22-3 THE YEAST CELL 



for the adaptation in the above instance, it is important to note that 

 in this particular case, because of the lethal effect of the phage, it is 

 not possible to determine the mutation rate in the continued presence 

 of the phage, but only prior to its application. Lysis takes place very 

 quickly. Only bacteria which are in the process of division at the 

 time of plating may sometimes complete the division, and the division 

 time is 19 minutes (in broth). Therefore, if the phage were able to 

 induce resistance in bacteria, it would have to be by an almost instan- 

 taneous induction. 



A particularly interesting case of adaptation is that studied by 

 Ryan (1946) of the adaptation of a leucineless Neurospora mutant ob- 

 tained after treatment with ultraviolet light. Genetic tests supplied 

 evidence that the adaptation was a mutation of the recessive leucine- 

 less allele back to the normal dominant wild type allele. The number 

 of adaptations which occurred increased as the concentration of leu- 

 cine was lowered. This is opposite to what would be expected if the 

 mutation were spontaneous and random. One would then expect a 

 larger number of adaptations in higher concentrations of leucine, 

 since the leucineless strain should be able to multiply niore rapidly 

 producing a larger number of nuclei and hence a greater chance of 

 mutation. 



When the non -synthesizing and the synthesizing nuclei were com- 

 bined in a heterocaryon and grown on nutrient agar without leucine, 

 the fimgus grew at the wild-type rate, showing that although both 

 types of nuclei were present, the sjmthesizing nuclei enabled the my- 

 celium to synthesize leucine and thus grow at the normal rate. How- 

 ever, when the same heterocaryon was grown on nutrient agar con- 

 taining a limiting concentration of leucine, the growth rate was that 

 of the non-S3mthesizing organism. It was shown further that on this 

 limiting medium the non-synthesizing nuclei actually displaced the 

 sjnithesizing nuclei in the hyphal tips during growth, so that when 

 hyphal tips are cut off, they contain only leucineless nuclei. This re- 

 placement accouints for the growth occurring at the leucineless rate. 



Two groups of leucineless cultures were grown, one on a high 

 and the other on a low concentration of leucine, so that when each 

 group had completed growth, the first consisted of a larger amoimt 

 of mycelium than the second. Then small clots of wild-tjrpe myce- 

 lium were inserted into the middle of the masses of leucineless 

 mycelium and allowed to grow to completion. The small clot of wild- 

 t]^ mycelium inserted into the small mass of leucineless mycelium 

 grew to a greater size than that inserted into the large mass. Appar- 

 ently the large mass of leucineless mycelium is more effective than 

 the small mass in suppressing the growth of the wild-type mycelium. 



From the foregoing data the conclusion was drawn that, in the 

 heterocaryon, there is direct competition between the synthesijsing 

 and the non-synthesizing nuclei, and that the greater the number of 



