have established the allelism of a single pair of galactose genes (G/g) in Saccharomyces. One of these alleles (G) controls the rapid fermentation of galactose while cells carrying the other (g) do not effect detectable fermentation of galactose under the conditions of the Durham tube fermentation test. (Occasional irregularities in which an excess of fermenter (G G G g) or an excess of nonfermenter (g g g G) alleles were obtained from single asci have been interpreted as the result of conversion of alleles in the heterozygote.) We have discovered occasional cultures in our pedigrees which resemble the cultures of S. Chevalieri described by Winge and Roberts, by fermenting galactose after a long exposure involving growth; the present paper deals with the behavior of these unusual slow fermenters of galactose. We have concluded that this involves mutation from the g to the G allele followed by selection of the mutant G population in the presence of galactose.