BACK-MUTATION AND PROGRESSIVE MUTATION 17-9 



Saccharomyces fragilis, with kidney-shaped spores, and the mal- 

 tose fermenter Saccharomyces cerevisiae, with round spores, was 





Fig. 17-5 An Ascus from a Hybrid between S. Cerevisiae 

 (Maltose-Fermenter , Round Spores) and S. Fragilis (Lactose- 

 Fermenter, Kidney— Shaped Spores) Containing Two Round and Two 

 Kidney-Shaped Spoies. They were nonviable. 



successful and yielded a diploid which sporulated to produce two 

 round and two kidney -shaped spores per ascus, fig. 17-5. The 

 spores were nonviable, however, and the culture has been lost. 



