Nutrition and phylogeny of predacious yeasts

Yeast predation was studied with respect to the range of its distribution a mong ascomycetous yeasts, the range of yeast species that can be affected, and nutritional aspects of the phenomenon. The yeasts identified as predato rs belong to the Saccharomycopsis clade as defined on the basis of rDNA seq uence relatedness. The 11 recognized species in the clade, plus three undes cribed but related Candida species, were shown to be incapable of utilizing sulfate as sole source of sulfur, and all but two (Saccharomycopsis capsul aris andSaccharomycopsis vini) were observed to penetrate and kill other ye asts under some conditions. Other unrelated sulfate transport-deficient yea sts (strains in the genera Pichia and Candida and the two known species of Starmera) are not predacious. The predacious species vary considerably as t o the optimal environmental conditions that favour predation. Some are inhi bited by the presence of rich nitrogenous nutrients, organic sulfur compoun ds, or higher concentrations of ammonium nitrogen, whereas other species ma y be stimulated under the same conditions. An attempt was made to correlate prey susceptibility to the excretion of substances that stimulate the grow th of predators, but no correlation was detected between the two phenomena. The range of susceptible prey covers both ascomycetes and basidiomycetes, and includes Schizosaccharomyces pombe, which was previously thought to be immune. The achlorophyllous alga Prototheca zopfii is not killed by predaci ous yeasts, but the initial steps of penetration have been observed in some cases. Predacious species attack other predacious species, and in some cas es, young cultures may penetrate older cultures of the same strain.