Genus Fijivirus

Type Species	Fiji disease virus	(FDV)

Distinguishing Features

Fijivirus particles have a double-shelled, icosahedral structure, with a spherical rather than angular appearance and short surface spikes (type A spikes) on each of the twelve vertices of the icosahedron. The outer shell is fragile and easily breaks down leaving the inner shell bearing twelve type B spikes. There are 10 genome segments. The viruses replicate in delphacid planthoppers. Nilaparvata lugens reovirus (NLRV) has the above properties but replicates only in insects, whereas other fijiviruses can also replicate in phloem cells of susceptible plants of the families Graminae (in which they induce small tumours or enations), or Liliaceae.

Virion Properties

Morphology

Virions are double-shelled, spherical, 65-70  nm in diameter with “A”-type spikes of about 11  nm in length and breadth, at the 12 vertices on the icosahedral (Fig. 18, left). Unless prefixed, viruses readily break down in vitro to give cores, about 55  nm in diameter, with 12 “B”-type spikes, about 8  nm long and 12  nm in diameter (Fig. 18, right). Some treatments (shaking with butan-1-ol or incubation with 1.9  M MgCl₂) produce smooth subcores (Fig. 18, center).

Physicochemical and Physical Properties

The physicochemical properties of the virions have not been determined.

Nucleic Acid

Fijiviruses have 10 dsRNA segments that are identified in Table 14 as genome segment 1 to 10, Segment 1 to 10, or S1 to S10, in order increasing electrophoretic mobility during PAGE. However, S2 and S3 of some viruses (NLRV), 8 and 9 (Oat sterile dwarf virus, OSDV) do not migrate in order of their Mr and may migrate in a reverse order during (1%) agarose gel electrophoresis (AGE). The individual genome segments have Mr of about 1.0 to 2.9 10⁶ (1,430-4,391  bp) with a total genome Mr of 19.58 10⁶ (28,699  bp, based on the complete RNA sequence analysis of NLRV: Table 15). The coding strands of each segment of Rice black streaked dwarf virus (RBSDV), Maize rough dwarf virus (MRDV), OSDV, and NLRV, contain terminal conserved nucleotide sequences (Table 14). Within the genus, only the 3-terminal sequence . . . GUC-3 is conserved. Adjacent to the conserved terminal oligonucleotide sequences, each genome segment possesses inverted repeats, which are several bases long, similar to those of phytoreoviruses and oryzaviruses, although the sequences involved differ in these other genera. Characteristic of the genus is the low G+C content of the genomic RNAs, mostly around 34-36%. The sizes and groupings of the 10 dsRNA species are characteristic and distinctive for the five groups of fijiviruses that are recognized.

Proteins

Six polypeptides, numbered respectively I to VI (Mr 139, 126, 123, 111, 97 and 64 10³), can be detected by SDS PAGE of purified MRDV. The spiked cores contain peptides I, II and III, while the smooth core contains peptides I and II. The B spikes should therefore be composed of peptide III. Peptides IV-VI form the outer capsid.

Three major proteins (Mr 130, 120, and 56 10³) and three minor ones (Mr 148, 65, and 51 10³) can be detected by SDS PAGE of purified virions of RBSDV. The Mr 120 10³ protein is the “B” spike protein. Smooth subcore particles consist of Mr 148, 130, and 65 10³ proteins. The Mr 56 10³ protein is the major component of the outer capsid shell and the Mr 51 10³ protein is a partial degradation of it. In NLRV protein, three major proteins (Mr 140, 135, and 65 10³), three intermediate (Mr 160, 110, and 75 10³), and one minor protein (Mr 120 10³) can be resolved. The Mr 135 10³ protein is the “B” spike. The Mr 65 10³ protein is the major component of the outer capsid shell and the Mr 140 10³ protein is the major core protein. In addition to the above structural proteins, there is thought to be an “A” spiked protein which has not yet been identified.

Lipids

Not known.

Carbohydrates

Not known.

