ADAPTATION TO PANTOTHENATE DEFICIENCY 



22-24 



mutation) and suggests that the entire population adapted rather 

 than a small fraction of it. The yeast containing recessive genes 

 "ab", grows slowly in the pantothenate -free medium, and must be 

 able either to grow at a very slow rate without pantothenate, or to 

 synthesize pantothenate at a very slow rate. Experiments in this 

 laboratory, with sugar fermentation have shown that in the presence 

 of the recessive so-called "nonfermenting" alleles, fermentation 

 actually does occur, but occurs much more slowly than when the 

 dominant gene is present. This corresponds to the well-known ex- 

 ample of the ci gene in Drosophila, in which increased dosage of 

 the recessive gene produces the same effect as the presence of a 

 single wild-type allele (Stern). It seems probable, therefore, that 

 the recessive allele does not involve complete absence of a reac- 

 tion, but rather an extremely slow rate of reaction. On this basis, 

 the slow growth on the pantothenate -minus medium during the pri- 

 mary adaptation may be due to very slow synthesis of pantothenate 

 by the recessive alleles. 



MUTATION IN THE ABSENCE OF PANTOTHENATE 



Rapidly growing mutants were isolated from the cuiture tube at 

 the points indicated (fig. 22-7). After 97 days, one colony which 

 grew more rapidly than the rest was detected in a sample of approx- 

 imately 44,000 cells plated on test agar plates. No colonies were 

 detected in two intermediate platings with a total of 60,000 to 70,000 

 cells each, but in the next plating another mutant colony was found. 

 This was 102 days after the original inoculation. 



Table 22-4 

 Rapidly-growing Mutants Detected in Culture II 



The cell population was estimated from a haemocytometer count 

 and the number of cells plated by a colony count of a diluted sample 

 plated on plus -pantothenate nutrient agar. 



