Viruses that occur naturally and multiply in fungi 'mycoviruses' were first discovered about 35 years ago. Interest in an antiviral activity associated with two Penicillium species led to the discovery of dsRNA-containing virus particles in fungi. It is now believed that viruses, mainly dsRNA viruses, are of common occurrence in fungi. The isometric dsRNA viruses with divided genomes, currently classified as belonging to the family Partitiviridae, were among the first mycoviruses to be studied and characterized. The main reason for the belated discovery of fungal viruses is that they are mostly associated with symptomless infections of their hosts. Because of similarities in biological properties and genome organization, the International Committee on Taxonomy of Viruses (ICTV) has recently grouped together the fungal partitiviruses (family Partitiviridae) and the plant cryptoviruses (family Cryptoviridae) into one family (Partitiviridae) and the family name Cryptoviridae was dropped.

Typically mycoviruses, including those in the family Partitiviridae, do not lyse their fungal host cells, and there is no evidence that they have an extracellular phase to their life cycles. For viruses with multiple dsRNA segments, the lack of conventional infectivity assays makes it difficult to ascertain the number of dsRNA segments required for infectivity. To elucidate the genome organization of these viruses thus requires that the complete nucleotide sequence of the dsRNA segments be known. To date, only a single virus, Atkinsonella hypoxylon virus 1 (AhV-1), in the genus Partitivirus has been completely sequenced.

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