Mapping the evolution of bornaviruses across geological timescales

It has always seemed likely that viruses originated early in the history of life. However, until the identification of endogenous viral elements (EVEs), there was little if any direct evidence for most virus groups ever having existed in the distant past (1). EVEs are virus-derived DNA sequences found in the germline genomes of metazoan species. Uniquely, they preserve information about the genomes of viruses that circulated tens to hundreds of millions of years ago. Comparative analysis of EVE sequences has now provided robust age calibrations for a diverse range of virus groups, completely transforming perspectives on the longer-term evolutionary interactions between viruses and hosts (2). As progress in mapping the complete genome sequences of species has accelerated, the abundance of “fossilized” viral sequences in eukaryotic genomes has become apparent. Increasingly, the challenge is not finding EVEs, but scaling analytical approaches to tackle the exponentially increasing volume of EVE sequence data (3, 4). In PNAS, Kawasaki et al. (5) utilize sophisticated computational approaches to implement a broad-scale analysis of the viral “fossil record,” focusing on a group of viruses called “bornaviruses.” Bornaviruses (family Bornaviridae ) are a poorly understood group of single-stranded negative sense RNA viruses (order Mononegavirales ) that infect vertebrates. Until 2015 the family contained only one genus ( Orthobornavirus ); however, recent progress in sampling viral diversity has led to the establishment of two novel genera ( Carbovirus and Curtervirus ) (6). The prototype member, Borna disease virus (BDV), infects a variety of mammalian species and is the causative agent of a neurological condition in horses referred to as Borna disease or “sad horse disease.” The name “Borna” derives from an 1895 outbreak that occurred in the vicinity of the town of Borna in Saxony, Germany, and decimated the Prussian cavalry (7). The association of BDV with neurological disorders in mammals, combined with … [↵][1]1Email: robert.gifford{at}glasgow.ac.uk. [1]: #xref-corresp-1-1