Synchronous divisions in mass cultures of the ciliate protozoon Tetrahymena pyriformis 



Discussion 



Chairman: J. F. Danielli 



Muriel Robertson. In the protozoological laboratory at the Lister Institute we have 

 repeated Dr. Zeuthen's work on the synchronization of cultures of the ciliate Tetra- 

 hymena pyriformis (W) with complete success. The variations in temperature must be 

 repeated an adequate number of times, and thereafter the organisms are kept at the 

 optimum of 28 C. rather than at 24° C. The nuclear division of the Tetrahymena is 

 being studied. There is no micronucleus in this strain. The macronucleus does un- 

 doubtedly increase in size as compared with the untreated ciliates, but it does not 

 at present appear that the number of chromosomes is increased. 



B. F. Folkes. These results suggest a very sharp temperature optimum for the synthesis 

 of some metabolic precursor required for the pre-division stage. This implies not only 

 a high energy of activation for these synthetic reactions but also one of two possi- 

 bilities: (a) an inactivation of the responsible enzymes at temperatures above the 

 optimum, or (b) the presence of some other reaction with an even higher energy of 

 activation which is competing for the same substrates or removing the reaction pro- 

 ducts. If we could distinguish between these possibilities we might be nearer the 

 identification of the metabolic precursor. In the case of reaction (a), holding at high 

 temperatures for increasing lengths of time should result in increasing periods of 

 delay of the pre-division stage on returning the organism to optimum temperature. 

 Have you any evidence of this ? 



E. £euthen. I think the evidence is that the holding of the organism at high tempera- 

 tures for varying lengths of time does not influence the delay with which cleavage 

 sets in after return to the optimum temperature. I agree that this makes hypothesis 

 (a) less likely, and that therefore the possibility (b), or an even more complicated 

 one, should be favoured. 



157 



