Genus Bymovirus

Type Species	Barley yellow mosaic virus	(BaYMV)

Virion Properties

Morphology

Virions are flexuous filaments of two modal lengths, 250-300 and 500-600  nm; both are 13  nm in width (Fig. 3).

Physicochemical and Physical Properties

Virion buoyant density in CsCl is 1.28-1.30  g/cm³.

Nucleic Acid

Virions contain two molecules of linear positive sense, ssRNA. RNA1 is 7.5-8.0  kb (Mr 2.6 10⁶) and RNA2 is 3.5-4.0  kb (Mr 1.5 10⁶) in size; RNA makes up 5% by weight of particles. Both RNA molecules have 3-terminal poly(A) tracts. There is little base sequence homology between the two RNAs except in the 5 noncoding regions. The CP gene is located in the 3-proximal region of RNA1.

Proteins

Virions have a single CP with Mr 28.5-33 10³. The CP of the type species, BaYMV contains 297 amino acids.

Genome Organization and Replication

The two RNA molecules appear to be translated initially into precursor polypeptides from which functional proteins are derived by proteolytic processing (Fig. 4).

Antigenic Properties

The viral proteins are moderately immunogenic; serological relationships exist among members except Barley mild mosaic virus (BaMMV). The CP amino acid sequence homology among members is 35-74%.

Biological Properties

Cytology

There are characteristic pinwheel-like inclusions and membranous network structures are formed in the cytoplasm of infected plant cells. No nuclear inclusions are found.

Host Range

The host range of member viruses is narrow, restricted to the host family Graminae.

Transmission

The viruses are transmitted by the plasmodiophoraceous fungus Polymyxa graminis; transmissible experimentally by mechanical inoculation.

List of Species Demarcating Criteria in the Genus

	Genome sequence relatedness. CP amino acid sequence identity less than approximately 80%, Nucleotide sequence identity less than 85% over whole genome, Different polyprotein cleavage sites.
	Natural host range. Host range may be related to species but usually not helpful in identifying species. May delineate strains.
	Pathogenicity and cytopathology. Different inclusion body morphology, Lack of cross-protection, Seed transmissibility, or lack thereof, Some aspects of host reaction may be useful (e.g., resistance genes, different responses in key host species).
	Mode of transmission. Different primary vectors, but vector species not of use in identification to virus species.
	Antigenic properties. Serological differences.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description number ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Barley mild mosaic virus (356)	[L49381, D83408, D83409, X75933, X82625, X90904, Y10973, Y10974]	(BaMMV)
Barley yellow mosaic virus (143)	[D01091, D01092, D01099, X69757]	(BaYMV)
Oat mosaic virus (145)		(OMV)
Rice necrosis mosaic virus (172)		(RNMV)
Wheat spindle streak mosaic virus (167)		(WSSMV)
Wheat yellow mosaic virus		(WYMV)

Tentative Species in the Genus

None reported.

List of Unassigned Viruses in the Family

None reported.

Phylogenetic Relationships within the Family

See Fig. 5.

Similarity with Other Taxa

Viruses in the family Potyviridae are similar to members of the families Comoviridae, Picornaviridae, and Hypoviridae. Genomes of member viruses of these taxa are positive sense ssRNAs. Most have a VPg at their 5-termini and a poly(A) tract at their 3-termini. Their genomes are expressed initially as high molecular weight polyprotein precursors which are processed by viral-encoded proteases. Gene products involved in replication are conserved in gene order and gene sequence.

Derivation of Names

Bymo: siglum from barley yellow mosaic.

Ipomo: siglum from Ipomea and mosaic.

Maclura: siglum from host genus name “Maclura”.

Poty: siglum from potato Y.

Rymo: siglum from ryegrass mosaic.

Tritimo: siglum from Triticum and mosaic.