CYTOPLASMIC EQUILIBRIUM 



The first evidence of the particulate transmission of plasmagenes 

 we find ill Epilohimn. Again, malc-steriHty appears when the nucleus 

 of one species, E. hirsutum, lies in the cytoplasm of another, E. luteum, 

 as a result of continued back-crossing of the hybrid E. luteum X 

 hirsutum with pollen of hirsutum. But this pollen must occasionally 

 carry over the operative particles of paternal cytoplasm in fertiliza- 

 tion, for in the fourteenth generation of back-crossing about one 

 plant in 400 has some shoots with fertile anthers. Self-pollination of 

 these plants gives fewer male-sterile progeny when the male-fertile 

 flowers are used, instead of the male-sterile flowers. The plasmagenes, 

 therefore, sort themselves out in the growth of the plant, although 

 they do so less rapidly than the plastids: they are probably more 

 numerous. 



Cytoplasmic Equilibrium 



In the infusorian Paramecium aurelia, plasmagenes have revealed 

 still more specific properties. Of the seven varieties which Somieborn 

 has described, four show nothing but mendelian inheritance of their 

 differences. In the other three, however, there is always a cyto- 

 plasmic element in inheritance. This has been most fuUy investigated 

 in the case of two antagonistic types: one of these, the Killer, 

 poisons the other, the Sensitive, type when they are put together 

 in the same water. They can, however, be crossed and it is some- 

 times found that the difference is wholly cytoplasmic in inheritance. 

 In other cases a nuclear gene also comes into play. 



Where this happens each of the two Fj individuals from con- 

 jugation, in spite of their exchange of nuclei, resembles in character 

 the cell from which it came, so revealing the cytoplasmic element in 

 transmission. The offspring of the Sensitive individual continue to be 

 entirely sensitive in the Fg ; but in the Killer line one quarter of the 

 Fg goes back to the Sensitive character of the male grandparent 

 (Fig. 42). Clearly the Killer character can be lost by virtue of a 

 change in the nucleus as well as in the cytoplasm. Or to put it 

 another way we have, as in flax, a character, Killer, which depends 

 on the simultaneous presence of a nuclear gene K and a plasmagene, 

 termed kappa. 



The relation between K and kappa is not confmed to their action, 

 for it affects the reproduction of kappa. Where Kis introduced into 



175 



