IV. THE SUPREMACY OF GENES 249 



be noted that kappa particle contains a large amount of DNA. Its 

 strong effect may be attributed to this nucleic acid in addition to its 

 large size. 



It is probably true to say that the gene system has been evolved 

 amazingly in higher organisms because it is the most ideal system for 

 their development. Therefore, presence of particles in cytoplasm such 

 as kappa, which can strongly interfere w^ith the gene, may not be 

 regarded as a normal feature. Also from this point of view the con- 

 cept of the parasitic nature of kappa appears to be reasonable. 



The results obtained by L'Heritier and his collaborators (41) in 

 their studies on the physical basis of sensitivity to carbon dioxide in 

 the fruit fly, Drosophila, are in many respects strikingly similar to 

 the results on kappa in Paramecium. Reciprocal crosses showed that 

 sensitives to carbon dioxide differed from resistants, not in any nuclear 

 gene, but in some cytoplasmic factor. This factor was named sigma 

 and at first regarded as a kind of plasmagenes, but there are also 

 some evidences for its parasitic nature, although the factor has not 

 been seen or counted. Presumably it is much smaller than kappa, 

 more alike in size to a small virus or a nuclear gene. 



Likewise, in higher plants a similar cytoplasmic factor causing 

 male sterility is known ; the individuals having the factor fail to form 

 functional pollen (42). This may also be of a parasitic nature. It has 

 been demonstrated that this factor is inherited through the cytoplasm. 

 Sigma is likewise inheritable ; the flies that developed from the in- 

 fected eggs are sensitive to carbon dioxide and transmit sigma to 

 subsequent generations through the egg cytoplasm. Discussions have 

 already been made on the fact that viruses are transmissible to 

 offspring through germ cells (Part III, Chapter X). It should be borne 

 in mind that not only viruses but also these parasitic Rickettsia-like 

 organisms can be inherited to progeny through host germ cells. 



Mitochondria are usually regarded as normal cytoplasmic particles, 

 but some workers consider them to be parasitic microorganism. This 

 is not an inconceivable concept, though nucleic acid in mitochondria 

 is RNA, so that they are always subjected to the genes, not disturbing 

 the beautiful control of the protoplasm by the latter. 



To sum up, since the character of organisms having evolved to a 

 certain degree is determined by genes, and since this mechanism of 

 character determination is* considered to be most suitable for the orga- 

 nisms, cytoplasmic particles which can strikingly interfere with the 

 function of genes may be, at least in the majority of cases, an abnor- 

 mal existence. 



However, it should be mentioned here that there is an interesting 

 evidence showing that the normal cytoplasm can exert a considerably 



