VAEIATION AND ITS CHEMICAL CORRELATES 129 



Strain. The terms variant and strain are usually employed in a general sense 

 while the term mutant is, in keeping with classical genetics, often reserved 

 for those cases in which a highly specific relationship between parent and 

 offspring is intended. For example, to say that the Holmes' ribgrass virus is 

 a strain or variant of tobacco mosaic virus is simply stating that this virus 

 belongs in the tobacco mosaic \arus group. On the other hand, to say that 

 the ribgrass virus is a mutant of tobacco mosaic virus would imply that it 

 arose from tobacco mosaic virus by a specific mutational event involving a 

 certain change in the genie constitution of the latter. There is no evidence 

 that the ribgrass virus is such a mutant of TMV, whereas there is abundant 

 evidence that it is a variant strain. Since in the general literature on viruses a 

 rigorous distinction among terms is not always observed, the reader may be 

 left to decide what is meant from the context. In the present treatment, the 

 terms will be used in the sense defined above. 



C. Frequency of Mutation 



Little effort has been made to determine quantitatively the frequency of 

 mutation of plant viruses. This is largely attributable to the unfavorable 

 sampling situation associated with the fact that thousands of virus particles 

 must be apphed to produce a single lesion for testing virus type. However, 

 Kunkel (1940) found that when infectious juice from mosaic-diseased 

 tobacco was rubbed on a leaf of a host which gave local lesions, about 1 

 lesion in 200 contained virus causing symptoms different from those of the 

 original mosaic when returned to the systemic host. This result was obtained 

 with virus inoculum obtained from plants about a month after the infected 

 plants showed symptoms (this would be equivalent to about 5-6 weeks after 

 inoculation); in a plant diseased for a short period of time the per cent of 

 mutants was found to be smaller, while in plants diseased for several months 

 as many as 1-2 % variants were, observed. In any case, the mutation fre- 

 quency, if representative of the total population, seems rather large when 

 compared with that of the T-phages (Luria, 1953), certain mutations of 

 which were found to occur with a frequency of 1 in 10^ to 1 in 10^ particles. 

 It would be expected, of course, that different strains of TMV would have 

 somewhat different mutation rates, and further, that other plant viruses 

 might differ appreciably from TMV in this respect. Good quantitative com- 

 parisons are lacking but one gets the impression from the number of strains 

 isolated (Kunkel, 1940) that common TMV mutates as frequently as or more 

 frequently than most plant viruses. Despite this situation, the mass culturing 

 of TMV and other plant viruses in appropriate plants results in a rather 

 constant product consisting predominantly of progeny just like the strain 

 inoculated. The inoculated strain apparently overgrows the mutant progeny, 



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