An Introduction to Fungal Viruses

Fungal viruses or mycoviruses are widespread in fungi and are usually associated with asymptomatic infections. Generally, mycoviruses are transmitted intracellularly during cell division, sporogenesis and cell fusion. It is rare that a DNA virus can replicate in and have a mycophagous fly as a vector. Their natural host ranges are generally limited to individuals within the same or closely related vegetative compatibility groups. Recent technological advances, however, allowed the establishment of experimental host ranges for a few mycoviruses. Although the majority of known mycoviruses have double-stranded RNA (dsRNA) genomes that are packaged in isometric particles, an increasing number of mycoviruses with single-stranded RNA genomes are being reported. The best characterized of the dsRNA mycoviruses belong to the family Totiviridae whose members have simple undivided dsRNA genomes that code for a capsid (or coat) protein and an RNA dependent RNA polymerase. Many as-yet-unclassified dsRNA viruses have similar genome organizations of different size ranges. The mycoviruses with unencapsidated (capsidless) RNA genomes (hypoviruses) and those with bacilliform (+) strand RNA genomes (barnaviruses) appear to have common ancestry with plant positive-sense RNA viruses with potyvirus and sobemovirus lineages, respectively. Mycoviruses that infect plant pathogenic fungi and cause debilitating diseases and/or reduce the virulence of their fungal hosts, provide valuable tools for studies on the molecular basis of fungal virulence and for the development of novel biocontol strategies, referred to as “virocontrol”. Furthermore, mycoviruses provide a versatile platform for studying virus/host interactions such as symptom induction and host defense/counter-defense as well as various types of virus/virus interactions.