Capsid-Less RNA Viruses

Capsid-less RNA viruses comprise an assemblage of diverse virus-like agents with genomes of variable size that share only the gene encoding the RNA-dependent RNA polymerase. Although double-stranded ribonucleic acid (RNA)is the dominant form of virus-specific RNA isolated from organisms infected by most capsid-less viruses, these viruses probably have evolved independently from distinct positive-strand RNA virus ancestors that have lost the ability to form capsids. The loss of capsid caused the loss of virus transmissibility via extracellular routes, limiting these agents to vertical transmission. The capsid-less RNA viruses include the families Narnaviridae, Hypoviridae, and Endornaviridae as well as several viruses scattered among diverse viral lineages. The majority of the known capsid-less viruses reproduce in fungi, and the rest in oomycetes and plants. The narnaviruses of the genus Mitovirus replicate within fungal mitochondria and show evolutionary affinity to RNA bacteriophages, suggesting their potential ancient origin from viruses of bacteria that gave rise to mitochondrial endosymbiont of the ancestral eukaryote. In contrast, it seems likely that the fungal hypoviruses and plant/fungal endornaviruses have emerged more recently, via reductive evolution of eukaryotic ancestral viruses of the Picornavirus-like and Alphavirus-like superfamilies, respectively.