236 Cytoplasm as Seat of Genetic Properties 



maize, dozens of chromosomal loci are known the mutation of which 

 affects the color or distribution of the plastids. This can hardly be 

 said to be a mutation of the plastids, since the genie material ob- 

 viously controls cytoplasmic conditions, which provide a different 

 environment for the plastids to grow in. An actual mutation of plastids 

 alone would result in a cell containing two types of plastids, normal 

 and mutant ones, with the consequence of hereditary behavior 

 identical with that found in Renner's and Schwemmle's work. In a 

 number of cases such a result has been claimed (see review by 

 Rhoades, 1946), not as a spontaneous occurrence in the plastids but 

 as a result of genie action. It is of course known that genie mutation 

 may be caused by the presence of "mutator" loci, as discussed above. 

 Thus it might be concluded that genie action could as well produce 

 plastid mutation (though the logic of such a conclusion might be 

 regarded as doubtful). 



The general trend of such experiments, as exemplified by Rhoades' 

 (1943) work on the iojap locus in maize, is that differently colored 

 sectors appear in the presence of the mutant locus, which, if isolated, 

 turn out to be henceforth independent of the genie constitution and 

 remain constant in breeding. Rhoades found many details in the 

 behavior of these plastids which would suggest that the ij locus affects 

 the cytoplasm rather than the plastids, the fate of the plastids being 

 controlled by the cytoplasmic change, meaning that the plastids had 

 not mutated, but only failed to develop normally when the cytoplasm 

 was little modified. They were changed irreversibly with a large 

 change in the cytoplasm. The latter would then be called mutation 

 of the plastids, but it cannot be distinguished from an irreversible 

 change in the cytoplasm alone. Only experiments of the type per- 

 formed by Schwemmle, hardly possible in maize, could distinguish 

 this alternative. Thus for the time being a genuine or induced 

 mutability of plastids may be considered as not yet proved. Recently 

 Rhoades (1950) reported that the same mutant iojap also induces a 

 cytoplasmic condition, male sterility, similar to that which we studied 

 before (see II 2 C b). We considered it there as one of many cases 

 of plasmon effect of a generalized nature, all being of the type of 

 inhibitions. In this discussion we showed that the facts do not prove 

 the presence of autonomous genetic units in the cytoplasm, but rather 

 indicate an inhibitory effect upon the respiratory enzyme system 

 located in the mitochondria. Rhoades (1950) tends to localize his 

 sterility effect in the mitochondria, so that our previous discussions 

 apply to this case also. Its special feature is, however, that the same 



