POPULATION GENETICS 



used in pure culture work with bacteria, a strain of influenza 

 virus could be maintained genetically constant despite the 

 fact that a highly favoured mutant was constantly being 

 produced. This type of experiment is, we believe, the only 

 one which can establish that a viral change in character is 

 mutational and not adaptive. 



In the light of all our experience, however, we believe that 

 the 0-D change is a genetic one in which a mutation or 

 series of mutations is manifested by a modification of the 

 components of the virus surface which react with, and act 

 enzymically on, the sialic-acid-containing receptors of 

 mammalian and avian cells respectively. 



Two other examples can be cited of this phenomenon by 

 which a strain of virus can be maintained in its original state, 

 although it is being propagated in a tissue on which various 

 possible mutants would grow much more effectively. 



There is a well-known strain of influenza virus which was 

 ' trained ' by Stuart Harris to grow in the brain cells of the 

 mouse — a thing no normal influenza virus can do. After 

 more than loo passages from one mouse brain to the next, 

 the quality of the virus diflfered in many respects from what 

 we find in viruses cultivated in, and adapted to growth in, 

 the allantoic cavity. In our experiments this virus was trans- 

 ferred to chick embryos, and two sets of transfers set in 

 progress. 



In the first, limit-dilution methods were used. In the 

 second, a relatively large amount of virus was used in each 

 transfer. The results were as we should expect. After fifteen 

 passages at limit dilution, the mouse-brain virus appeared to 

 have the same qualities it started with. The mass transfer 

 series, however, had changed in two important characters 

 which brought it much closer to the nature of normal 

 influenza virus strains (Burnet, 1 95 1 ) . 



The second example depends on the fact that when many 

 influenza strains are first isolated and persuaded to grow in 

 the cavities of the chick embryo, they are in what we call the 



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