13-8 THE YEAST CELL 



TRANSFORMATION OF SACCHAROMYCES INTO TORULOPSIS 

 (CRYPTOCOCCUS, TORULA) 



The four cultures in fig. 13-1, two of which were originally a 

 and two a, arising from the four ascospores of a single ascus, were 

 carried in culture in the laboratory for over a year and were ob- 

 served to produce a great variety of morphological mutants during 

 that period. That they were still haploid was shown by the shape 

 and size of the cells (fig. 13-4), After this extended subculturing 

 the cultures were mated to each other in all possible pairs, follow- 

 ing exactly the same procedure shown in fig. 13-1. These matings 

 all failed to produce either copulations or diploid cells. No asci 

 were obtained by transferring the mated cultures to our presporu- 

 lation medium and gypsum slants. These cultures have become 

 completely neutralized as to sex and have been transformed into 

 typical members of the genus Torulopsis. They are incapable 

 of producing diploid sporulating cultures either by legitimate or 

 illegitimate copulation. This undoubtedly is due to mutation either 

 of the principal a/a alleles or of modifying genes which inhibit 

 copulation. 



The origin of Torulopsis from Saccharomyces was suggested 

 by Satava (1934), and Winge and Laustsen (1939) suggested -that the 

 genus Zygosaccharomyces also originated from Saccharomyces. 

 Winge and Laustsen (1937) paired all the Torulopsideae available to 

 them in all combinations and failed to obtain any copulations. 



THE INSTABIUTY OF THE MATING TYPES ON SUBCULTURE 



The fact that many of the cultures from single ascospores which 

 we carried in culture for a year or more were unable to copulate 

 with freshly isolated tester strains capable of copulation suggests 

 the view that standard Saccharomyces capable of copulation may 

 be transformed into neutral Torulopsidae (Cryptococci). Apparently, 

 the genes controlling copulation become altered by mutation under 

 the selection pressure exerted on haploid cultures. When cultures 

 are isolated in the haploid state, the survival value of the genes 

 controlling copulation disappears and mutations which might be 

 deleterious to copulative ability may be favored. The new mutation 

 might give an advantage in growth rate which would be detrimental 

 to copulative strength or specificity. 



Another type of transformation with respect to capability for 

 copulation is found among single ascospore cultures. These vari- 

 ants produce an abundance of copulation tubes as soon as the cul- 

 ture reaches full growth. The cells from cultures of this type also 

 copulate with the cells of any other cultures of either mating type 

 and produce diploid zygotes, irrespective of whether or not they 

 are mated with either a or a strains. The variants have lost the 

 characteristic of discriminating between the a and a types. 



