Insect retroelements provide novel insights into the origin of hepatitis B viruses.

The origin of hepadnaviruses (Hepadnaviridae), a group of reverse-transcribing DNA viruses that infect vertebrates, remains mysterious. All the known retrotransposons are only distantly related to hepadnaviruses. Here we report the discovery of two novel lineages of retroelements, which we designate hepadnavirus-like retroelement (HEART1 and HEART2), within the insect genomes through screening 1,095 eukaryotic genomes. Both phylogenetic and similarity analyses suggest that the HEART retroelements represent the closest non-viral relatives of hepadnaviruses so far. The discovery of HEART retroelements narrows down the evolutionary gap between hepadnaviruses and retrotransposons and might thus provide unique insights into the origin and evolution of hepadnaviruses.