CYTOPLASMIC EQUILIBRIUM 



with Killer behaviour. There is no need to postulate a second type 

 of plasmagene associated with Sensitive. This state is merely the 

 absence of Killer and is associated consequently with absence, or at 

 least over-low presence, of kappa. We must, however, recognize 

 the more general possibility of two different plasmagenes, or alter- 

 native phases of the same plasmagene, being associated with the 

 alternative characters. We should then be forced to interpret changes 

 in plasmagene constitution as being, at least in part, due to the 

 suppression of one plasmagene or phase by the successful competition 

 of the other. Such suppressiveness might at first sight resemble the 

 dominance of nuclear genes ; but it must be different in that it will 

 preclude, at least if complete, all sorting out of the particles or 

 determinants that are suppressed. The suppressor must displace the 

 suppressed. 



Whether in competition with some alternative type of plasmagene 

 in this way, or whether reproducing without any direct and imme- 

 diate competition, the level or concentration of a plasmagene 

 must depend on its rate of reproduction relative to those other 

 constituents which determine the division of the cell itself. We 

 have seen how the increase and decrease of kappa are governed in 

 this way. In yeast we can go even further and see how the supply 

 of a particular substance from outside can govern the concentration 

 of a plasmagene. 



Both Saccharomyces carlshergensis and S. cerevisiae, when placed in 

 a medium containing melibiose, are unable to ferment this sugar. 

 But carlshergensis soon becomes habituated or "adapted" to doing 

 so, while cerevisiae is quite incapable of adaptation. When the two 

 species are crossed, the diploid hybrid is found to be adaptable, and 

 segregation into adaptable and unadaptable haploids occurs in its 

 asci. It would appear that one, or possibly more, genes govern the 

 difference in adaptability between the species. 



The gene itself cannot, however, be responsible for the immediate 

 production of all the enzyme in an adapted individual. Rather, as 

 Spiegelmann points out, we should regard the allelomorph, which 

 confers adaptability, as constantly seeding the cytoplasm with small 

 quantities of the enzyme, or of a precursor which, in conjunction 

 with some substance available from the cytoplasm, can give rise 

 to the enzyme. In the absence of melibiose, the enzyme or precursor 



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