Selectively maintained paleoviruses in Holarctic water fleas reveal an ancient origin for phleboviruses

The ecological model, Daphnia pulex (Cladocera: Daphniidae), is broadly distributed in Holarctic freshwater habitats and has been the subject of multidisciplinary study for over half a century, but never has a natural RNA virus infection been reported in daphnids. Here we report on a group of paleoviruses related to RNA dependent RNA polymerase in the genome of D. pulex. Phylogenetic analysis suggests that these paleoviruses are derived from a viral lineage within the genus Phlebovirus. Comparison of the genomic sequences flanking individual paleoviruses reveal that some are orthologous viral insertions having been present in the common ancestor of the D. pulex species complex, which is millions of years old. Still, we detected some sites that have the signature of purifying selection. In contrast, other paleoviruses in this group seem to be unique to specific host lineages and even contain undisrupted open reading frames, suggesting either more recent acquisition, or selective maintenance.