HYBRIDIZATION 



10-2 



is almost impossible if spore-to-spore mating is practiced, for un- 

 less the parents are classified, heterozygosis is merely a matter of 

 chance. Furthermore, the same parent can be preserved indefin- 

 itely in lyophil tubes. 



Fig. 10-1 Copulations Between a and a Haploid Cells of S. 

 cerevisiae, by Phase— Di fference Microscopy. 



MATING TYPES 



Haplophase cultures of S. cerevisiae fall into two main groups 

 which we have designated a and a. Each haploid culture is made up 

 of potential gametes which can be mated with other cells of appro- 

 priate genetic composition from another culture. As the pure hap- 

 lophase cultures age, illegitimate copulations occur between two 

 cells of the same reaction in the same culture, but these matings 

 rarely result in the production of characteristically large diploid 

 cells. Diploid cells produced by illegitimate copulations are gen- 



