VIRUSES, PUOVIRUSES AND THIi CONFLICT OF SYSTEMS 



susceptibility. Many of these, like the 50 tested strains of Tobacco 

 Mosiac, have arisen under experiment and, we may say, by mutation 

 analogous, as Muller first pointed out, to that of nuclear genes. The 

 question arises as to whether this mutation is in some sense free and 

 uncontrolled, or whether some control is exercised by the host over 

 its frequency or direction such as we saw in the property of 

 mutafacience. The answer seems to be that all degrees of control 

 occur. 



Bacteria, infected by viruses, mutate to become resistant to them. 

 The viruses can then mutate, becoming adsorbable on the bacteria, 

 so as to be once more successfully virulent. In this situation, described 

 by Luria, we seem to have free mutation on both sides followed 

 by the survival of the fittest mutants. On the other hand there are 

 many types of genetic change in which viruses, like bacteria, react 

 so characteristically to a particular type of change of host or diet 

 that control seems to be indicated. Of this kind is perhaps the 

 weakening or attenuation of rabies, secured by Pasteur when he 

 inoculated a series o£ rabbits with the virus, passing it from one to 

 another. This weakening varies in rate with the different strains of 

 rabies. There is also the fortification of other viruses by rapid passage 

 through higlily susceptible hosts, as described by Holmes and Pirie. 

 Similar changes have lately been induced in plant viruses. 



Certain recent studies of mutation in viruses and bacteria (which 

 in this respect seem to be analogous) give us an opportunity of 

 discovering the means of mutation itself. Two experiments have 

 a parallel significance. Different strains o£ Pneumococcus differ in the 

 specificity of their antigens, which in this case are polysaccharides 

 carried in the capsule. Avery and others found that if the capsule 

 is lost, the specificity of a strain A can be transferred to a strain B, 

 by feeding it with dead cells of A, or, even more precisely, by 

 feeding it with desoxyribose nucleic acid from the capsules of A. 

 We have already seen that this nucleic acid is necessary for the 

 reproduction of nuclear genes. We now see that it is necessary for 

 the specificity of this reproduction. It provides the pattern. 



Similar transformations can be produced amongst the viruses 

 causing different kinds of tumour in rabbits. Dead myxoma virus, 

 in a large excess, can be used to convert living fibroma virus into 

 one producing myxoma of the same strain-type as the dead virus. 



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