342 CLASS ASCOMYCETEAE 



depending upon the type of asexual reproduction characteristic of the 

 species. Eventually some of these cells enlarge a little and the nucleus 

 divides (probably meiotically) into four and the four ascospores are 

 produced. These in their turn conjugate by twos. Yeasts of this type are 

 apparently diploid during their whole vegetative life, the ascospores ^lone 

 being haploid. Winge (1935) showed that sometimes a pair of ascospores 

 may be so situated in the ascus that they do not unite. In that case each 

 buds vegetatively. The colonies thus formed consist of haploid cells 

 which are smaller and more rounded than the normal diploid cells of the 

 species. Soon one or more pairs of these haploid cells, of varying degrees of 

 relationship, fuse and the resulting diploid cell multiplies by budding to 

 produce the larger more elongated cells characteristic of the normal vege- 

 tative development of the species. 



A third category of yeasts has been assumed to produce its asci par- 

 thenogenetically. Saccharomyces cerevisiae Meyen, the common yeast of 

 bread and beer, does not show conjugation of the ascospores within the 

 ascus nor is such conjugation observed immediately prior to ascus 

 formation. However the Lindegrens (1943, 1949) have shown that the four 

 ascospores of a normal four-spored ascus when germinated separately in 

 the proper culture media produce colonies of haploid cells resembling 

 those mentioned above in the case of failure of a pair of ascospores to 

 conjugate within the ascus. Mostly these remain of the haploid type 

 although occasionally conjugation between two cells may occur to form an 

 "illegitimate" diploid colony. This may, under conditions favorable for 

 ascosporogenesis, produce asci but these usually contain only two asco- 

 spores which are mostly incapable of germination. These illegitimate 

 diploid colonies when perpetuated in conditions favorable for asexual 

 multiplication may be of industrial value since these strains are frequently 

 very constant and free from mutations. When the haploid colonies from 

 the four ascospores are mated vigorous diploid colonies develop in certain 

 matings and these under conditions favorable for ascospore formation 

 produce four-spored asci whose ascospores are capable of germination. 

 These matings show that two of the four ascospores in the normal ascus 

 are of one mating type and two of another. The haploid strains from the 

 original four ascospores may develop into lines that under no mating 

 combinations are able to produce ascospores. In other words they may by 

 mutation become asporogenous yeasts to which the name Torulopsis 

 (Toriila) has been given in the past (see below). Saccharomyces paradoxus 

 Batsch, as shown by Guilliermond (1936), is variable as to the point 

 where sexual reproduction occurs. Under some conditions the diploid 

 phase vegetative cells become asci and produce ascospores which con- 

 jugate in the ascus. From the zygotes thus formed arise by budding the 

 usual vegetative cells of the yeast. Sometimes in the same species under 



