How ancient are the tobamoviruses?

The amino acid sequences of the virion proteins of 7 definitive tobamoviruses have been compared. The results show that the proteins may be arranged unequivocally in a single dendrogram which, by analogy with similar comparisons of other sets of homologous proteins, probably reflects their phylogeny. The magnitude of the differences between these sequences suggests that the tobamoviruses are at least as old as flowering plants. The sequence differences are significantly correlated with serological differences and with differences in amino acid composition, and so the latter was used to compute a classification of 13 tobamoviruses that also probably reflects their phylogeny. The possible ages of several other virus groups are discussed.