344 CLASS ASCOMYCETEAE 



CO (TO 



A B 



Fig. 114. Saccharomycetales, Family Schizosaccharomycetaceae. Schizosaccharo- 

 myces odosporus Beijer. (A-E) Stages in the conjugation of two equal haploid cells 

 and the formation of the ascus and ascospores. (Courtesy, Guilliermond: Arm. fermen- 

 tations, 2:129-151, 257-277.) 



other conditions the ascospores do not conjugate but bud and form small 

 colonies of adhering haploid yeast cells. The cells of a single colony arising 

 from a single ascospore conjugate by pairs, thus giving rise to the diploid 

 phase again. This species does not have the two mating types which are 

 possessed by S. cerevisiae. A very full discussion of the various types of 

 sexuality in the yeasts is given by Guilliermond (1936). (Figs. 112, 113.) 



Family Schizosaccharomycetaceae. The only genus is Schizo- 

 saccharomyces. Only two species are definitely recognized by Stelling- 

 Dekker (1931), Sch. pom^e Lindn., with mostly four (or fewer) ascospores, 

 and capable of fermenting saccharose, and Sch. odosporus Beijer., with 

 eight ascospores and incapable of fermenting saccharose. In both species 

 conjugation of two vegetative cells initiates ascus formation. Sch. 

 versaiilis Wick. & Duprat (1945), by its production of hyphae under 

 anaerobic conditions, shows relationship to Endomyces. (Fig. 114.) 



Family Saccharomycetaceae. In this family are found most of the 

 industrial strains of yeasts. The genus Saccharomyces is the most impor- 

 tant. Many species have been described, but the differential characters 

 have been largely based upon their fermentation abilities with different 

 kinds of sugar. In the light of the recent genetic studies of S. cerevisiae it 

 seems likely that many of these represent merely segregations or muta- 

 tions of genetic characters which have been perpetuated nonsexually. 

 Saccharomycodes differs from Saccharomyces mainly in the fact that the 

 ascospores conjugate within the ascus so that the whole life history is 

 diploid. The genus Hansemda (Willia) resembles Saccharomyces vege- 

 tatively but the ascospores are spherical or ellipsoidal with an equatorial 

 ridge or spherical and flattened on one side with a rim at the edge, giving 

 the appearance of a hat. In Deharyomyces the ascus arises from isogamous 

 or heterogamous conjugation and the spherical ascospores are roughened 

 with fine warts. They are usually one (rarely two to three) per ascus. The 

 genus Nemato.spora contains two or three species of yeasts that are para- 

 sitic in plant tissues but easily cultivated in various culture media. 

 Asexual reproduction is by budding. In some species short mycelia are 

 produced. The single yeast-like cells or the cells of the mycelium may be- 

 come transformed into asci within which are produced mostly eight (rarely 

 four or fewer) ascospores. These are usually in two bundles of four spores 



