The phylogenetic structure of the cluster of tobamovirus species serologically related to ribgrass mosaic virus (RMV) and the sequence of streptocarpus flower break virus (SFBV).

Ribgrass mosaic virus (RMV), turnip vein-clearing virus (TVCV) and Youcai mosaic virus (YoMV; formerly designated as oilseed rape mosaic virus; ORMV) belong to the genus Tobamovirus and are arranged in one out of three subgroups because of their common host range, serological cross-reactivity and amino acid composition of their coat proteins. The recently defined species Wasabi mottle virus (WMoV) is closely related to the same subgroup. The distinction of the four species is difficult and the lack of sequence information of a wide range of isolates has led to an unclear nomenclature. To clarify this situation we sequenced the coat protein genes from 18 isolates which were serologically related to members of the species of this cluster. The size of the coat protein was conserved with the exception of one isolate which revealed an N-terminal extension due to the mutation of three stop-codons. Phylogenetic analysis of these CP ORFs resulted in a tree with three clusters each containing at least one of the approved species RMV, TVCV and 1ptYoMV/WMoV in which our isolates were distributed. The tree was congruent and did support the present taxonomic status of species within this subgroup. For practical purpose we developed a subgroup 3 specific primer pair and a species differentiating restriction fragment length polymorphism (RFLP). Sequencing of the genome of Streptocarpus flower break virus (SFBV) which is serologically distantly related to the subgroup 3 viruses revealed a distinct genome organization. Therefore we propose that this virus should be regarded as a member of a species not belonging to any of the subgroups so far established.