Genome Organization and Replication

Genome organizations and coding assignments of fijiviruses are summarized in Table 15. Most of the genome segments are monocistronic. Some segments possess two ORFs but expression of the second ORF has not been demonstrated in vivo in insect or plant cells. For viruses other than NLRV, replication occurs in the cytoplasm of phloem-related cells in association with viroplasms composed partly of fine filaments. During infection, tubules, about 90  nm in diameter, accumulate. Sometimes these are incompletely closed and form scrolls.

Antigenic Properties

Proteins of some fijivirus species are serologically unrelated, but some proteins of other species (MRCV, MRDV, Pangola stunt virus (PaSV) and RBSDV) are distantly related.

Biological Properties

All the plant-infecting fijiviruses induce hypertrophy of the phloem (both expansion and multiplication of cells) leading to vein swellings and sometimes galls (enations or tumours) derived from phloem cells, especially on the backs of leaves. MRDV in maize induces longitudinal splitting of the roots. Other effects include the suppression of flowering, plant stunting, increased production of side shoots, and induction of a dark green coloring. In insect hosts, no particular tissue tropism or severe disease is recognized. Viruses are transmitted propagatively by delphacid planthoppers (Hemiptera, Delphacidae, e.g., Perkinsiella, Laodelphax, Toya, Sogatella, Javesella, Ribautodelphax, Dicranotropis, Delphacodes, Sogatella, Unkanodes). Following virus acquisition from infected plants, the latent period is about two weeks and leads to a lifelong capacity for virus transmission to plants. No transovarial transmission, or seed transmission of virus has been identified. Mechanical transmission from plant to plant can only be demonstrated with difficulty. Virus is spread by offsets in vegetatively propagated crops (e.g., pangolagrass and sugarcane). Viruses can over-winter in diapausing planthoppers, in certain weed species and in autumn-sown cereals.

Generally, fijiviruses are widespread in nature although apparently absent from North America and not reported from Africa, or confirmed from India. FDV has been reported from Australia and the Pacific islands. RBSDV occurs in Japan, Korea, and China. PaSV occurs in northern countries of South America, Oceania, Taiwan and northern Australia, and OSDV occurs in northern Europe. GDV has been found only in southern France. MRDV is found in Scandinavia and in areas bordering the northern and eastern Mediterranean. MRCV occurs in Argentina.

NLRV was found in the planthopper Nilaparvata lugens, which occurs in south-east Asia. Experimentally, it infects a second hopper, Laodelphax striatellus. There is no evidence that NLRV can multiply in rice plants, a natural host of N. lugens, but the virus is transmitted from hopper to hopper through contaminated rice plants and moves through the phloem and/or xylem of rice plants once injected by the viruliferous hoppers.

List of Species Demarcation Criteria in the Genus

Fijivirus species can be clustered into “groups” of more closely related species. Thus, some species are distinct but others (RBDSV, MRDV, PaSV, MRDV) appear to be more closely related to each other than to any other Fijivirus species. Nucleotide and amino acid sequence relatedness between RBDSV, MRDV, PaSV and MRCV is much greater than it is among other species of fijiviruses. Further information about these viruses may necessitate a revision of the species status of these viruses, but they are retained as distinct species in accordance with previous practice.

For the family Reovirdae as a whole, the prime determinant for inclusion of virus isolates within a single species is “an ability to exchange (reassort) genome segments during co-infection, thereby exchanging genetic information and generating viable and novel progeny virus strains”. However, data is not available that could extend this criterion to judgments about fijiviruses.

Within the genus, the 3-terminal conserved nucleotide sequence of . . . GUC-3 is common among MRDV, RBSDV, OSDV and NLRV. Values of nucleotide and amino acid sequence relatedness among some species (for example MRDV and RBSDV, both in group 2) are much higher (>85% nucleotide sequence homology among genome segments coding for major capsid proteins) than those among viruses from different groups (<55% homology) (Fig. 19). Between RBSDV or MRDV (group 2) and either OSDV or NLRV, there are detectable but low levels of amino acid sequence homology in some proteins. cDNA probes from some but not all of the genome segments, can cross-hybridize with corresponding segments among different viruses within group 2, whereas no cross-hybridization has been found between viruses species in different groups. NLRV does not have a counterpart to ORF2 present in the corresponding RBSDV S7, MRDV S6, and OSDV S7.

