240 Cytoplasm as Seat of Genetic Properties 



(1924) spoke of plasmagenes, and Harder (1927) followed him. 

 Correns countered this idea with the statement that we should speak 

 of plasmagenes only if many different ones could be found to be 

 sorted out in the course of cell divisions. If, however, the entire cyto- 

 plasm is involved, as in all the early work, the logical assumption is 

 that the cytoplasm is involved in all genie actions; that is, the cyto- 

 plasm is a specific substrate or plasmon. We saw how the successors of 

 Wettstein changed this simple idea, to which Correns clung, into that 

 of the plasmon-sensitive genes, which I criticized as a rather oblique 

 view of the facts. 



The great new upswing of the interest in cytoplasmic heredity 

 came when viruses entered the field of genetic study and still more 

 when Sonneborn's work on Paramecium introduced what looked like 

 corpuscular plasmatic entities of heredity, plasmagenes. Cytoplasmic 

 heredity via plasmagenes became so popular that in textbooks and 

 symposia plasmagenes were presented as established facts, and genet- 

 icists who remained cautious were considered fossils. Some geneticists 

 and even embryologists ceased speaking of cytoplasm in genetic dis- 

 cussions, but used the term "plasmagenes" for whatever property of 

 the cytoplasm they discussed. Tlien Sonneborn in his indefatigable 

 search for a full knowledge of the killer effect, the cornerstone of the 

 theory of plasmagenes, found that the plasmagenes were visible virus- 

 like bodies of the Rickettsia type. This discovery dampened the 

 enthusiasm for plasmagenes, and we are now again in a phase which 

 recognizes the plasmon in a certain sense but is skeptical about 

 plasmagenes. I have always been among the skeptics, and the fore- 

 going presentation amply illustrates the reasons for it. 



Most informative and eloquent discussions of the subject in its 

 present state have been made by Sonneborn (1949, 1950, I95la,b, 

 1955). He realizes that in many cases the cytoplasm acts as a sub- 

 strate, affecting quantitatively gene-controlled reactions. Self-dupli- 

 cating elements in the cytoplasm exercise their own effects interacting 

 with those of the genes. He expresses this interaction in terms of 

 reaction norm. "What is transmitted during reproduction is genetic 

 material with a particular reaction norm. The reaction norm denotes 

 different responses under different conditions, without change in the 

 responding genetic materials. The cytoplasmic genetic materials con- 

 stitute or control part of the conditions to which the genes respond, 

 and the nuclear genetic materials constitute or control part of the 

 conditions to which the cytoplasmic genetic materials respond. The 

 phenotype is the result of interaction between the two components 



