ADAPTATION TO PANTOTHENATE DEFICIENCY 22-38 



The supposedly "random" gene mutations initiating the secon- 

 dary adaptation are not necessarily fortunate accidents which occur 

 without previous history in the phylogeny of the organism; they may 

 be the result of a standard mechanism developed by selection. 



CONCLUSIONS 



Adaptation from pantothenate dependence to pantothenate inde- 

 pendence of a haploid yeast was found to be due to mutation of a 

 single gene. The new mutant gene is not a back mutation to the 

 dominant gene originally present, but is a mutation at a different 

 locus. The new mutant enables the yeast to sjmthesize pantothenate 

 by an alternate route to that controlled by the original wild-type 

 synthesizing gene. 



There are two steps in the adaptation to pantothenate indepen- 

 dence: (1) An adaptation to slow growth in minus pantothenate me- 

 dium, which is transmitted vegetatively. (2) Gene mutation to pan- 

 tothenate synthesis. The gene mutation was identified as a single 

 gene by independent segregation of the wild -type and the mutant sjni- 

 thesizer. However, the growth rate of the mutant increased on se- 

 rial transfer of the adapted culture prior to making the hybrid in 

 which segregations occurred. The mutant locus for pantothenate 

 synthesis may be capable of different levels of synthesizing capacity, 

 and the environment may act to select the most active level, there- 

 by increasing the rate of growth on serial transfer. 



Mutation to sjmthesis, in this case, seems to occur at random 

 both in the presence and in the absence of pantothenate. From the 

 limited amount of data obtained so far, there appears to be no dif- 

 ference in mutation rates in the presence and in the absence of pan- 

 tothenate. However, in the presence of intermediate or high con- 

 centrations of pantothenate the non-sjmthe sizing cells overgrow the 

 mutants and, therefore, adaptation occurs only in very low concen- 

 trations of pantothenate. 



The primary adaptation to the medium, permitting very slow 

 growth, has not been shown experimentally to occur at random. The 

 fact that the generation times of cultures, during the primary adapta- 

 tion, varied from approximately 40 hours to 80 hours is consistent 

 with what would be expected with spontaneous mutations. However, 

 there are two facts which make it seem most likely that the primary 

 adaptation consists of a change in all or a large portion of the cells 

 some time after inoculation into the deficient medium. (1) The change 

 occurs with considerable regularity in a relatively small number of 

 cells while gene mutations ordinarily occur extremely rarely and with 

 great irregularity. (2) Samples of a few hundred cells taken at ran- 

 dom from a culture growing slowly following the primary adaptation 

 showed little variation in rate of growth. This slow growth may be 



