Identifications of novel avian and mammalian deltaviruses filled the large evolutionally gaps and revealed inter-family transmission of deltaviruses

Deltaviruses are unique satellite viruses requiring envelope protein from helper viruses to form infectious particles. Although human hepatitis delta virus (HDV), a satellite virus of a hepadnavirus, has been the only member of the genus Deltavirus until 2018, recent studies identified several deltaviruses in vertebrate and invertebrate animals. However, the evolution of deltaviruses, such as the origin of HDV and co-evolution with helper viruses, is still unclear due to the large phylogenetic gaps between the deltaviruses. In this study, by a combination of screening of publicly available RNA-seq datasets and RT-PCR, we identified five full-length, circular genomes of deltaviruses from passerine birds, woodchucks, and white-tailed deer. By mapping pubic RNA-seq data to the newly identified deltaviruses and RT-PCR, we found that the passerine deltaviruses have been circulating among the animal populations, including a bird caught from wild. Interestingly, the passerine deltaviruses show more than 98% nucleotide identities to each other and were detected from birds spanning at least four families in the order Passeriformes, suggesting the recent inter-family transmission events and the potential of broad host ranges of deltaviruses. Phylogenetic analyses showed that the newly identified deltaviruses are relatively closely related to known vertebrate deltaviruses and that newly identified mammalian deltaviruses are the closest relatives of HDV, suggesting the mammalian origin of HDV. Interestingly, there is no evidence that these mammalian deltaviruses express the large isoform of delta antigen that contains a farnesylation site required for HDV to utilize hepadnavirus envelope proteins. Also, there is no evidence of hepadnavirus infections in deltavirus-positive individuals. Therefore, the satellite-helper relationship between HDV and hepadnavirus may have been established relatively recently, after the divergence of the newly identified non-HDV mammalian deltaviruses and the HDV lineage. Taken together, our findings provide novel insights into the evolution and diversity of deltaviruses and raise the importance of further surveillance for deltaviruses in a wide range of animals.