The Biology of Senescence 



pre-destined fate is early death. The rejuvenating function of 

 conjugation is distinct from, and in addition to, its function as 

 a producer of variation by redistribution of genes. Among the 

 clones produced (by conjugation) there are seemingly, in some 

 species, some clones of such vigour that they may continue 

 vegetatively for an indefinite period, without decline or death' 

 (Jennings, 1945). 



Some authors have regarded the increased proportion of 

 weak and non- viable conjugants of old clones as the outcome of 

 an accumulation of unfavourable mutations. Comparable effects 

 (Banta, 1914; Banta and Wood, 1937) have been described in 

 clones ofDaphnia. This was long since suggested by Raffel (1932) 

 on the basis of Paramecium experiments. The type of lineal 

 'senescence' which occurs in Paramecium is in some respects 

 analogous to the processes which are familiar in inbred stocks 

 reproducing sexually, from Drosophila to domestic cattle (Regan, 

 Mead and Gregory, 1947), and described under the general 

 title of inbreeding depression, but differs from it in that in clones 

 the accumulation of mutations, rather than the segregation of 

 existing genes and the loss of the advantages of heterozygy, have 

 been held to be involved. The mortality among the progeny 

 of autogamy in Paramecium is directly related to the length of 

 time during which autogamy has been previously suppressed 

 (Pierson, 1938). The time scale of the group 'life-cycle' is 

 modified by a great many physical and chemical agents — on 

 the other hand, methylcholanthrene, normally a mutagenic 

 agent, delays the decline of Paramecium clones (Spenser and 

 Melroy, 1949). 



The real mechanism of clonal senescence in Paramecium, how- 

 ever, has now been brilliantly elucidated by Sonneborn and 

 his co-workers (in press). It depends, as other workers have 

 foreseen (Faure-Fremiet, 1953) on the peculiar mechanism in 

 ciliates whereby the germinal and vegetative functions of the 

 nucleus are divided between two separate structures. When 

 Paramecium divides after a sexual process, the new nucleus of 

 each daughter cell again divides into two. One of these products, 

 the micronucleus, which reaches the anterior end of the cell, 

 has the normal diploid number of chromosomes, and is appar- 

 ently concerned solely with conveying the genotype: it is, in 



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