Members of a single Fijivirus species may be identified by:

1.	An ability to exchange genetic material by genome segment re-assortment during dual infections, thereby producing viable progeny virus strains.
2.	A relatively high amino acid sequence homology. Members of different species have low amino acid sequence similarity (less than 40% for counterparts corresponding to those encoded by RBSDV S7, S8, S9, and S10),
3.	Showing cross-hybridization of some segments using RNA or cDNA probes. Essentially, viruses within a species should have cross-hybridization signals in all genome segments as assessed by using cDNA probes under standard conditions. Members of different species will show a lack of cross-hybridization at high stringency. Some species (MRDV, RBSDV) share 94% nucleotide sequence homology in genome segment 10, which encodes a major outer shell protein (highly conserved genome segment). The most practical and reliable criterion for distinguishing species is a lack of cross-hybridization using cDNA probes to less conserved genome segments. Hybridization using RBSDV segment 5 and segment 6 cDNA probes to detect the homologous sequences is more than 20 times more sensitive than hybridization using their counterparts from MRDV.
4.	Showing serological cross-reactions within a species; viruses species in different groups do not cross-react, those in group 2 do so to a limited extent that is dependent on the proteins being compared.
5.	Having differences in the near terminal conserved sequences (Table 14), although the 3-terminal trinucleotide is identical in at least RBSDV, OSDV and NLRV.
6.	The identity or family of the plant host species, or the absence of a plant host, as well as the insect vector or insect host species may help to indicate virus species.

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, insect vector and host names { }, genome sequence accession numbers [ ], and assigned abbreviations ( ) are:

Species in the Genus

Fijivirus group 1
Fiji disease virus	{Perkinsiella saccharicida, P. vastatrix, P. vitiensis: Graminae}	(FDV)
Fijivirus group 2
Rice black streaked dwarf virus	Seg 7: [S63917] Seg 8: [S63914] Seg 9: [AB011403] Seg 10: [D00606] Seg 6: [X55701] Seg 7: [L76562] Seg 8: [L76561] Seg 10: [L76560]	(RBSDV)
{Laodelphax striatellus, Ribautodelphax albifascia, Unkanodes sapporona: Graminae}
Maize rough dwarf virus		(MRDV)
{Laedelphax striatellus, Toya propinqua, Sogatella vibix, Javesella pellucida, Ribautodelphax notabilis: Graminae}
Mal de Rio Cuarto virus		(MRCV)
{Delphacodes kuscheli: Graminae}
Pangola stunt virus		(PaSV)
{Sogatella furcifera S. kolophon: Graminae}
Fijivirus group 3
Oat sterile dwarf virus	Seg 7: [AB011024] Seg 8: [AB011025] Seg 9: [AB011026] Seg 10: [AB011027]	(OSDV)
{Javesella pellucida, J. discolor, J. dubia, J. obscurella, Dicranotropis hamata: Graminae}
Fijivirus group 4
Garlic dwarf virus		(GDV)
{unknown: Liliaceae}
Fijivirus group 5
Nilaparvata lugens reovirus	Seg 1: [D49693], Seg 2: [D49694], Seg 3: [D49695], Seg 4: [D49696], Seg 5: [D49697], Seg 6: [D49698], Seg 7: [D49699], Seg 8: [D26127], Seg 9: [D49700], Seg 10: [D14691]	(NLRV)
{Nilaparvata lugens, Laodelphax striatellus: no plant hosts}

Tentative Species in the Genus

None reported.

Phylogenetic Relationships Within the Genus

See Fig. 19.