100-My history of bornavirus infections hidden in vertebrate genomes

Significance Many viral diseases have emerged in recent decades, but prehistoric viral infections remain poorly understood. In some cases, nucleotide sequences of ancient viruses, which infected ancestral animals, have been integrated into their genomes during evolution. Such “molecular fossil records” of viruses help researchers trace past viral infections. Here, we reconstructed the infection history of an RNA virus, the bornavirus, for approximately 100 My in vertebrate evolution, using molecular fossils of ancient bornaviruses. Our analyses using ancient bornaviral sequences from over 100 vertebrate species genomes indicated that bornaviruses infected a broader range of host lineages during their long-term evolution than expected from extant bornaviral host ranges. Our findings highlighted the hidden history of this RNA viral infection over geological timescales.