Order Tymovirales

This chapter focuses on Tymovirales order, which contains viruses that mostly infect plants, have a single molecule of positive sense ssRNA, and which are united by the similarities in their replication-associated polyproteins. The virions within the families Alphaflexiviridae, Betaflexiviridae, and Gammaflexiviridae are flexuous filaments and are usually of 12–13 nm in diameter and of about 470 to 1000 nm in length, depending on the genus. They have helical symmetry and in some genera there is clearly visible cross-banding. Almost all members have a single coat protein of 18–44 kDa and in case of genuses Lolavirus and some marafiviruses, there are two structural proteins, which are different forms from the same gene. The largest protein encoded is a replication-associated polyprotein of about 150–250 kDa close to the 5' end of the genome and which is translated directly from the genomic RNA. Most members of the order infect plants, but a few species are from plant pathogenic fungi. Host range and transmission are often characteristic of individual genera. In phylogenetic analysis of the replication protein, each genus and family forms a distinct, well-supported branch. The viruses with flexuous virions in the families Alphaflexiviridae and Gammaflexiviridae are more closely related to those with icosohedral particles in the family Tymoviridae than they are to members of the Betaflexiviridae.