202 Cytoplasm as Seat of Genetic Properties 



course of several cell generations is used up and replaced by the 

 original condition. This may also be expressed as the final victory of 

 the genically controlled condition over a self-diluting, temporary, 

 induced cytoplasmic change. 



A very different explanation has been proposed by Sonneborn 

 (1947-1954) on the basis of his (and his students') work on antigenic 

 reactions in Farameciura, which show relations of apparent cyto- 

 plasmic properties to genically controlled processes. (We shall hear 

 more of this when discussing so-called plasmagenes.) In Sonneborn's 

 own words (1951a): "Dijfferent genes of one and the same nucleus 

 may be capable of determining two or more alternative, mutually 

 exclusive traits [i.e., antigenic types] only one of which can come to 

 full phenotypic expression in any one cell. The decision of which of 

 these potentiahties will be realized and which suppressed is dictated, 

 at least in part, by conditions outside the nucleus. Once made, the 

 decision is binding during subsequent cell multiplication for a shorter 

 or longer time, and under certain conditions, permanently. Thus two 

 cells with the same genes and under the same conditions may have 

 different alternative traits and maintain them during cell reproduction. 

 In other words, once these traits are developed, they are cyto- 

 plasmically inherited; but the genes seem to determine which ones 

 can be developed." These conclusions were derived from experiments 

 showing the presence of different antigens on the cilia of different 

 lines of Paramecium. These genetically controlled, antigenic types can 

 be changed by thermic or nutritional conditions into one another 

 directly or only after generations of cytoplasmic inheritance. Each 

 type, once developed, perpetuates itself during vegetative and sexual 

 reproduction, until external conditions bring about a shift to another 

 one of the alternative types. This implies, according to Sonneborn, 

 that "cytoplasmic mechanisms control the persistence of activity of a 

 certain gene (or genes) and the suppression of the activity of the 

 other genes at other loci for alternative antigenic traits." 



One of the explanations offered for the facts is that the under- 

 lying conditions are two independent chains of reactions producing 

 two products, each through an intermediate product. Under normal 

 conditions the amount of these intermediate products will reach a 

 stable equilibrium. If, however, these intermediates influence each 

 other (e.g., one inhibits the other at certain concentrations), transitory 

 external changes (like temperature in the experiments) will produce 

 a condition in which only one or the other of the substances is func- 

 tioning. Both these states will be very stable and therefore will be 



