Extranuclear Genes and Nuclear Genes 



411 



FIGURE 45-1. Normal {above) and 

 kappa-containing {right) Paramecium. 

 {Courtesy ofT. M. Sonneborn.) 



which are located in the cytoplasm of certain 

 strains of the protozoan, Paramecium. Hun- 

 dreds of kappa particles can be seen un- 

 stained in a single cell (Figure 45-1). They 

 contain DNA and are self-reproducing. 

 Individuals containing kappa are called 

 killers, since animal-free fluid obtained from 

 cultures of killer paramecia will kill sensitive 

 (kappa-free) individuals. 



Mutant kappas are known which produce 

 diff'erent poisons. Kappa is hberated into 

 the medium once it develops a highly re- 

 fractile granule, which sometimes appears as 

 a "bright spot" under the microscope. One 

 "bright spot" kappa particle is enough to kill 

 a sensitive individual. Kappa has a specific 

 relationship to its host, in that a particular 

 dominant host gene must be present in order 

 for kappa to maintain itself, i.e., reproduce. 

 Killer individuals that are homozygotes for 



the recessive host allele partition the non- 

 dividing kappa among the two daughter cells, 

 and this continues in successive divisions until 

 a daughter receives none and is a kappa- 

 sensitive cell. 



Just what is kappa? In size and shape it 

 resembles a bacterium and, like a bacterium, 

 is known to be infective. But, kappa diff"ers 

 from bacteria in certain staining reactions 

 and in internal morphology, particularly 

 when kappa develops the bright spot, the 

 refractile granule. Even though kappa looks 

 like no known bacterium, the fact that it is 

 infective and is not typically found in para- 

 mecia suggests that it is a foreign organism 

 of some kind. Nevertheless, kappa furnishes 

 our first example of genes which are extra- 

 nuclear but intracellular in location. 



The special significance of kappa is that it 

 furnishes a model of how a parasitic or 



