This article was originally published in the Encyclopedia of Virology, Volumes 1–5 published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non- commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Ghabrial S A, Ochoa W F, Baker T S and Nibert M L. Partitiviruses: General Features. Encyclopedia of Virology, 5 vols. (B.W.J. Mahy and M.H.V. Van Regenmortel, Editors), pp. 68-75 Oxford: Elsevier. Author's personal copy 68 Partitiviruses: General Features multiply mainly in areas of the fungal mycelium that are Further Reading less involved in fungal growth might be one of the reasons that partitiviruses, although they replicate to relatively high Bruenn JA (1993) A closely related group of RNA-dependent RNA levels, are usually associated with a symptomless infection polymerases from double-stranded RNA viruses. Nucleic Acids Research 21: 5667–5669. of their respective fungal hosts. Recently, the latency of a Ghabrial SA (1998) Origin, adaptation and evolutionary pathways of partitivirus infection was demonstrated by direct experi- fungal viruses. Virus Genes 16: 119–131. mental evidence in the case of ascomycete Rosellinia necatrix Ghabrial SA, Buck KW, Hillman BI, and Milne RG (2005) Partitiviridae. In: Fauquet CM, Mayo MA, Maniloff J, Desselberger U, and Ball LA and the RnV-1-W8 partitivirus. Protoplasts of R. necatrix (eds.) Virus Taxonomy: Eighth Report of the International Committee were transfected with purified particles of RnV-1-W8. The on Taxonomy of Viruses, pp. 581–590. San Diego, CA: Elsevier resulting mycelium contained transmissible RnV-1-V8 Academic Press. ICTVdB Management (2006) 00.049.0.01. Partitivirus. In: Bu¨ chen- virions but showed no observable symptoms associated Osmond C (ed.) ICTVdB – The Universal Virus Database, version 4, with the presence and replication of this virus. In con- New York: Columbia University. trast, in the H. annosum study (see previous section), Kim JW, Choi EY, and Kim YT (2006) Intergeneric relationship between the aspergillus ochraceous virus F and the penicillium stoloniferum germination frequency of basidiospores was significantly virus S. Virus Research 120: 212–215. reduced (P < 0.05) by the presence of partitivirus dsRNA Strauss EE, Lakshman DK, and Tavantzis SM (2000) Molecular in the parental fruit bodies. Use of isogenic fungal lines characterization of the genome of a partitivirus from the basidiomycete Rhizoctonia solani. Journal of General Virology 81: may be needed to verify these results. 549–555. Tavantzis SM and Bandy BP (1988) Properties of a mycovirus from See also: Fungal Viruses; Chrysoviruses; Partitiviruses: Rhizoctonia solani and its virion-associated RNA polymerase. Journal of General Virology 69: 1465–1477. General Features; Totiviruses. Partitiviruses: General Features S A Ghabrial, University of Kentucky, Lexington, KY, USA W F Ochoa and T S Baker, University of California, San Diego, La Jolla, CA, USA M L Nibert, Harvard Medical School, Boston, MA, USA ã 2008 Elsevier Ltd. All rights reserved. Glossary differ from the ‘allowed’ symmetries where monomers are either equivalently or quasi-equivalently related. Cryo-transmission electron microscopy Virus capsid Generally a protective shell, composed Transmission electron microscopy of unstained, of multiple copies each of one or more distinct protein unfixed, frozen-hydrated (vitrified) specimens, subunits, that encapsidate the viral genome. preserved as close as possible to their native state. Hyphal anastomosis The union of a hypha with another resulting in cytoplasmic exchange. Icosahedral symmetry Arrangement of 60 identical Introduction objects or asymmetric units adopted by many isometric (spherical) viruses, with a combination of In the early 1960s, interest in the antiviral activities asso- two-, three-, and fivefold rotations equivalently ciated with cultural filtrates of Penicillium spp. led to the relating the units about a point in space. discovery of double-stranded RNA (dsRNA) isometric Mycoviruses Viruses that infect and multiply in viruses in these and other filamentous fungi. Fungal fungi. viruses, or mycoviruses, are now known to be of common ‘T ¼ 2’ symmetry The so-called ‘forbidden’ occurrence. The isometric dsRNA viruses isolated from triangulation symmetry, in which 120 chemically Penicillium spp. were among the first to be molecularly identical protein monomers, form 60 identical, characterized and were shown to have segmented gen- asymmetric dimers