13-10 THE YEAST CELL 



In our genetical studies we have selected vigorously against 

 this character and in our breeding stocks a high degree of stable 

 mating type specificity has been achieved. 



The genes controlling copulation in yeasts are much more sta- 

 ble than the alleles at the AB loci in the Hymenomycetes. Two cul- 

 tures of Coprinus lagopus collected from different dung heaps are 

 completely interfertile (Hanna, 1925) due to mutations at the AB 

 loci. Vandendries (1937) extended this study to show that sterility 

 factors can be demonstrated when Hymenomycetes from different 

 continents are mated. The self -sterility alleles in Neurospora are 

 extraordinarily stable (Lindegren, 1932, 1934), we found no muta- 

 tions of + to - or vice versa in nearly 100,000 single ascospore 

 cultures. However, single ascospore cultures of Neurospora often 

 lose their fertility when carried in culture in precisely the same 

 way that yeasts do, probably because of genes modifying the +/- 

 factors. The fact that multiple mutants of Neurospora are less 

 fertile than the wild type suggests that most mutant genes modify 

 the +/- alleles to reduce fertility (Lindegren, Beanfield, and Bar- 

 ber, 1939). 



Haploid yeasts are astonishingly variable. Subculturing a hap- 

 lophase yeast produces a large variety of morphological mutants. 

 If the a/a alleles are modified by these variations, changes in 

 fertility might occur. This would be especially true in a yeast 

 culture, since the selection pressure is great and constantly 

 varying, due to the effect of the growth of the organisms on the 

 substrate. Diploid cultures are protected against variation pos- 

 sibly because the presence of paired chromosomes minimizes the 

 probability of depletion mutations occurring (see Chapter 15). Il- 

 legitimate diploids are stable by another device. They generally 

 fail to produce viable haploid ascospores but produce diploid as- 

 cospores which germinate to form clones indistinguishable from 

 the parent type. 



