A replication-competent deltavirus from the marsupial fat-tailed dunnart Sminthopsis crassicaudata.

Deltaviruses are circular, negative-sense RNA agents that replicate autonomously but depend on heterologous envelope glycoproteins for spread. Only partial sequences of deltaviruses had been reported from marsupials. By reanalysing public metatranscriptomes from the Australian fat-tailed dunnart (Sminthopsis crassicaudata), we assemble the first complete marsupial deltavirus genome and test its replication in human and animal cells. The fat-tailed dunnart deltavirus (FtDDeV) is a 1,680-nt circular RNA that folds into a canonical unbranched rod-like structure and encodes a 195-aa delta antigen (FtDDAg). Genomic and antigenomic HDV-like ribozymes are present and conserve catalytic core motifs. Phylogenetic analyses cluster FtDDAg with the Tasmanian devil sequence, and both are quite close to RDAg from the neotropical rodent species Proechimys semispinosus. A dimeric FtDDeV cDNA replicon supports time-dependent DAg accumulation in human, simian, rodent and Tasmanian devil cells, with faster kinetics in rodents and marsupial cells. FtDDAg accumulation patterns in host nuclei show characteristic viral hubs, observed with other deltaviruses. No obvious coinfecting helper viruses were detected in FtDDeV-positive libraries. Our study extends the confirmed host range of deltaviruses to marsupials and provides a replication-competent clone to investigate helper usage, host restriction and deltavirus evolution.