Hidden diversity and expanded host range of sarthroviruses, including terrestrial vertebrates

The Sarthroviridae are a family of highly compact satellite RNA viruses comprising one recognised species, extra small virus (XSV). Macrobrachium rosenbergii nodavirus (MrNV) is the associated helper virus of XSV and their co-infection has been linked to white tail disease in freshwater prawns globally, although the role of XSV is remains unclear. Here, we describe the discovery and characterisation of ten novel, highly divergent sarthrovirus species from a range of hosts and environments within a small geographical region in Australia. These comprise novel sarthroviruses associated with marine sponges, seal and dingo faeces, environmental marine sediment samples and Indo-Pacific geckos (Hemidactylus garnotii). All the novel viruses possess only a capsid protein, consistent with the genome of XSV, yet exhibit substantial sequence divergence. Notably, some sarthrovirus variants seem to utilise different replication systems despite being genetically identical and present in the same host species. Sequences from nodaviruses, which could plausibly act as helpers, were associated with some, but not all, the sarthroviruses identified here. Phylogenetic analyses support the expansion of the Sarthroviridae into multiple distinct lineages, comprising at least seven genera. Collectively, these findings reveal a broader ecological distribution and evolutionary diversity of sarthroviruses and highlight the possibility of alternative replication strategies and tissue tropism in diverse animal host. SignificanceSarthoviruses are small ([~]800 nucleotides) satellite RNA viruses associated with a nodavirus of crustaceans that acts as a helper. To date, the only known sarthovirus is extra small virus (XSV), which also represents the sole species within the Sarthroviridae. Here, we report the detection of ten divergent sarthroviruses sampled from diverse animal hosts, including vertebrates, that expand the family to 11 species and at least seven genera. These viruses were detected from various host taxa and environmental samples from a confined geographical region in eastern Australia, suggesting that they are ecologically connected. Notably, we did not detect nodaviruses in all samples containing sarthroviruses, suggesting that different viruses may act as helpers for sarthovirus replication.