Senescence in Protozoa 



other words, the 'germ-plasm'. The other portion, the macro- 

 nucleus, controls the metabolism of the cell. It becomes highly 

 polyploid, and at subsequent cell divisions, while the micro- 

 nucleus divides evenly in the normal manner of nuclei, the 

 macronucleus distributes its chromosomes at random to the 

 daughter macronucleus arising from it. Because of the enormous 

 number of sets which it contains, every cell in the earlier divi- 

 sions has a fair chance of getting its quota, but with the passage 

 of time more and more daughters receive an unbalanced set and 

 a reduced physiological repertoire, and a chromosome once lost 

 cannot be restored from the micronucleus except by sexual 

 division — conjugation or autogamy. In the later stages of clonal 

 senescence even sexual division is affected and abnormal or 

 non-viable products increase. Sonneborn has shown that this is 

 not due to the accumulation of mutations, since it can be pre- 

 vented by periodic autogamy, even though this does not alter 

 the genotype: it appears to be due to injury inflicted upon the 

 micronucleus itself through the abnormal intracellular condi- 

 tions produced by the defective macronucleus. In ciliates the 

 germ-plasm has to live in the cell where the processes of somatic 

 maintenance are carried out, and it is therefore unusually 

 exposed. This is probably a unique situation — it does not even 

 apply in other ciliates — and the division of function between 

 vegetative and germinal nuclei is confined to this group. The 

 existence of presumed cytoplasmic mutations, although there is 

 no evidence to relate them to metazoan senescence as such, 

 might be far more relevant to it than studies of protozoan clones. 

 A kindred subject, that of somatic aneuploidy, is discussed in 

 6-1-3 (p. 168). It is in any case probably misleading to identify 

 the decline of protozoan cultures with the metazoan senescence 

 which it superficially resembles; it is doubtful if analogies can 

 properly be drawn between acellular organisms and metazoan 

 cells, and the only relevance of the whole question of 'ageing 5 

 in protozoan clones to ageing in the metazoan body lies in the 

 the light which it might possibly throw upon the effects of cell 

 division in renewing expendable enzyme systems. There is no 

 special reason, upon the present evidence, why the 'senescence' 

 of Paramecium should continue to figure as extensively as it has 

 done in treatises devoted to gerontology. 



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