HyBRIDIZATlON 10-6 



the species is limited; for in the diplophase (which constitutes al- 

 most the entire life cycle in Saccharomycodes) mutations do not ap- 

 pear so frequently. It is relatively weak mating strength evidenced 

 in Saccharomyces which makes an extension of the haplophase pos- 

 sible. The greater vigor of the diplQphase enables it to outgrow 

 the haplophases after the diplophase has been produced. The vari- 

 ety of genes affecting mating strength results in the production of 

 a large variety of sterile haploids (which are essentially "blind al- 

 leys" in terms of the continuity of the species), and many of these 

 haploids become stabilized in the form of the genus Torulopsis. 

 Some freshly isolated haplophase cultures are incapable of mating 

 and these would normally be placed in one of the asporogenous 

 genera. 



SELECTION VERSUS HYBRIDIZATION 



Previous to the development of a system of hybridizing yeast 

 all improvements resulted from selection. One can carry on a con- 

 tinuous selection with the unstable haplophase culture and get con- 

 tinued improvement for a long time. Such continued selection gen- 

 erally involves much mutation and the resulting culture may be- 

 come sterile and incapable of hybridizing. It is, therefore, diffi- 

 cult to carry out a program of yeast improvement involving inten- 

 sive selection followed by hybridization. Matings are most easily 

 made between haplophase cultures that have been recently derived 

 from single ascospore cultures. Since haploid yeast cultures may 

 become sterile when subcultured vigorously, the test for mating 

 type is best performed with single ascospore cultures obtained 

 from freshly isolated spores. Each haplophase culture is either 

 sterile or belongs to either the a or a mating type; a culture which 

 copulates with both a and a haplophases is very unusual. In spite 

 of occasional illegitimate copulations, the mating types are differ- 

 entiated primarily by a single pair of alleles, which may lose their 

 potency presumably by mutation and which are only rarely trans- 

 formed into each other, i.e., a into a. We have evidence, however, 

 that this does occur for some illegitimate diploids which we tested 

 produced both a and a cultures, indicating that even in illegitimate 

 copulations, mutation probably precedes the act of copulation. 



SPAN OF LIFE CYCLE 



A short generation time is an advantage in genetic al studies. 

 The following schedule reveals the minimum time intervals involved 

 in yeast breeding: 

 Days 



Isolation of ascospores. 



1 Transfer of haplophase colony to agar. 



