The Enigma of Double-stranded RNA (dsRNA) Associated with Mushroom Virus X (MVX)

New variants of pathogenic fungal viruses are emerging and they are enigmatic in revealing their molecular identity and of their origin. Double-stranded RNAs, some in non-encapsidated forms are increasingly becoming causal agents for sporadic diseases and are consistently associated with a complex profile of dsRNAs, presumably of (multiple) viral origin present in the host while the same are conspicuously absent in healthy (looking) counterparts. The emergence of an unusual Agaricus bisporus mushroom 'patch disease' first reported in 1996, later termed as 'mushroom virus X' (MVX), exhibited a wide range of symptoms (e.g. barren patches beside healthy looking mushrooms, arrested pins, premature opening, brown, off-colour and distortions in shape). A variable compendium of novel 26 (dsRNA) elements, ranging in sizes between 20.2 kb to 0.64 kb, several of them (~17/26) in non-encapsidated form have been shown to occur in the diseased mushroom fruiting bodies and are thought to comprise multiple viruses. Ten years on, this devastating disease is now more widespread and prevalent in a number of European countries (e.g. The Netherlands, Ireland) ranging from occasional to severe outbreaks leading to crop losses. Impressive data has been accumulated on the symptoms, but the potential aetiological sources, biochemical and molecular characterizations corresponding to the symptoms vis-a-vis MVX linked dsRNAs still remain either elusive or unclear. We have overviewed mainly the molecular findings of research groups working on MVX in these countries together with our own work on MVX in Northern Ireland. To date, the results reviewed suggest that with the exception of 4 low molecular weight dsRNA bands (sizes 2.0. 1.8, 0.8 and 0.6 kb) which consistently were found synchronous to mushroom off-colour/browning symptoms in the UK and the Netherlands, other individual MVX dsRNAs or their banding patterns clearly lack credible relationship with other symptoms of the MVX disease complex. The issues in the molecular characterisation of the MVX dsRNAs include the disparate results on the molecular sequences obtained for some of these by the different research groups, the varying molecular methods or approaches adopted by them for deciphering the nucleotide sequences of the novel dsRNAs that are different from previously encountered mushroom viruses. The future outlook and general consensus among mushroom researchers worldwide is for an urgent need to recruit international taskforce and re-focus on clarifying the symptom vis-a-vis dsRNAs in the enigmatic MVX disease complex. As crossing the cellular membrane is a key step to infection process, we have also attempted to draw parallels with other viruses in terms of the potential cell entry mechanisms for MVX dsRNAs. In the light of MVX disease and A. bisporus being a commercial crop worldwide in agri-food markets, and taking cue from its nearest Basidiomycete model mushroom, Coprinus cinereus whose genome mapping is completed, we also propose that it may be timely for the international research groups to renew efforts to prop up a network for sequencing the host A. bisporus mushroom genome (~38 MB) for a better understanding of host-pathogen relationships.