506 A. TSUGITA AM) II. 1 KAKXKEL-COMiAT 



showing a conii)08ition so grossly unlike the pai'cntal strain and most 

 of the mutants resulting from it. 



While no explanation can be ori'ei'cd for these observations, a review 

 of what we know concerning the composition of various natural strains 

 may supply some basis of understanding. As previously discussed, natural 

 TMV strains are known to occur in groups: the M strain is indistinguish- 

 able from the connnon strain, and YA, GA, and J14D1 differ by 2 to 3 

 residues. Then suddenly we are faced with several extremely different 

 strains, bearing little relationship to TMV and in only a few instances 

 resembling one another. Yet no strains of intermediate compositions are 

 known, although this would be expected if the latter arose by a series 

 of 20 or more mutational events from common TMV. Thus, the situation 

 is similar for both series, that of the naturally observed and of the 

 chemically evoked strains, even though this fact does not facilitate an 

 understanding of the mechanism of either. The possibility must naturally 

 be considered that any isolated mutant is actually due to a spontaneous 

 mutation, rather than to the chemical treatment employed. This possi- 

 bility cannot be excluded in any individual case, and therefore no con- 

 clusions should be based on single isolates. Yet, obviously, it would be 

 hardly less surprising to find spontaneous mutation to lead to such 

 marked changes. Further the infrequency of detection of spontaneous 

 mutation makes this appear unlikely on statistical grounds, since the 

 unexpected observation of gross changes in protein composition upon 

 treatment with three different mutagens were made repeatedly. 



Finally, there exists the possibility that the various chemical agents 

 through their inactivating action might select for a more stable strain 

 or family of strains present in traces in common T]\1V preparations, 

 This might seemingly account for the six observed strains of unusual 

 composition, all of which resulted from ])reparations inactivated to 95% 

 or more. Nevertheless, such an explanation appears improbably because 

 common TMV generally appears more stable and more viable than its 

 strains. Furthermore, the fact that the RNA rather than the whole 

 virus was subjected to the inactivating reactions makes this interpreta- 

 tion even more unlikely, for it appears veiy improbable that the RNA 

 differs greatly in stability from one strain to another. 



In the course of our chemical analyses of many mutants, these were 

 repeatedly passed through singk^ lesion hosts and through large groups 

 of A^. tabacum. Occasionally, and particularly in hot weather, the virus 

 progeny so isolated was not identical in composition or symptomatology 

 with the parental strain. At times, altered symptoms were detected on 

 part of the inoculated plants and these plants were found to contain the 

 chemically different strains. Of the six instances in wliich such phenom- 



