Senescence in Protozoa 



cell division is a pair of rejuvenated and infant cells rather than 

 a mother and a daughter of different seniority. 



The indeterminacy of cell lineages has lately been attacked 

 with some ferocity, though on grounds of political philosophy 

 rather than experimental evidence (Lyepeschinskaya, 1950). 

 The question legitimately arises, however, particularly in cili- 

 ates, how far the renewal of structures at mitosis is evenly 

 distributed between the resulting cells. Child (1915) noticed 

 that in Stentor one of the progeny retains the old, while the other 

 forms a new, peristome. From experiments he concluded that 

 this made no difference to the age status of the inheritors, both 

 being equally 'young'. The criterion of 'youth', however, was 

 high susceptibility to cyanide poisoning. The critical experi- 

 ment of making a genealogical table to determine the order of 

 death of the fission products over several generations on the 

 pattern of Sonneborn's (1930) Stenostomum experiments does not 

 appear to have been carried out. 



True senescence, and a marked difference in age status 

 between mother and progeny, certainly appears to occur in 

 suctorians. Korschelt (1922) noticed this in several forms {Acan- 

 thocystis, Spirochona, Podophrya, etc.), while in Tokophrya the 

 parent organism's life-span can be measured, and is increased 

 by underfeeding (Rudzinska, 1952). In a far greater number of 

 cases, there are signs that the copying process at division only 

 produces a new structure additional to one which already 

 exists, not two new, or one new and one manifestly renovated, 

 structure. The theoretical interest of this process (in Euglypha) 

 and its bearing on protozoan 'age' has been noticed before 

 (Severtsov, 1934). In such cases, either the structure does not 

 deteriorate with time, or it is maintained continuously during 

 life, or its possession must ultimately confer a disadvantage on 

 one or other of the division products. 



Whereas in some protozoa organelles, axostyles, flagella and 

 cilia are visibly resorbed or shed at fission, and new ones pro- 

 duced for each fission product, in others, especially in ciliates, 

 maternal organelles, flagella and other structures are shared 

 between the progeny, being taken over by one daughter cell 

 while copies are developed in the other. Of two closely-related 

 species of Spirotrichonympha infesting termites, for example, one 

 i 115 " 



