242 PROTOZOOLOGY 



susceptibility to the immobilizing action of a given serum. Strains of 

 Tetrahymena pyriformis differ in antigenic reactions, as has already 

 been mentioned (p. 227). Sonneborn and his co-workers have studied 

 serological reactions in Paramecium aurelia (Sonneborn, 1950). 



When a rabbit is inoculated intraperitoneally with a large number 

 of a strain of P. aurelia, its serum immobilizes in a high dilution, the 

 organisms of the same strain, but not of other strains. Such a sero- 

 logically distinct strain is called a serotype or antigenic type. It was 

 found that a clone originating in a homozygous individual gives 

 rise to a series of various serotypes. Race 51 gave rise to eight sero- 

 types: A, B, C, D, E, G, H and J, and race 29, to seven serotypes: 

 A, B, C, D, F, H and J. When a serotype is exposed to its antiserum, 

 it changes into other types, which course Sonneborn was able to 

 control by temperature and other conditions. For example, serotype 

 D (stock 29) may be changed by its antiserum to type B at 32°C. 

 and to type H at 20°C, types B, F and H are convertible one into the 

 other and all other types can be transformed to any of the three ; and 

 serotypes A and B (stock 51) are convertible one into the other, and 

 other types can be changed to A or B. The antigenic types are in- 

 herited, if the cultures are kept at 26°-27°C. with food enough to 

 allow one division a day. When induced or spontaneous changes of 

 serotype occur, crosses made among different serotypes of the same 

 strain reveal no effective gene differences among them ; thus all sero- 

 types of a strain possess apparently an identical genie constitution. 

 Sonneborn finds serotype A of stock 29 is not exactby the same as the 

 type A of stock 51. When these are crossed, it is found that the dif- 

 ference between two antigens is controlled by a pair of allelic genes 

 of which the 51A-gene is dominant over the 29A-gene. On the basis 

 of these observations, it has been concluded that nuclear genes con- 

 trol the specificity of the physical basis of cytoplasmic inheritance 

 in these antigenic traits, and hereditary transformations of serotype 

 are cytoplasmic "mutations" of hitherto unknown type. 



In the inheritance of the killer trait and of serotype, both traits 

 are cytoplasmically determined and inherited; hereditary changes 

 are brought about by environmental conditions; and the traits are 

 dependent for their maintenance upon nuclear genes. However, the 

 specific type of killer trait is controlled by the kind of kappa pres- 

 ent, not by the genes, while the specific type of A antigen is de- 

 termined by the nuclear genes. The transformation of the killer 

 to the sensitive is made irreversible, but that of serotypes is not. 

 The various types of killer character are not mutually exclusive, 

 as different kinds of kappa can coexist in the same organism and 



