27-5 



THE YEAST CELL 



position would not normally occur. They called it a cytoplasmic ef- 

 fect due to deficiency in an autonomous cytoplasmic apparatus. The 

 yeast produced viable 4-spored asci whose single ascospore cultures 

 were capable in turn of producing viable 4-spored asci. This is an 

 exception to the otherwise general rule that single ascospore cultures 

 usually produce a high frequency of non-viable spores. Diploids arose 

 from the ascospores in two different ways (fig. 27-1); (a) The spore 

 germinated to produce a bud, and subsequently both the original spore 

 and its daughter cell produced buds. Copulations occurred between 

 the most distantly related cells in the cluster of four and a diploid 

 budded out of each zygote. These diploids had developed from zygotes 

 produced by the fusion of cells whose cytoplasms had undergone com- 

 plete division before fusion occurred. The spores from the zygotes 

 had a viability of about 70 per cent, (b) Occasional haploid spores 



O Co Co 



(QOm 



SPOREIS 



7 0^0 VIABLE 



a. DIVISION and COPULATION 



(QOOQi 



SPORES 

 2% VIABLE 



b. DIRECT DIPLOIDIZATION 



Fig. 27-1 The difference between (a) spore viability pro- 

 duced by copulation of two fully grown haploid cells and (b) spore 

 viability of a diploid produced by direct diploidization . 



were observed to germinate directly to produce diploid zygotes. It 

 was assumed that the nucleus in the original haploid spore had divided 

 without a concurrent division of the cytoplasm and the two haploid nu- 

 clei had fused to produce a diploid nucleus. 



