Role of Preformed Structure in Cell Heredity L99 



two sets of organelles and replacement by one new set (Faure-Fremiet, 

 1945; Tartar, 1954a). Still other processes have heen observed. 



The question is whether, by one process or another, doublet- are 

 destined inevitably to revert to singlets. The answer is "No." Apparent 

 ultimate mass reversion has been shown to be due to one or more of 

 several causes which by no means involve 100 per cent reversion or 

 even a high rate of reversion. Chatton (1921) pointed out tbat the 

 singlets which arose with low frequency in Glaucoma multiplied 

 faster than the doublets and simply overgrew the culture. This has 

 been confirmed in a number of other ciliates, for example, for Col- 

 pidium by Sonneborn (1932). As he pointed out, the same result 

 would occur even in the absence of differential reproductive rates, but 

 more slowly, because doublets keep producing singlets occasionally, 

 while the latter do not revert to doublets. This one-wav change leads 

 to higher and higher proportions of singlets in the culture. Chatton 

 (1921), Margolin (1954), and Uhlig (I960) have further shown that 

 the rate of singlet production by doublets depends upon the cultural 

 conditions; conditions that permit rapid multiplication reduce the 

 frequency of reversion. Uhlig. ascribing Tartar's (1954a) limited 

 maintenance of doublet Stentors to suboptimal cultural conditions, 

 succeeded in maintaining them for hundreds of fissions and apparently 

 could do so indefinitely. Indeed, under appropriate conditions, doub- 

 lets have been maintained indefinitely by culling out the revert ant 

 singlets (Dawson, 1920, on Oxytricha; Sonneborn. 1932, on Colpid- 

 ium; Uhlig, 1960, on Stentor: the present work on Paramecium; and 

 by others on other ciliates ) . Moreover, Sonneborn ( 1932 ) showed 

 that selection for morphologically more "perfect" or symmetrical 

 doublets in Colpidium could reduce the frequency of reversion from 

 one in seven line-days to zero in 864 line-days. Doublets are not 

 destined inevitably to revert to singlets; aside from occasional pro- 

 duction of singlets, they can in general keep reproducing true to type 

 by fissions indefinitely. 



However, occasional production of singlets is genetically important 

 in two respects. First, exceptions to doublet self-reproduction are 

 useful, as teratological phenomena usually are, in revealing some of 

 the processes involved in the normal maintenance of type ( see page 

 192). Second, the fact that revertant singlets and their doublet sister 

 lines of descent carry products of division of one and the same 

 ancestral nucleus implies strongly that the difference between the two 

 cell types could hardly be due to a genotypic difference in their nuclei 

 ( Sonneborn. 1932 ) . 



