MATING TYPE DI TERAIINATION 277 



4. Chiio's rcniiirknhlc ohscrvntions concerning mating types and 

 kapi");! concentration indicate some precise quantitative distinctions 

 betw een the mating types, hut the nature of the distinctions remains 

 a puzzle. 



NUCLEOCYTOPLASMIC INTERACTIONS IN THE B SYSTEM 



There can be no doubt that a cytoplasmic component occurs in 

 the group B system of mating type determination, nor that this cyto- 

 plasmic component acts on the developing macronuclei so as to deter- 

 mine their manner of development. Differences of interpretation are 

 possible only in regard to the manner in which the cytoplasmic spe- 

 cificity is maintained. Either the cytoplasmic conditions are self- 

 perpetuating, or they are to some extent under the control of the 

 nuclei. 



Interpretations regarding the means of perpetuation of the cyto- 

 plasmic conditions center around the subsequent behavior of the 

 caryonides which have changed mating type at nuclear reorganiza- 

 tion. Sonneborn (unpublished) observed two kinds of changed cary- 

 onides in regard to their stability at subsequent reorganizations. The 

 first group were those that maintained the changed type with as 

 great stability as normal clones and included nearly all caryonides 

 that had changed from mating type VIII to mating type VII and the 

 majority of those which had changed from type VII to type VIII. 

 Many caryonides that had changed from type VII to type VIII 

 showed a considerable amount of reversion from VIII to VII at the 

 next autogamy. More rarely, a caryonide of type VII showed a con- 

 siderable change to type VIII at autogamy. Since in these clones the 

 cytoplasmic conditions usually associated with one mating type were 

 at least partially maintained in the presence of a macronucleus con- 

 trolling the other mating type, Sonneborn concluded that the cyto- 

 plasmic conditions necessary for the inheritance of mating type are 

 self-maintaining and at least to some extent independent of the nu- 

 clear condition. Although failing to manifest its activity at one nuclear 

 reorganization, the cytoplasmic determiner maintains itself and is 

 manifested at a subsequent reorganization. 



This interpretation is called into question first of all by recent 

 investigations regarding the nature of the caryonides which show con- 

 siderable reversion at reorganization. In a study of the inheritance of 



