Family: Nanoviridae

Genus: Nanovirus

Distinguishing features

Nanoviruses have been reported almost exclusively from legumes (Fabaceae), although natural infections in plants in the Apiaceae, Solanaceae, Caricaceae and surprisingly, the first record in a monocotyledonous plant (Liliaceae) have recently been reported (Yang et al., 2016, Lal et al., 2020, Choi et al., 2019, Vetten et al., 2019). They are transmitted by several aphid species, with Aphis craccivora being a major natural vector of many of these viruses. 

Virion

See discussion in family description. 

Genome organization and replication

Experimental and circumstantial evidence suggests that nanovirus genomes typically consist of eight different ssDNA components, referred to as DNA-R, DNA-S, DNA-M, DNA-C, DNA-N, DNA-U1, DNA-U2 and DNA-U4 (Figure 2. Nanoviridae). The latter three DNAs are characteristic for viruses of the genus Nanovirus. Whereas the proteins encoded by DNA-U1 and DNA-U2 influence symptom severity and affect transmission efficiency, DNA-U4 is dispensable (in faba bean) for both symptom development and aphid transmission (Grigoras et al., 2018). Nanovirus proteins have low levels of amino acid sequence identities (14 to 58%) with those of babuviruses, and, in contrast to babuviruses, nanovirus DNA-R transcripts are terminally redundant (Grigoras et al., 2008). 

Biology

Nanoviruses have largely overlapping but relatively narrow host ranges, including over 50 legume species (Vetten 2009, Franz et al., 1997, Yang et al., 2016). However, milk vetch dwarf virus (MDV) has natural hosts reported in the Apocynaceae, Solanaceae and Caricaeae (Lal et al., 2020) and, surprisingly, the monocotyledonous genus Lilium (Choi et al., 2019). Additionally, sequences resembling those of a nanovirus have been described from parsley (Apiaceae) (Vetten et al., 2019). Nanoviruses are transmitted by several aphid species. Aphis craccivora appears to be a major natural vector of these viruses as it is the most abundant aphid species on legume crops in afflicted areas and was among the most efficient vectors under experimental conditions. Other aphid vectors are Acyrthosiphon pisum and Aphis fabae; for MDV  Aphis gossypii and Megoura viciae are also vectors. 

Antigenicity

Antisera to faba bean necrotic yellows virus (FBNYV) and subterranean clover stunt virus (SCSV) cross-react weakly in western blots and immunoelectron microscopy, and not at all in DAS-ELISA, suggesting that the serological relationship between these two viruses is distant (capsid protein (CP) amino acid sequence identity ca. 57%) (Franz et al., 1996). However, MDV, faba bean yellow leaf virus (FBYLV) and faba bean necrotic stunt virus (FBNSV) cross-react with some monoclonal antibodies (MAbs) to FBNYV (Abraham et al., 2012, Franz et al., 1996). Therefore, species-specific MAbs are required for the differentiation and specific detection of viruses belonging to these closely related species, which share CP aa sequence identities of 83–88%. 

Species demarcation criteria

Criteria for species demarcation are:

• Differences in natural host range
• Differences in the identity and number of vector aphid species
• Different reactions to antibodies between members of different species
• Members of different species generally have an overall nt sequence identity of <80% across all eight combined genome components

Member species

Related, unclassified viruses