Evolution of negative strand RNA viruses: New hints from viruses infecting plants

Negative strand RNA (nsRNA) viruses are among the most threatening pathogens of humans, animals and plants. Many nsRNA viruses have segmented genome and may infect two unrelated hosts, such as invertebrates and vertebrates or invertebrates and plants. Many novel invertebrate-restricted viruses have been reported in the last few years, with phylogenetic evidence supporting that the dual host tropism in nsRNA viruses originated from the adaptation of ancestral invertebrate-restricted viruses to divergent hosts. However, the underlying evolutionary routes of such virus trans-kin- gdom adaptations are difficult to envisage. In an attempt of identifying the causal agent of a severe citrus disease, first reported more than eighty years ago and named citrus concave gum (CG), a novel nsRNA virus has been identified and shown to be closely associated with the disorder. The new plant virus, tentatively named citrus concave gum-associated virus (CCGaV), has a bipartite genome that has been com- pletely sequenced. Although CCGaV should be classified in a novel genus, molecular signatures in the viral genomic RNAs and phylogenetic reconstructions support evo- lutionary links with members of the genus Phlebovirus, which are arthropod-transmit- ted viruses infecting mammals, and with some phlebo-like viruses exclusively infecting arthropods. Implications of these findings in the adaptation of ancestor nsRNA virus(es), likely infecting arthropods, to plant hosts will be examined. The impact of specific molecular and serological detection methods developed in the frame of this study on sanitation and certification programs of citrus propagation material will be also discussed.