268 



YEASTS AND YEAST-LIKE FUNGI 



morphologically different from their parents. They are small and 

 round and not so vigorous as the parent cells. These proliferating 

 haploid yeast cells may conjugate with each other or with an asco- 

 spore that has not fused. From either of such unions or a union of 

 two spores or nuclei within the ascus, there develops the ordinary 

 yeast cell, which is diploid. Eventually on the proper substrate 

 these develop ascospores, after reduction division. Ascospore-form- 

 ing yeasts are then of two types, one of which consists of those in 



which the usual growth form is 

 haploid and in which spores form 

 immediately after conjugation of 

 the ordinary cells (Fig. 12) . These 

 have been called haplobiontic and 

 are typified by Zygosaccharomy- 

 ces or Schizosaccharomyces. The 

 other type consists of yeasts which 

 are called diplobiontic. In these 

 the usual cell is diploid and the 

 union of cells (and presumably of 

 nuclei) takes place in the ascus 

 or among the budding progeny of 

 the haploid spores. Since the 

 diploid cells are large and vigorous, 

 those cells which have conjugated 

 tend to crowd out the unfertilized 

 haploid cells in the culture. The terms haplobiontic and diplobiontic 

 are not well chosen because they have been used in another sense by 

 biologists. 



Lindegren and Lindegren ^"' "• ^-' '^ have continued the work of 

 Winge and Laustsen. They have confirmed the work of the Danish 

 investigators, but find that yeasts such as Saccharomyces are hetero- 

 thallic. If a single spore is isolated and grown on media, the small 

 haploid cells which develop do not conjugate with each other at all 

 readily. If and when they do conjugate, the ascus which eventually 

 develops has usually only two spores. These spores are mostly non- 

 viable. If these germinate, the cells resulting from this union are 

 still less likely to conjugate. However, when pure cultures, having 

 developed from single spores of opposite sex, are mixed, mating takes 

 place readily. The resulting proliferating diploid cells eventually 

 form spores (usually four) which are found to be viable. Thus the 

 heterothallism of these yeasts is only partial, but in nature it must 

 be usual. There are only two sexes and the sexes are morphologically 



a 



Fig. 111. Asci and ascospores of 

 yeasts: a, Zygosaccharomyces sp. 

 from soil; b, Saccharomyces cerevisiae 

 var. ellivsoidcus ; t, Hansenula anovi- 

 ala; d, Pichia sp. from soil. 



