Evolutionary and phenotypic analysis of live virus isolates suggests arthropod origin of a pathogenic RNA virus family
Significance Knowledge of the origin and evolution of viruses provides important insight into virus emergence involving the acquisition of genes necessary for the infection of new host species or the development of pathogenicity. The family Bunyaviridae contains important arthropod borne pathogens of humans, animals, and plants. In this study, we provide a comprehensive characterization of two novel lineages of insect specific bunyaviruses that are in basal phylogenetic relationship to the rodent borne hantaviruses, the only genus within the Bunyaviridae that is not transmitted by arthropod vectors. These data, together with ancestral state reconstruction of bunyavirus hosts for major virus lineage bifurcations, suggest that the vertebrate infecting viruses evolved from arthropod specific progenitors. The evolutionary origins of arboviruses are unknown because their typical dual host tropism is paraphyletic within viral families. Here we studied one of the most diversified and medically relevant RNA virus families, the Bunyaviridae, in which four of five established genera are transmitted by arthropods. We define two cardinally novel bunyavirus groups based on live isolation of 26 viral strains from mosquitoes (Jonchet virus [JONV] eight strains; Ferak virus [FERV] 18 strains) Both viruses were incapable of replicating at vertebrate typical temperatures but replicated efficiently in insect cells. Replication involved formation of virion sense RNA (vRNA) and mRNA, including cap snatching activity. SDS/PAGE, mass spectrometry, and Edman degradation identified translation products corresponding to virion associated RNA dependent RNA polymerase protein (RdRp) glycoprotein precursor protein, glycoproteins Gn and Gc, as well as putative nonstructural proteins NSs and NSm. Distinct virion morphologies suggested ancient evolutionary divergence, with bunyavirus typical morphology for FERV (spheres of 60 120 nm) as opposed to an unusual bimorphology for JONV (tubular virions of 60 x 600 nm and spheres of 80 nm) Both viruses were genetically equidistant from all other bunyaviruses, showing <15% amino acid identity in the RdRp palm domain. Both had different and unique conserved genome termini, as in separate bunyavirus genera. JONV and FERV define two novel sister taxons to the superclade of orthobunyaviruses, tospoviruses, and hantaviruses. Phylogenetic ancestral state reconstruction with probabilistic hypothesis testing suggested ancestral associations with arthropods at deep nodes throughout the bunyavirus tree. Our findings suggest an arthropod origin of bunyaviruses.
Proceedings of the National Academy of Sciences