A new clade of pararetroviruses distantly related to hepadnaviruses and nackednaviruses

Group VII of the Baltimore classification comprises reverse-transcribing, non-integrated DNA viruses, also known as pararetroviruses. These include the hepadnaviruses, a family of small enveloped DNA viruses that infect vertebrates, but also a sister family of non-enveloped fish viruses, the nackednaviruses. Here we describe the complete sequence of a new pararetrovirus found in the feces of an insectivorous bat. This virus encodes a core protein and a reverse transcriptase but no envelope protein. A database search identified a viral sequence from a permafrost sample as its closest relative. The two viruses form a cluster that occupies a basal phylogenetic position relative to hepadnaviruses and nackednaviruses, with an estimated divergence time of 500 million years. These findings may lead to the definition of a new viral family and support the hypothesis that ancestral animal pararetroviruses were non-enveloped.