Two negative-stranded RNA viruses with unusual molecular features recently identified in citrus in Italy

In an attempt of identifying the causal agent of a severe citrus disease first described in the early 1930s and named citrus concave gum, a new negative stranded RNA (nsRNA) virus was identified by high-throughput sequencing (HTS). Being associated with the disease, this virus was tentatively named citrus concave-gum-associated virus (CCGaV). With a bipartite genome composed of an RNA1 encoding the RNA-dependent RNA polymerase (RdRp) in the negative strand and an ambisense bicistronic RNA2 coding for the nucleoprotein (NP) and the putative movement protein (MP), CCGaV showed structural and phylogenetic features supporting the proposal of classifying it as a representative member of a new species to be allocated in the new genus Coguvirus in the Order Bunyavirales (https://talk.ictvonline.org/files/proposals/taxonomy_proposals_plant1/m/plant01/7405). A second nsRNA virus, resembling CCGaV has been recently identified by HTS in citrus and provisionally denoted citrus bunya-like virus (CBLV). The molecular and structural features of CBLV genomic RNAs, the signatures of the proteins it encodes and phylogenetic analyses support the classification of CBLV and CCGaV in the same genus and suggest a possible modular genome evolution for these viruses. These findings, supporting the hypothesis that plant-infecting nsRNA viruses most likely have evolved from an invertebrate-infecting ancestor, several times as independent events, will be discussed. Specific methods for detecting both CCGaV and CBLV and results of a field survey on the distribution of both viruses in Southern Italy will also be presented and discussed.