CYTOPLASM, NUCLEUS AND ENVIRONMENT 



We should expect, on this view, that the enzyme could persist 

 and multiply in the cell eve aM^ay from the gene which was its 

 ultimate origin, always provided that the supply of melibiose was 

 maintained. That this is indeed the case is suggested by an experiment 

 carried out by Spiegelmann, Lindegren and Lindegren (Fig. 43). 

 They crossed the two species of yeast and allowed the diploid hybrid 

 to form asci in the presence of melibiose. Instead of the ascospores 

 segregating into ferm ntors and non-fermentors, all were fermen- 

 tors so long as melibiose was supplied. When the melibiose was 

 removed, however, so that the ability to ferment was lost, only 

 half of the lines were able to readapt to its fermentation. The habit 

 can persist with the plasmagene in the cytoplasm, in the correct 

 chemical conditions; but once the plasmagene is lost it can be re- 

 initiated only with the help of a nuclear gene. Thus, unless the 

 experimental evidence is at fault, we must conclude that in yeasts a 

 plasmagene which is produced or created by a nuclear gene can 

 reproduce itself indefinitely without that nuclear gene. 



Cytoplasm, Nucleus and Environment 



The experiments with Paramecium and yeast together tell us a 

 great deal about the properties of plasmagenes. Those with Para- 

 mecium are most comprehensive in showing us how anything which 

 affects the rate of cell fission correspondingly affects the plasmagene 

 concentration. In the yeast experiments a precise chemical footing 

 has been achieved, and two new relationships revealed. One is that 

 between the nuclear and cytoplasmic determinants. We have always 

 supposed that the nucleus can pass out materials into the cytoplasm. 

 We now fmd that those materials can sometimes cut adrift. They 

 can propagate themselves independently, with this proviso: they 

 seem to require more specific precursors or food materials than 

 does the nucleus as a whole, which is assisted in this respect 

 by the co-ordinated supply and demand of its constituent genes. 

 Plasmagenes are therefore liable to extinction by changes either 

 in the nuclear genes or (in micro-organisms) in the environment. 



The substance produced by a nuclear gene and necessary for the 

 development of a particular plasmagene may be described as a 

 specific precursor. It might be supposed that, in requiring such a 



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