Fungi harbor a vast diversity of mobile genetic elements (MGEs). Recently, novel fungal MGEs, tentatively referred to as 'ambiviruses,' were described. 'Ambiviruses' have single-stranded RNA genomes of about 4-5 kb in length that contain at least two open reading frames (ORFs) in non-overlapping ambisense orientation. Both ORFs are conserved among all currently known 'ambiviruses,' and one of them encodes a distinct viral RNA-directed RNA polymerase (RdRP), the hallmark gene of ribovirian kingdom Orthornavirae. However, 'ambivirus' genomes are circular and predicted to replicate via a rolling-circle mechanism. Their genomes are also predicted to form rod-like structures and contain ribozymes in various combinations in both sense and antisense orientations-features reminiscent of viroids, virusoids, ribozyvirian kolmiovirids, and yet-unclassified MGEs (such as 'epsilonviruses,' 'zetaviruses,' and some 'obelisks'). As a first step toward the formal classification of 'ambiviruses,' the International Committee on Taxonomy of Viruses (ICTV) recently approved the establishment of a novel ribovirian phylum, Ambiviricota, to accommodate an initial set of 20 members with well-annotated genome sequences.

Department of Agricultural Science and Plant Protection Mississippi State University Mississippi State Mississippi USA

Department of Forest Protection and Wildlife Management Faculty of Forestry and Wood Technology Mendel University in Brno Brno Czech Republic

Department of Plant Protection School of Agriculture The University of Jordan Amman Jordan

Institut Pasteur Université Paris Cité Archaeal Virology Unit Paris France

Institute for Genomics Biocomputing and Biotechnology Mississippi State University Mississippi State Mississippi USA

Institute for Sustainable Plant Protection CNR Bari Italy

Institute for Sustainable Plant Protection CNR Torino Italy

Instituto de Biología Molecular y Celular de Plantas Universidad Politécnica de Valencia CSIC Valencia Spain

Integrated Research Facility at Fort Detrick Division of Clinical Research National Institute of Allergy and Infectious Diseases National Institutes of Health Fort Detrick Frederick Maryland USA

National Center for Biotechnology Information National Library of Medicine National Institutes of Health Bethesda Maryland USA

Natural Resources Institute Finland Helsinki Finland

Nuffield Department of Medicine University of Oxford Oxford United Kingdom

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