264 CELL HEREDITY 



Generally little or no cytoplasm is transferred at conjugation, but 

 under special coiiditions cytoplasmic transfer can he induced by a pro- 

 longed persistence of the paroral bridge. 



It is also possibli' to induce meiosis in unmated individuals, and this 

 process, called autogamy, leads to the segregation of any heterozygous 

 genetic factors present, with the consequent establishment of genetically 

 homozygous animals. The details of the process are rather similar to 

 those of conjugation, as can be seen in Figure 9.10. Genetic segregation 

 in conjugation and in autogamy are diagrammed in Figure 9.11. 



Inheritance of the killer trait is shown in Figure 9.12. When no cyto- 

 plasm is transferred during conjugation, the killer trait is transmitted 

 to those progeny receiving their cytoplasm from the killer-type parent, 

 whereas in the event of cytoplasmic exchange, all progeny become 

 killers. Thus, the inheritance of the killer property strictly follows 

 transmission of cytoplasm, and is independent of gene segregation, pro- 

 vided the genotype will support the cytoplasmic factor, kappa. 



Further intensive studies of kappa have supported the inferences of 

 the first crossing experiments, namely, that kappa is a cytoplasmic deter- 

 minant responsible for the killer trait. It is a large body some 0.2 n in 

 diameter, containing DNA and protein, and it can be isolated from killer 

 cells and introduced into sensitive cells, converting them to killers; 

 thus kappa is an infectious agent. Furthermore, kappa can divide at a 

 different rate from that of the organisms themselves, giving rise to ani- 

 mals with increased numbers of particles and, what is more interesting, 

 to animals with no particles at all. 



Two single gene pairs have been identified which influence kappa. 

 The better known of these, K, is necessary for maintenance of kappa, 

 and cells containing only k lose their kappa particles and become sensi- 

 tive, as shown in Figure 9.12. 



Kappa may mutate to one of several forms which differ in the manner 

 of killing sensitive animals, and segregation has also been demonstrated 

 of two different mutant forms of kappa out of one animal. 



A great deal has been written about the significance of kappa. Ini- 

 tially it was introduced as a particularly clear example of cytoplasmic 

 inheritance, but when its size and chemical composition were discovered, 

 it was relegated to the more specialized position of intracellular sym- 

 biont. More recently, as new particles are found in many organisms 

 which are both infectious agents and the source of new and useful host 

 phenotypes, the distinction has lost its meaning. Kappa is an extreme 

 example of a class of genetic determinants which are not essential cell 

 constituents, but which produce new traits in the host organism when 

 they are present. Other examples include CO2 sensitivity and altered 



