Family: Phenuiviridae

Genus: Ixovirus

Distinguishing features

Eight viruses are assigned to the genus Ixovirus. Ixoviruses were first discovered in black-legged ticks in USA. Unlike other phenuivirids, the genome of ixoviruses comprises only two segments of negative-sense RNA and has only two genes, encoding a large protein (L) and a nucleocapsid protein (N). The apparent lack of a membrane-bound virus particle distinguishes ixoviruses from other viruses in the family Phenuiviridae. Based on well-supported Maximum Likelihood or Maximum Clade Credibility trees inferred from complete L protein sequences, viruses classified in the genus Ixovirus form a monophyletic cluster clearly distinguished from other phenuivirids (Tokarz et al., 2014, Pettersson et al., 2017, Tokarz et al., 2018).

Virion

Morphology

Virion morphology is unknown.

Nucleic acid

The ixovirus genome encompass two segments of negative-sense RNA. The terminal nucleotides of each segment occur in a canonical, conserved sequence (in coding sense) 5′-ACACAAAGGC…GCCAUUGUGU-3′ and may form the panhandle structures typical of other members of the class Bunyaviricetes. The genomic RNAs contain non-coding regions flanking a single ORF which, based on comparisons with other negative-sense RNA viruses, is predicted to be contained in the virus-complementary strand. The L segment (6.7–6.9 kb) encodes a protein with a predicted molecular mass of 248.2–255.9 kDa that is homologous with the bunyaviral RNA-directed RNA polymerase (RdRP) domain. The S segment (1.4–4.2 kb) encodes a protein of 44.2–58.3 kDa that is homologous with the tenuivirus/phlebovirus N domain (Tokarz et al., 2014, Pettersson et al., 2017, Tokarz et al., 2018).

Genome organization and replication

Unlike other phenuivirids, ixoviruses have only two genes, encoding L and N (Figure 1 Ixovirus). The glycoprotein that is thought to be related to vector transmission, is not confirmed in ixoviruses. Details of virus replication are unknown (Tokarz et al., 2014, Pettersson et al., 2017, Tokarz et al., 2018).

Figure 1 Ixovirus. Genome organization of an ixovirus. Coloured boxes depict ORFs that encode N, nucleocapsid protein and L, large protein.

Biology

Ixovirus RNAs have been found in black-legged ticks [Ixodes scapularis (Say, 1821)], castor bean ticks [I. ricinus (Linnaeus, 1758)], common seabird ticks (Ixodes uriae White, 1852), taiga ticks (Ixodes persulcatus Schulze, 1930), western blacklegged ticks (Ixodes pacificus Cooley & Kohls, 1943) in Antarctica, Belgium, Bulgaria, China, Norway, Russia, and USA. Vertebrate hosts have not been identified (Tokarz et al., 2014, Pettersson et al., 2017, Tokarz et al., 2018).

Species demarcation criteria

The criteria demarcating species in the genus are:

•　Less than 95% identity in the amino acid sequence of the L protein.

Related, unclassified viruses

* Sequences do not comprise the complete genome segment.

Virus names and virus abbreviations are not official ICTV designations.