In this article we will discuss about:- 1. Discovery of Mycoviruses 2. Morphology of Mycoviruses 3. Replication.
Discovery of Mycoviruses:
M. Hollings of Glasshouse Crop Research Institute, USA for the first time gave experimental evidence of viruses in cultivated mushroom Agaricus bisporus causing die-back disease in 1962. The most characteristic and consistent features of mushroom virus diseases are the loss of crop and the degenerat on of mycelium in the compost.
Before it, no experimental evidence regarding presence of viruses in fungi was at hand though viruses were thought to be the cause of certain abnormalities in fungi. Several terms have been proposed to denote such viruses, viz., fungal viruses, mycophages, dsRNA plasmids, mycoviruses and virus-like particles (VLPs); the last two terms have been frequently used by the microbiologists.
Since their discovery, mycoviruses have been reported from all major taxonomic groups of fungi, the number of fungal genera ranging from about 50 to 60. Some-important fungi containing mycoviruses are Agaricus bisporus (25-50 nm), Alternaria tenuis (30-40 nm), Aspergillus foetidus (40-42 nm), A. glaucus (25 nm), A. niger (40-42 nm).
Penicillium brevicompactum (40 nm) P. chrysogenum (35 nm), P. funiculosum (25- 30 nm) P. notatum (25 nm), P. stoioniferum (40-45 nm), Peziza ostrachoderma (17 x 350 nm), Endothiaparasitica (300 nm), Laccaria laccata (28 nm), Stemphilium botryosum (25 nm; VLPs), Saccharomyces cerevisiae (40 nm) etc.
Within brackets against the name of each fungal species is given the size of the virion. However, it is interesting to note that most of the species of Penicillium and Aspergillus have been found to be attacked by mycoviruses while the latter arc not found so frequently in other fungal genera. So far very few mycoviruses have been fully characterized, and most are only the ‘virus like particles’ (VPLs) in electron micrographs.
Morphology of Mycoviruses:
Mycoviruses recorded so far show morphologically variable forms, viz., bacilliform, rod- shaped, filamentous and herpes types. But majority of the known mycoviruses are typically isodiametric ranging usually from 25 and 50 nm in diameter and particle weight from 6-13 x 106 dalton.
The most outstanding feature common to mycoviruses is possession of double-stranded ribonucleic acid (dsRNA) usually segmented into 1-8 segments with a total molecular weight of 2 to 8.5 x 106 dalton.
The dsRNA segments are separately enclosed into identical capsids. This feature of mycoviruses differentiates them from plant and animal dsRNA viruses in which the genetic material segments are, usually, all enclosed in a single virion.
Replication of Mycoviruses:
Replication of mycoviruses inside the fungal cell has been reviewed by Buck (1979, 1980). He has reported some host enzymes capable of transcribing the ssRNA and dsRNA in laboratory conditions and probably dsRNA in vivo.
Highly specific virus-coded RNA polymerases are necessary for effective in vivo transcription and replication of dsRNA. Such polymerase has been reported in some dsRNA mycoviruses. It is thought that the polymerases remain confined within the virion during the replicative cycle of mycoviruses.
The mechanism of infection and transmission of mycoviruses is still obscure. They have been found in fungal spores and it is believed that they are transmitted through the spores. The presence of viral-RNA in the fungal cells does not appear to affect any cellular properties such as antibiotic production.
For example Penicillium notation contains a dsRNA mycovirus, but penicillin production by the fungus is not affected at all. In recent years the dsRNA mycoviruses have attracted the attention of scientists since they have ability to induce interferon production in animal cells. Also, they do not appear to the animal cells be toxic unlike other chemicals that induce interferon production.