Statistically significant alignment was generated between the amino acid sequences of the (putative) shell (S) domains of the capsid proteins of small RNA plant viruses with icosahedral capsids in the tombusvirus, carmovirus, dianthovirus, sobemovirus and luteovirus groups. Inspection of the alignment showed good correspondence between the experimentally defined beta-strands and alpha-helices of the capsid proteins of tomato bushy stunt, southern bean mosaic and turnip crinkle viruses, allowing prediction of the secondary structure elements in proteins with unresolved tertiary structure. It is concluded that this set of viral capsid proteins forms a tight evolutionary cluster. Comparison of the alignment of the proteins of this family with the sequences of other capsid proteins of icosahedral RNA viruses revealed more distant similarities to the satellites of tobacco necrosis, panicum mosaic, tobacco mosaic and maize white line mosaic viruses, as well as to nepo- and comoviruses. The tentative phylogenetic tree derived from the capsid protein alignment separated into three main lineages: (I) carmo-, tombus- and dianthoviruses, (II) southern bean mosaic, tobacco necrosis and maize chlorotic mottle viruses, and (III) luteoviruses. Comparison of this tree topology with the tentative evolutionary schemes for the respective virus RNA-dependent RNA polymerases suggested that gene shuffling is the universal trend in the evolution of small RNA plant virus genomes.
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