Mycovirology

Mycoviruses (ancient Greek μύκης mykes: fungus and Latin virus) 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. To be a true mycovirus, they must demonstrate an ability to be transmitted - in other words be able 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 codiverge with their hosts, i.e. their phylogeny is largely congruent with the one of their hosts. However, many virus families containing mycoviruses have only sparsely been sampled.

Mycovirology is the study of viruses infecting fungi, also called mycoviruses. It is a special subdivision of the general field of virology and describes 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 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 had to be via hygienic practises to stop the spread of the virus.

However, the best known mycovirus is Cryphonectria parasitica hypovirus 1 (CHV1). CHV1 is exceptional amongst mycoviral research due its success as biocontrol agent against 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 relative small number of vegetative compatibility groups (VCGs) on this continent. In contrast, in North America the distribution of the hypovirulent phenotype is often prevented because fungal hyphae cannot fuse and exchange their cytoplasmic content due to an incompatibility reaction. In the United States of America at least 35 VCGs were found. A similar situation seems to be present in China and Japan where 71 VCGs were identified so far.

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