Strains and mutants of tobacco mosaic virus are both found in virus derived from single-lesion-passaged inoculum.

An analysis has been made of variants found in preparations of the common (U1) strain of tobacco mosaic virus (TMV) which had been obtained after the inoculum had been passaged through a necrotic local lesion. These lesions were induced with a minimal number of virions (3000 or 30,000) per inoculation point. The relationships of the variants to the U1 strain were analyzed both by nucleic acid hybridization using complementary DNA in nuclease S1 assays and by two-dimensional RNA fingerprinting. Two classes of variants were obtained: Members of one class, because of their close sequence homology to the U1 strain, were considered to arise by mutation of the U1 strain. These variants were isolated from chlorotic or necrotic spots which developed on the leaves of U1-infected plants. The other class represented distantly related strains of TMV which are presumed to have been passaged as contaminants along with the U1 strain, even though the virus from which the inoculum was obtained was itself generated via a single local lesion from minimum dose inoculation. Such data thus cast suspicion on the fidelity of local lesion passage as a means of getting biologically "pure" virus, viz (1) contaminants could be passaged along with the major virus (normally they would not be seen, but they might be expressed under the appropriate environmental conditions), and (2) the high rate of transcriptional infidelity known to occur during RNA synthesis results in a heterogeneous viral RNA population but with one predominant genotype. Occasionally, mutants with a new phenotype may be expressed (as chlorotic or necrotic spots on a leaf of mosaic symptom plants, for example).