arranged with icosahedral omes. Those with bipartite genomes are currently classi- symmetry. Monomers in a ‘T ¼ 2’ structure occupy fied in the genus Partitivirus in the family Partitiviridae. two distinct, nonequivalent positions and therefore Interestingly, plant viruses (also called cryptoviruses) with Encyclopedia of Virology, Third Edition (2008), vol. 4, pp. 68-75 Author's personal copy Partitiviruses: General Features 69 bipartite dsRNA genomes and very similar properties to sedimentation coefficients of these virions range from the fungal partitiviruses were discovered in the late 1970s 101S to 145S (S20w in Svedberg units). Generally, each and are presently classified in the genera Alphacryptovirus virion contains only one of the two genomic dsRNA seg- and Betacryptovirus in the family Partitiviridae. The fungal ments. However, with some viruses like the fungal partiti- partitiviruses and plant partitiviruses are discussed else- virus PsV-S, purified preparations can contain other where in this encylopedia. The goals of this article are to distinctly sedimenting forms that include empty particles examine the similarities and differences between these two and replication intermediates (see the next section). groups of viruses and to discuss future perspectives of All fungal and plant partitiviruses examined to date partitivirus research. have been shown to possess virion-associated RNA- dependent RNA polymerase (RdRp) activity, which cata- lyzes the synthesis of single-stranded RNA (ssRNA) Biological Properties copies of the positive strand of each of the genomic dsRNA molecules. The in vitro transcription reaction Fungal and plant partitiviruses are both associated with occurs by a semi-conservative mechanism, whereby the latent infections of their respective hosts. There are no released ssRNA represents the displaced positive strand of known natural vectors for any partitivirus. Fungal parti- the parental dsRNA molecule and the newly synthesized tiviruses are transmitted intracellularly during cell divi- positive strand is retained as part of the duplex. sion and sporogenesis (vertical transmission) as well as following hyphal anastomosis, that is, cell fusion, between compatible fungal strains (horizontal transmission). In Genome Organization and Replication some ascomycetes (e.g., Gaeumannomyces graminis), virus is usually eliminated during ascospore formation. Experi- Virions of members of the partitivirus family contain mental transmission of fungal partitiviruses has been two unrelated segments of dsRNA, in the size range of reported by transfecting fungal protoplasts with purified 1.4–2.3 kbp, one encoding the capsid protein (CP) and the virions. The plant cryptoviruses, on the other hand, are other encoding the RdRp. The two segments are usually of not horizontally transmitted by grafting or other mechan- similar size and are encapsidated separately, that is, each ical means, but are vertically transmitted by ovule and/or particle generally contains only one dsRNA segment. The pollen to the seed embryo. Thus, whereas sexual repro- genomes of at least 16 members of the genus Partitivirus duction of the host is required for the survival of plant have recently been completely sequenced (Table 1). In partitiviruses, it is detrimental to the continued existence contrast, the complete genome sequences of only three of the fungal partitiviruses that infect some ascomycetes. alphacryptoviruses and no betacryptoviruses have yet Transmission of fungal partitiviruses through asexual been determined (Table 1). The genomic structure of spores, however, can be highly efficient, with 90–100% Atkinsonella hypoxylon virus (AhV-1), the type species of of single conidial isolates having received the virus. In the genus Partitivirus, comprising segment 1 (2180 bp, summary, transmission of fungal partitiviruses by asexual encoding the RdRp) and segment 2 (2135 bp, encoding the spores and plant partitiviruses by seed provide the primary CP), is schematically represented in Figure 1. or only means for disseminating these viruses. The presence of one or more additional dsRNA seg- Both fungal and plant partitiviruses are generally asso- ments is common among members of the family Partiti- ciated with symptomless infections of their hosts. While viridae. For example, in addition to the two genomic cryptoviruses are present in very low concentrations in segments, preparations of AhV-1 contain a third dsRNA plants