PLASMAGENES 40I 



which, in the presence of the substrate, could persist indefinitely even if 

 the corresponding gene had been removed from the cell by crossing. 

 Their original evidence, obtained in yeasts, w^as later shown to be inade- 

 quate, but there still remains some evidence which may indicate that such 

 plasmagenes exist (Spiegelman 195 1). 



Ephrussi (1953), (Ephrussi and Hottinguer 195 1) has also found evidence 

 which suggests the existence of cytoplasmic self-duplicating particles 

 concerned with enzymatic adaptation, hi his case, yeast cells cultivated in 

 the presence of the nuclear poison acriflavine are shown often to give rise 

 to colonies which grow abnormally slowly and in which the cells have 

 lost certain respiratory enzymes and have an abnormal cytochrome 

 system. The change is quite stable through many generations of vegetative 

 division. Crossing experiments show that it is immediately dependent on 

 a cytoplasmic, not on a nuclear, factor; but again further investigation 

 has demonstrated that the plasmagene concerned is itself under the even- 

 tual control of a gene. The enzyme changes induced by the acriflavine are 

 not themselves adaptive, but they are closely similar to undoubtedly 

 adaptive changes which yeast cells exhibit when cultivated in the absence 

 of oxygen. 



It is questionable whether it is really appropriate to employ the word 

 plasmagenes for any of the gene-initiated factors considered in this section. 

 The character they share with the true plasmagenes is a certain ability to 

 multiply in the cytoplasm. It is not clear, however, that any case is known 

 in which a gene-initiated c}^toplasniic factor acquires complete autonomy 

 in its powers of reproduction. Certainly the Paramecium antigen determin- 

 ants can only persist for a very hmited period after the removal of the 

 gene. The situation of the factors studied by Billingham and Medawar is 

 obscure, since in the piebald guinea-pigs they studied, the originally colour- 

 less cells into which the factor passes probably possess the same genotypic 

 constitution as the coloured cells out of which it comes, the difference be- 

 tween the cells being one which arises durmg differentiation rather than 

 of a truly genetic nature. Beale (1954), who has studied these phenomena 

 as closely as anyone, has recently expressed a lack of satisfaction with the 

 term plasmagene for such factors. Haldane (1954) is apparently of a 

 similar opinion and has suggested calling them mnemons. For con- 

 venience in the present discussion, however, I shall continue to refer to 

 them as gene-initiated plasmagenes. 



5. The role of plasmagenes in differentiation 



It will be noticed that the overwhelming majority of the evidence for 

 the existence of plasmagenes come from studies on micro-organisms. It 



