Strains of tobacco mosaic virus (TMV) obtained from systemically-infected leguminous plants in Nigeria and India changed their properties greatly when propagated in different hosts. From systemically-infected tobacco, they closely resemble type TMV and share many antigens with it, but from systemically-infected French bean, they differ from it by at least as much as any previously described strains. The different forms of the viruses share few antigens, have different amino acid constitutions, electrophoretic behaviour and resistance to inactivation by ultraviolet radiation, and they produce different kinds of local lesion in . Isolates derived from single local lesions behave like the original bulk cultures, and as the changes are also reversible they seem unlikely to occur simply because different hosts select different strains from existing mixtures. The electrophoretic patterns, particularly of preparations from beans, show that infected plants contain more than one anomalous product; both forms of the viruses may be produced in all hosts, but there is no evidence that sap from systemically-infected beans contains the tobacco forms in amounts needed to cause infection. The change after transfer to a new host apparently occurs because mutants produced in that host are favoured over the infecting form because they move more readily and become systemic. The bean forms multiply more extensively in tobacco at above 30° than at 20° and above 30° seem not to produce forms that give a systemic mosaic; many tobacco plants at 20° develop only local lesions, and systemic infection, indicating the occurrence of the tobacco forms, happens more often in young seedlings than when mature plants are inoculated.


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