Chapter 1 

 SOME IMPORTANT YEASTS AND THEIR LIFE CYCLES 



Yeast is a general term for any growth that appears in a fer- 

 menting fluid. Whenever grape juice is collected, the growth that 

 accumulates and settles on the bottom is yeast; the film that forms 

 on the top of a pickle barrel is also yeast. The term does not have 

 any botanical connotation; it merely describes a sediment or a film, 

 containing a great variety of bacteria and filamentous fungi along 

 with true yeasts. True yeasts are non -filamentous, single celled 

 microorganisms that generally grow in sugary solutions; many of 

 them produce spores. Their non -filamentous nature distinguishes 

 them from the other fungi; their larger size distinguishes them from 

 bacteria. There are two genera that are very easily differentiated; 

 (1) Saccharomyces and (2) Schizosaccharomyces. 



The cells of Saccharomyces cerevisiae exist in both haplophase 

 and diplophase (fig. 1-1). We found that this species possesses two 

 mating types in the haplophase, and that haploid cells of different 

 mating type copulate to produce diploid cells. Kruis and Satava 

 (1918) in Prague, and Winge (1935) in Copenhagen showed that the 

 standard vegetative cells of S. cerevisiae are diploid, produced by 

 copulation of two spores or gametes derived from spores. The dip- 

 loid nuclei undergo reduction at spore formation to produce four 

 haploid ascospores. The large, ellipsoidal vegetative yeast cell is 

 produced by the fusion of two round haploid gametes derived from 

 ascospores. Winge established the basic facts of this life cycle by 

 a classical series of observations on the germination of ascospores 

 and fusion of haploid cells. When spores of Saccharomyces are 

 planted in culture medium each spore germinates to produce a 

 cluster of round haploid cells which undergo a period of vegetative 

 growth of variable length. There may be an extended haplophase 

 before copulation begins and cell fusion reconstitutes the diplophase. 



Schizosaccharomyces (fig. 1-2) was originally described by Bei- 

 jerinck (1894). This organism has long, cylindrical cells which be- 

 come asci (spore sacs) usually containing eight haploid spores each. 

 The spores often fuse in pairs on germination to produce a long dip- 

 loid cell. The Schizosaccharomyces described by Beijerinck (1898) 

 produced nonsporulating variants rather readily, suggesting that 

 Schizosaccharomyces may have a lengthy haplophase like Saccharo- 

 myces. Further Investigation of this interesting possibility may 

 yield profitable results. 



Saccharomyces increases in numbers by budding; Schizosac- 



1-1 



