Mycovirus

Mycoviruses (Ancient Greek: μύκης mykes ('fungus') + Latin virus), also known as mycophages, are viruses that infect fungi. The majority of mycoviruses have double-stranded RNA (dsRNA) genomes and isometric particles, but approximately 30% have positive-sense, single-stranded RNA (+ssRNA) genomes. Mycoviruses (Ancient Greek: μύκης mykes ('fungus') + Latin virus), also known as mycophages, are viruses that infect fungi. The majority of mycoviruses have double-stranded RNA (dsRNA) genomes and isometric particles, but approximately 30% have positive-sense, single-stranded RNA (+ssRNA) genomes. True mycoviruses demonstrate an ability to be transmitted to infect other healthy fungi. Many double-stranded RNA elements that have been described in fungi do not fit this description, and in these cases they are referred to as virus-like particles or VLPs. Preliminary results indicate that most mycoviruses co-diverge with their hosts, i.e. their phylogeny is largely congruent with that of their primary hosts. However, many virus families containing mycoviruses have only sparsely been sampled. Mycovirology is the study of mycoviruses. It is a special subdivision of virology and seeks to understand and describe the taxonomy, host range, origin and evolution, transmission and movement of mycoviruses and their impact on host phenotype. The first record of an economic impact of mycoviruses on fungi was recorded in cultivated mushrooms (Agaricus bisporus) in the late 1940s and was called the La France disease. Hollings found more than three different types of viruses in the abnormal sporophores. This report essentially marks the beginning of mycovirology. The La France Disease is also known as X disease, watery stripe, dieback and brown disease. Symptoms include: Mushrooms have shown no resistance to the virus, and so control has been limited to hygienic practises to stop the spread of the virus. Perhaps the best known mycovirus is Cryphonectria parasitica hypovirus 1 (CHV1). CHV1 is exceptional within mycoviral research for its success as a biocontrol agent against the fungus C. parasitica, the causative agent of chestnut blight, in Europe, but also because it is a model organism for studying hypovirulence in fungi. However, this system is only being used in Europe routinely because of the relatively small number of vegetative compatibility groups (VCGs) on the continent. By contrast, in North America the distribution of the hypovirulent phenotype is often prevented because an incompatibility reaction prevents fungal hyphae from fusing and exchanging their cytoplasmic content. In the United States, at least 35 VCGs were found. A similar situation seems to be present in China and Japan, where 71 VCGs have been identified so far. The majority of mycoviruses have double-stranded RNA (dsRNA) genomes and isometric particles, but approximately 30% have positive-sense, single-stranded RNA (+ssRNA) genomes. So far there is only one true example of a single-stranded DNA (ssDNA) mycovirus. A geminivirus-related virus was found in Sclerotinia sclerotiorum conferring hypovirulence to its host. The updated 9th ICTV report on virus taxonomy lists over 90 mycovirus species covering 10 viral families, of which 20% were not assigned to a genus or sometimes not even to a family. Isometric forms predominate mycoviral morphologies in comparison to rigid rods, flexuous rods, club-shaped particles, enveloped bacilliform particles, and Herpesvirus-like viruses. The lack of genomic data often hampers a conclusive assignment to already established groups of viruses or makes it impossible to erect new families and genera. The latter is true for many unencapsidated dsRNA viruses, which are assumed to be viral, but missing sequence data has prevented their classification so far. So far, viruses of the families Partitiviridae, Totiviridae, and Narnaviridae are dominating the 'mycovirus sphere'. Mycoviruses are common in fungi (Herrero et al., 2009) and are found in all four phyla of the true fungi: Chytridiomycota, Zygomycota, Ascomycota and Basidiomycota. Fungi are frequently infected with two or more unrelated viruses and also with defective dsRNA and/or satellite dsRNA. There are also viruses that simply use fungi as vectors and are distinct from mycoviruses because they cannot reproduce in the fungal cytoplasm.