152 C. A. KNIGHT 



letlia] mutants. Likewise, the incorporation of 5-bromouracil into the deoxy- 

 ribonuleic acid of T2 bacteriophage has proved to be a nonheritable change 

 (Litman and Pardee, 1956). However, it was observed that unusually large 

 numbers of plaque-type mutants resulted from infecting cells with T2 in the 

 presence of sulfanilamide and 5-bromouracil; this was attributed to a disturb- 

 ance of nucleic acid metabolism. 



B. Relation of Radiations to Production of Variants 

 Investigation of the effects of radiation on plant viruses have largely been 

 concerned with inactivating doses of radiation (Pollard, 1954; Kleczkowski, 

 (1957), but some irradiation experiments have been directed towards the 

 production of mutants. Gowen (1941) reported that exposure of preparations 

 of ordinary TMV and of the aucuba mosaic strain to low-energy X-rays 

 increased the frequency of mutants observed in subsequent tests in plants. 

 However, Gowen's data show that the X-ray treatment caused substantial 

 inactivation of the viruses and that the two strains were not equally resistant 

 to the radiation. Hence, the occurrence of a selective phenomenon rather 

 than mutagenesis is a possibility which does not seem excluded, particularly 

 since the data presented also show that unirradiated samples of each strain 

 contained some of the other strain. Moreover, the lesion counts showing 

 changes in proportions of the strains with increased exposure to radiation 

 were not made on the same host, but on two different hosts of unequal 

 sensitivity, thus compounding the normal difficulties of quantitative plant 

 virus assays. Other workers (Kausche and Stubbe, 1940; Pfankuch et at., 

 1940) have concluded that mutants were not produced by treatment of 

 purified TMV with X-rays or gamma rays, although it was reported that 

 irradiation of the infected plants caused an increase in mutation. Likewise, 

 attempts to produce mutants by irradiation of bacteriophages (where quan- 

 titative detection methods are much better than with viruses of higher plants) 

 have thus far been fruitless, although here again, irradiation of the host may 

 be mutagenic (Later jet, 1949). 



Thus, at present, it must be concluded that mutations are not produced in 

 plant viruses by treatment in vitro with chemicals or by irradiation. It is 

 clear, however, that investigation along these lines has not been extensive 

 and that the discovery of infectious preparations of virus nucleic acids may 

 provide new material with which to make a fresh approach.^ 



V. Summary and Perspectives 

 It is abimdantly evident that plant viruses mutate to yield a variety of 

 variant strains. These strains are characterized by diverse biological proper- 

 ties, usually best manifested by the types of disease symptoms they cause in 



^ Recently, Gierer and Mundry (1958) have reported production of mutants by 

 treating TMV or its RNA with nitrous acid. 



