PLASMAGENES 



391 



Thus the persistence of kappa is strictly gene-dependent. The particles 

 can be seen in microscopical preparations, following suitable staining. 

 Unlike most normal cytoplasmic particles, they contain DNA, and it 

 seems likely that they should be regarded as invasive exogenous organisms, 

 perhaps comparable to large viruses or rickettsias, rather than as normal 

 parts of the Paramecium. 



A rather similar case, and this time in a higher organism, is the so- 

 called CO2 genoid in Drosophila (L'Heretier 195 1). This is a cytoplasmic 

 factor which causes the individuals carrying it to be highly sensitive to 

 COg, which produces in them an irreversible anaesthesia at a concentra- 



/</U£fi SENSITIVE 



® ® ®® ®® ® (g) 



MILLMIfS scNsmves 



Figure 18.3 

 Cytoplasmic inheritance of kappa. 

 In the figure on the lefi:, conjugation is occurring between a killer (with 

 genotype KK) and a sensitive (with genotype ]i\i). In the third figure they 

 are exchanging nuclei and in the fourth and fifth the macronuclei are being 

 reconstructed. Both daughters (now ready to divide) have the genotype Kk, 

 but only that with the original kappa cytoplasm will be a killer. (From 

 Beadle 1949, after Soimeborn and others.) 



tion wliich has little effect on normal flies. The agent is easily transmitted 

 by the cytoplasm of the eggs, and can also pass through the sperm, al- 

 though its passage through the male is very irregular, presumably because 

 of the small quantity of cytoplasm carried by male gametes. There are 

 no genes known with a clear-cut effect on its propagation, but it certainly 

 multiphes more easily in some genetic stocks than others, so that it too 

 may be considered as gene-dependent. There seems no reason to think 

 of the Drosophila genoid as anything other than an exogenous virus. 



For many other viruses diseases, there is as yet little evidence of nuclear 

 control of susceptibility or resistance. On general grounds, however, it is 

 probable that there is always some variation in this respect and this will 

 probably be under the control of numerous nuclear genes, each of small 

 effect. Again, the physiological result of infection with the exogenous 

 particle in some cases clearly depends on the nuclear constitution of the 



