Family: Adenoviridae
Genus: Ichtadenovirus
Distinguishing features
The single known member of this genus, white sturgeon adenovirus 1 (WSAdV-1), is the only confirmed fish adenovirus (Benkő et al., 2002). It was isolated and propagated on a white sturgeon cell line. This host is very divergent from those of other adenoviruses, and phylogenetic calculations and a unique genome organization further distinguish this virus from all other adenoviruses (Kovács et al., 2003, Doszpoly et al., 2019). The WSAdV-1 genome is longer than that of any known adenovirus. The fiber gene was not found in its usual position towards the right end of the genome, but a fiber gene homolog have been discovered at the left end (Doszpoly et al., 2019) (Figure 2 Adenoviridae).
Virion
Morphology
Although electron micrographs of WSAdV-1 have been produced, the morphology (including the number of the fibers per vertex) is not known. Bacterial expression of the putative fiber gene has failed thus far.
Physicochemical and physical properties
Not known.
Nucleic acid
The WSAdV-1 genome is 48,395 bp, and therefore is the longest of any known adenovirus (Doszpoly et al., 2019). The nucleotide composition is 42.6% G+C. The inverted terminal repeats (ITRs) are 126 bp.
Proteins
WSAdV-1 has orthologues of the 16 family-common genes, and these proteins are presumed to have roles similar to those in other adenoviruses. However, the virus has a further 31 ORFs that presumably code for at least this number of additional proteins. Most have no similarity to any known adenovirus proteins; the two exceptions distantly resemble the parvovirus NS proteins, homologues of which occur in certain avian adenoviruses. There are two encoded esterases that are homologous to Escherichia coli O-acetylesterase.
Lipids
None reported.
Carbohydrates
See discussion under family properties.
Genome organization and replication
At the left genome end, there is a gene (ORF0) encoding a hypothetical protein with unknown function. Downstream from this gene, there is fiber gene (Doszpoly et al., 2019) surrounded by two esterase genes. To the right of these genes is the conserved region with 16 genes shared by the family (starting with IVa2). Further to the right is a region extending to the right terminus that accounts for >42% of the entire genome. A total of 28 ORFs potentially encoding proteins of >50 amino acid residues are present in this region (Figure 2 Adenoviridae) (Doszpoly et al., 2019). Only two WSAdV-1 genes are predicted to be spliced: those encoding pTP and 33K (which are also spliced in all other adenoviruses). The other family-common genes are thought not to be spliced, while the genus-specific genes/ORFs need further study.
Biology
The virus seems to be non-pathogenic for fish.
Antigenicity
See discussion under family properties.
Species demarcation criteria
There is currently a single species in the genus (Ichtadenovirus acipenseris) and no related, unclassified adenoviruses are known; demarcation criteria have not been defined.