14-4 THE YEAST CELL 



The yield of the single ascospore cultures from 4-spored asci is 

 variable, indicating that the original culture is heterozygous for 

 the factors affecting the ability of the cells to grow in the pre- 

 scribed medium. 



The second generation originated from culture Db, which gave 

 a high yield and produced viable 4-spored asci on gypsum. Seven 

 of eight 1-spored asci germinated, and the yield of the cultures 

 from 1-spored asci was approximately the same as that of the cul- 

 tures from which they originated. Fourteen 4-spored asci were 

 dissected and the fifty-six single spores isolated; only thirty-three 

 spores produced viable cultures. Both yield and ascospore pro- 

 duction were rather variable among these cultures. Only two of 

 the thirty -three single ascospore cultures produced 4-spored asci; 

 the others generally produced 1- or 2-spored asci, or failed to 

 sporulate. 



Sixteen intact 4-spored asci were isolated and planted in hang- 

 ing drops. All germinated to produce cultures. The resulting co- 

 lonies were suspended in water and plated on agar. Individual co- 

 lonies were selected and tested for yield. Although only 19 per 

 cent of single ascospore cultures gave a yield of 6.0 or more, 45 

 per cent of the cultures from self -fertilized asci fell in thjs cate- 

 gory. The following shows the data summarized from fig. 14-0: 



Class Range Single Colonies 



Asco- from 



spore Self-ferti- 



cultures lized asci 



4.0 - 4.4 6 1 



4.5 - 4.9 10 3 



5.0 - 5.4 23 17 



5.5 - 5.9 42 34 



6.0 - 6.4 16 34 



6.5 - 6.9 3 11 



The highest-yielding colonies from the self -fertilized asci were 

 placed on gypsum, and all but one of them produced an abundance 

 of spores. Three of them (Db5d, Db5h, and Db5i) produced viable 

 4-spored asci and were used to progagate the third generation. 



Only two of the 34 single ascospore cultures in the second gen- 

 eration (DblA and Db2A) produced 4-spored asci. Five 4-spored 

 asci originating from these single ascospore cultures were isolated 

 but no viable cultures were obtained. Sixteen 1-spored, eleven 2- 

 spored, and four 3-spored asci from these single ascospore cul- 

 tures also failed to produce cultures. A few of the asci from single 

 ascospore culture Db25b were viable but the low viability of the 

 ascospores from single ascospore cultures arising from normally 

 heterozygous diploid cultures seems clearly established by these 



