History, Evolution, And Diversity Of Mycoviruses

Abstract

Mycoviruses are significant widespread viruses found to be infecting all major groups of plant pathogenic fungi. Mycoviruses require living cells to replicate like other animal and plant viruses. These viruses are transmitted through cell division or during spore formation. Most of the mycoviruses have double strand RNA genome, and few are known to have positive single strand RNA genome, DNA genomes are rear among mycoviruses with one exception of Sclerotinia sclerotiorum hypovirulence-associated DNA mycovirus 1 having DNA genome. Some Mycoviruses have been reported which are responsible for causing abnormal pigmentation, irregular growth and some are involved in altering their host sexual reproduction. These viruses are thought be evolved from plant viruses and so their diversity is classified in taxonomic groups like other viruses. The importance of mMycoviruses arises due to their most significant ability that is they reduced virulence of their host (hypovirulence). In this review, weI explored different aspects of mMycoviruses.

Introduction

Mycoviruses usually infect the fungal species or they are found in association with pathogenic fungal species. These viruses are most commonly recognized in all major phyla of the fungi. These viruses are mostly present in then in latent stage and rarely cause disease [1]. Few mycoviruses are found to cause considerable changes in the host fungi like irregular growth, mutated sexual reproduction and abnormal pigmentation. The most important feature they exhibit is hypovirulence (ability to reduce the virulence of pathogenic fungi) [1-4]. The presence of mycoviruses adds a new dimension in experimental mycology.

History & of mycoviruses.

History of mycoviruses goes back to As as early as 1950 when a disease was reported by Siden and Hauser [5] in cultivated mushroom Agaricus bisporus. This mushroom produced misshaped fruiting bodies, premature distortion of mushroom tissue and consequently reduction in yield [2]. Hollings and coworkers subsequently isolated tghree more morphologically distant viruses associated with diseased mushroom [6,7,8]. In years between 1962 and 1965, considerable evidence for mycoviruses in A.bisporus were reported. Prior to 1968, evidence for the mycoviruses other than that of A.bisporus had been reported. Presumptive evidence for yeast virus was published as early as 1936 [9]. Many other viruses were discovered in fungal groups. These mycoviruses were discovered in two species of Penicilium (Penicilium stoloniferum and Penicilium funiculosum). Mycoviruses are found to share their characteristics with both animal and plants but they also show some distinct feature or their own such as they lack an extracellular route of infection, sporulation, transmission through cell division(intracellularly) and absence of movement proteins which essential for animal and plant viruses to complete their life cycle.

Several reports by taxonomists showed that the genome of most mycoviruses consist of double stranded RNA (dsRNA) while 30% of genome of mycoviruses is positive single stranded RNA (+ssRNA) [10]. Gemini group virus related to mycoviruses have been recently reported [2].

Diversity of mycoviruses

Mycoviruses have been detected in major phyla of the fungi including Ascomycota, Basidiomycota, Zygomycota, Deuteromycota and Chytridiomycota. Viruses in Basidiomycota During 1950s, an epidemic disorder of mushrooms was seen by mushroom growers and was called as “The disease in the crop”. The infected mushroom showed distorted shape and their yields were greatly reduced. Blattny and Pilate [11] described in Czechosovakia a sporophore abnormality of Laccaria laccata and its form amethystina. Filtrate obtained from infected sporophore were watered on normal growing sporophore, it induced abnormal sporophores in next years.

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Whereas normal sporophore continued to grow and produce normal yielding fruiting bodies [12]. Similar abnormalities were noticed in Cantharellus infundibuliformis, C.cibarius, and Armillaria mellea, but no virus particles were seemed [12].

Mycoviruses of Rhizoctonia species

An important soilborne necrotrophic fungal pathogen Rhizoctonia solani belongs to basidiomycetes. However different-sized dsRNA factors were found in natural populations from AG1 to AG-13, the sequences of only four mycoviruses were characterized. Two dsRNA (M1 and M2; 6.4 kb and 3.6 kb in size, respectively) were isolated from strain Rbs 1A1 of R.solacni AG-3. M1 is phylogenetically related to Bromoviruses of plants and found to be associated with enhanced vigor and virulence. While M2 is related to Mitoviruses and associated with hypovirulence [86,87]. The Rhs 717 virus which was islolated from Rhs 717 strain of R.solacni AG-2 was a Partitivirus [88].

The Diversity of Viruses Infecting Rhizoctonia solani

The first dsRNA element in R.solani was initially described by Butler and Castano[125]. Since then numerous studies were performed to discover the diverse world of viruses infecting R. solani.

Viruses in Imperfect fungi

This group of fungi is very important because of their importance in antibiotic production. Antiviral properties are found in extracts obtained from several species during past decades for example Penicilium stoloniferum[13], P.funiculosum[14], P.cyclopium[15], P.cyaneo-fulvom[16], and Aspergillus niger[17]. Lampson et al [18] identified virus like particles in P.funiculosum in some statolon preparation [19]. Department of Biochemistry imperial College London showed that Penicilium stoloniferum and P.funiculosum were infected by virus containing dsRNA that was found to induce interferon production in mice [20].

Viruses in Ascomycota

With exceptions of mushroom virus, all the other discussed viruses are isometric. The only rod- shaped particles were found in ascomycetes Peziza astracoderna [21]. Some purified preparation obtained from apothecia contained rigid rods 17x350 nm, resembling to tobacco mosaic virus TMV. But their inoculation did not produce TMV symptoms. The virus was serologically different from TMV [22]. The virus like particles were also seen in abnormal yeast [23]. They were 100 nm in diameter with double membrane. They are different from any other fungal virus.

Mycoviruses of Cryphonectria parasitica

Ascomycetes fungus Cryphonectria parasitica caused chestnut blight which has been a disaster for American chestnut (Castanea dentata) in both North America and Europe [69]. CHV1 is hypovirulence-associated mycovirus of the C.parasitica and it was firstly analyzed in 1991 [70]. Keeping in view the biological feature of CHV1, International Committee on Taxonomy of Viruses (ICTV) established a new family Hypoviridae [71]. Hypoviridae contains just a single Hypovirus genus. Family Hypoviridae contains four species isolated from C.parasitica that belong to genus hypovirus; Cryphonectria hypovirus 1 (CHV1)[72] , Cryphonectria hypovirus 2 (CHV2) [71], Cryphonectria hypovirus 3 (CHV3) [73] ans Cryphonectria hypovirus 4 (CHV4) [74]. The impact on the virulence of C.parasitica by these species was different. Three viruses other than these hypoviruses were isolated from C.parasitica. Cryphonectria parasitica mitovirus 1 (CpMV1) belonging to genus mitovirus of family Nanoviridae, was isolated from hypovirulent strain NB631 [75]. The other two viruses Cryphonectria parasitica mycoreovirus 1 (CpMyRV1) and Cryphonectria parasitica mycoreovirus 2 (CpMyRV2) were isolated from two hypovirulent strains 9B21 and C18 respectively [76].