The hidden diversity of ancient bornaviral sequences from X and P genes in vertebrate genomes

Abstract

Endogenous bornavirus-like elements (EBLs) are heritable sequences derived from bornaviruses in vertebrate genomes, which are strongly suggested to originate from transcripts of ancient bornaviruses. EBLs have been detected by sequence similarity searches, such as tBLASTn, and thus EBLs derived from small and/or rapidly evolving viral genes, such as viral X and P genes, are difficult to be detected. Indeed, no EBLs derived from X and P genes of orthobornaviruses were detected. Thus, although previous studies comprehensively analyzed the presence of EBLs in vertebrate genomes, there may still be undetectable EBLs. In this study, we developed a novel strategy to detect such "hidden" EBLs by focusing on the nature of readthrough transcription of orthobornavirus. We showed a series of evidence supporting that there are EBLs derived from orthobornaviral X and P genes in mammalian genomes. Therefore, this study contributes to a deeper understanding of the evolution of ancient viruses and of virus-host relationships. Further, our data also suggest that endogenous viral elements detected thus far are just the tip of the iceberg, and thus further studies are required to understand ancient viruses more accurately.