Family: Apasviridae

Apoorva Prabhu and Christian Rinke

The citation for this ICTV Report chapter is the summary to be published as Prabhu, A and Rinke, C. ICTV Virus Taxonomy Profile: Apasviridae 2026, Journal of General Virology, (in press)

Corresponding author: Apoorva Prabhu ([email protected])
Edited by: Mart Krupovic and Evelien Adriaenssens
Posted: January 2026

Summary

The family Apasviridae consists of dsDNA viruses that are associated with marine archaea in the order Poseidoniales, formerly known as Marine Group II Euryarchaeota. Viruses in the family Apasviridae are “magroviruses”, a collective term named after marine group II viruses. Discovered using metagenomics, the virions are predicted to have an icosahedral capsid and a helical tail, which are characteristic of viruses in the viral class Caudoviricetes. Like other magroviruses, viruses in the family Apasviridae have genomes that are among the largest known for archaeal viruses, and include genes comprising morphogenetic and DNA replication modules, encoding proteins such as major capsid protein and family B DNA polymerase. The family includes one genus (Agnivirus) and a single species (Agnivirus brisbanense).

Table 1 Apasviridae. Characteristics of members of the family Apasviridae

Virion

Morphology

Members of this family, like many other archaeal viruses have been discovered using metagenomics (Nishimura et al., 2017, Philosof et al., 2017, Xu et al., 2023, Zhou et al., 2023, Prabhu et al., 2025). The complete genome of magrovirus_A_01 (species Agnivirus brisbanense) encodes a morphogenetic module, comprising head and tail proteins, such as major capsid protein (MCP), portal protein, and tail fibre protein, that are typical of viruses in the class Caudoviricetes (Figure 1 Apasviridae). Beyond the identified proteins, their relatively close sequence similarity to tailed haloviruses suggests that virions of apasvirids likely possess icosahedral heads and helical tails (Baquero et al., 2020, Liu et al., 2021).

Nucleic acid

Members of the order Magrovirales, are dsDNA viruses with a genome of 90–110 kbp, making these among the largest genomes described for archaeal viruses (ref). The complete genome of Agnivirus brisbanense has been assembled as a circular molecule (i.e., based on the presence of direct terminal repeats) of 108,255 bp and encodes 99 predicted proteins (Prabhu et al., 2025) (Prabhu et al., 2025). The genes comprising the morphogenetic module and DNA replication and repair modules are predicted to be transcribed in opposite directions (Figure 1 Apasviridae).

Figure 1 Apasviridae. Genome organisation of magrovirus_A_01 (species Agnivirus brisbanense), a member of the family Apasviridae. Boxes indicate open reading frames as annotated on GenBank accession OR863078. Selected protein products are labelled.

Genome organization and replication

The genome of magrovirus_A_01 contains a replication module encoding proteins for DNA replication and repair, namely DNA polymerase family B, archaeal-like DNA primase, PD-(D/E) XK endonuclease, restriction enzyme-like nucleases, helicase and an ATP-dependent DNA ligase. The genome does not encode an integrase, and hence the virus is predicted to lead a lytic lifestyle.  

Biology

Apasvirids have been discovered using metagenomics and have yet to be cultured. Other than the morphogenetic and replication modules, apasvirid genomes encode a HTH-luxR transcriptional regulator involved in quorum sensing, a phosphatase, and a pyruvate carboxyltransferase which belongs to the biotin-dependent enzyme family (Prabhu et al., 2025). These auxiliary genes may enhance the host metabolic capability, e.g., by scavenging phosphorus and biotin from the environment.

Derivation of names

Agnivirus: from the Hindu fire-god Agni, as the fire of life on earth was born out of Apas

Apasviridae: from Apas, the water goddess in ancient Indian mythology

Demarcation criteria for taxa within the family

There is only one genus in the family. Additional genera would be expected to share ≥ 60% of proteins according to demarcation criterion set forward for archaeal viruses (Liu et al., 2021). Similarly, additional species in the family would be expected to display less than 95% overall genome sequence identity with viruses in other species, consistent with the demarcation criterion of other archaeal viruses within the class Caudoviricetes.

Relationships within the family

The family includes a single species, Agnivirus brisbanense (Figure 2 Apasviridae). However, comparative genomics with unclassified viruses in IMG/VR and from the Brisbane River Estuary dataset (PRJNA1024631) reveals a cluster of viruses sharing 20–40% of their genes, and these may represent additional genera.

Figure 2 Apasviridae. Relationships of the taxa connected to the family Apasviridae.

Relationships with other taxa

Members of the family Apasviridae share < 60% of proteins with their nearest relatives of other families within the same order (Aoguangviridae), according to the demarcation criterion set forward for archaeal viruses (Liu et al., 2021). Members of the order Magrovirales show close similarities with viruses of the family Druskaviridae (class Caudoviricetes), which infect hyperhalophilic archaea (Zhou et al., 2023). Collectively known as magroviruses, these viruses have been divided into multiple clades of which viruses in clade A are apasvirids (Prabhu et al., 2025), viruses in clade B are placed in the family Aoguangviridae (Zhou et al., 2023) and those in clade E in the family Krittikaviridae (Prabhu et al., 2025). Viruses in clades C, D and X could be members of additional families, but complete circular genomes are not yet available.