Senescence in Protozoa 



with an Ascomycete kept in continuous vegetative reproduc- 

 tion. On the other hand, Belar (1924) maintained Actinophrys sol 

 in isolation culture, without the occurrence of paedogamy, for 

 1244 generations over 32 months, and observed no decline in 

 the rate of cell division. Beers (1929) kept Didinium nasutum for 

 1384 generations without conjugation or endomixis. Hartman 

 (1921) kept Eudorina elegans in active asexual reproduction for 

 8 years. Woodruff's oldest culture of Paramecium aurelia persisted 

 for over 15,000 generations but was undergoing autogamy. The 

 conclusion must be that some clones are stable while others are 

 not. 



More light is thrown on this problem by the work of Jennings 

 (1945) upon clones of Paramecium bursaria. He found that in this 

 species the life-cycle fell into well-defined phases of growth, 

 sexual reproduction by conjugation with other clones, and 

 decline. The length and character of these phases differed sub- 

 stantially from clone to clone. In the decline phase the death 

 of individual cells, and especially of the progeny of conjugation 

 between old clones, becomes very common. The vitality and 

 viability of the progeny of conjugation, even when the con- 

 jugant clones are young, varies greatly, and a very high pro- 

 portion of ex-conjugants normally die. This mortality is highest 

 among the progeny of conjugation between related clones. Of 

 20,478 ex-conjugants, 10,800 (52-7 per cent) died before under- 

 going their fifth successive cell division, while 29-7 per cent died 

 without dividing at all. Most conjugations produced some non- 

 viable clones, some weakly clones capable of limited survival, 

 and a few exceptionally strong clones, some of which appeared 

 capable of unlimited asexual reproduction. It is from these 

 strong races that the population of laboratory cultures is 

 normally obtained. 



Jennings concluded as follows: 'Death did not take origin in 

 consequence of organisms becoming multicellular ... it occurs 

 on a vast scale in the Protozoa, and it results from causes which 

 are intrinsic to the organism. Most if not all clones ultimately 

 die if they do not undergo some form of sexual reproduction. . . . 

 Rejuvenation through sexual reproduction is a fact . . . yet 

 conjugation produces, in addition to rejuvenated clones, vast 

 numbers of weak, pathological or abnormal clones whose 



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