522 Heredity in Protozoa 



of a dominant gene K and a self-reproducing plasmagene, kappa. Non- 

 killer strains may be genotypes KK, Kk or kk which lack the factor kappa. 

 Although gene K may occur in the absence of kappa, maintenance of 

 kappa in the cytoplasm depends upon the presence of gene K, perhaps 

 in the macronucleus as well as the micronucleus. Sonneborn (81) sug- 

 gested that kappa particles are distributed as single molecules throughout 

 the body. Later calculations of Freer (61), however, indicate a size of 

 0.3-3.0[j. for kappa particles. Comparable particles, present only in killer 

 strains, have been identified as granules stainable by Feulgen and Giemsa 

 techniques and containing desoxyribonucleic acid (61, 63). Treatment of 

 P. aurelia with nitrogen mustard inactivates kappa and at the same time 

 reduces the number of these granules (18). Kappa particles apparently 

 lose the power to reproduce at temperatures above 33.5° (62, 83). Some 

 of these characteristics suggest close chemical similarity between kappa 

 and paramecin. Under optimal conditions, kappa in variety 2 of P. 

 aurelia is quadrupled daily. Consequently, it is possible to decrease 

 the kappa content by increasing fission-rate, or even to eliminate kappa 

 completely and permanently from strains of variety 2 (62). This appar- 

 ently is not possible for the kappa of variety 4 which increases fast 

 enough to keep up with rapid fission (79, 83). By depressing the fission- 

 rate, the kappa content of a low-kappa strain may be increased pro- 

 gressively, at a rate which apparently varies with the strain or with the 

 kind of kappa. Mutations of kappa have been reported in variety 4 of 

 P. aurelia. The mutant plasmagenes stimulated production of a new kind 

 of paramecin with a different lethal action on sensitive cilates (13). 



The behavior of kappa in autogamy and conjugation has been de- 

 scribed (77, 79). After autogamy in a A^A-kappa line, the persistence of 

 the killer trait for a few generations in homozygous recessive (^^-kappa) 

 lines suggests that kappa can maintain the production of paramecium 

 even without gene K. Sooner or later, however, the recessives become non- 

 killers, presumably because kappa cannot increase in the absence of gene 

 K. The disappearance of kappa thus shows a "lag" analogous to that 

 described for inheritance of size in conjugation. If gene K is reintroduced, 

 by crossing the recessive with a homozygous killer {KK) strain, before all 

 the kappa has been lost, the hybrid (J'i^A'-kappa) descendants remain 

 killers. If the cross, kk x ii^iiC-kappa, is made after the recessive has be- 

 come a non-killer strain upon exhausting its original supply of kappa, 

 the heterozygous {Kk) descendants of the non-killer conjugants remain 

 non-killers. Such results are said to demonstrate that gene K cannot 

 initiate the production of kappa after it has disappeared from the cyto- 

 plasm. Therefore, cytoplasmic inheritance is very important in trans- 

 mission of the killer trait. 



It has been reported more recently that, in certain crosses of X/^-kappa 

 y. kk, both exconjugant lines become killers (A^/f -kappa). This result is 



