Heterothallism in Saccharomyces rouxii.

Over the years a large number of osmophilic yeasts have been isolated from honey, sirups, and highly salted fermented foods of the Orient. AMany have been considered as new species by the persons who isolated them. Strains vary from one another by minute differences in physiology and morphology. The osinophilic yeasts were formerly classified in the subgenus Zygosaccharomyces, a subdivision of the genus Saccharomyces reserved for those species which exist exclusively or partially as haploid cells in the vegetative phase. In keeping with recent trends, the haploid and diploid species of a single phylogenetic line are classed together, with no division into subgenera. Thus the osmophilic Zygosaccharomyces have been placed in the genus Saccharomyces where, according to the opinion of the authors, they currently occupy low positions on their phylogenetic line. Lodder and Kreger-van Rij (1952) reduced the previously large number of osmophilic species to two-Saccharomyces mellis, found almost exclusively in honey, and Saccharomyces rouxii, isolated commonly from Oriental foods, wines, jellies, sun dried fruits, and other substrates with high sugar content. The distinction between the two species is not sharp. It rests mainly on the ability of S. rouxii to assimilate and ferment maltose and on the inability of S. mellis to have any action on this sugar. The fermentation of maltose by S. rouxii in our experience is weak, and assimilation is variable. Indecisive speciation at this low level of development continues at higher levels also, and in the diploid species related to Saccharomyces cerevisiae the phylogenetic line likewise may be more accurately viewed as a series of hybrids which they probably are, than as a series of clearly defined species.