Fungal Viruses

Fungal viruses or mycoviruses are widespread in fungi and are usually associated with symptomless infections. Mycoviruses are transmitted intracellularly during cell division, sporogenesis, and cell fusion, and they lack an extracellular phase to their life cycles. Their natural host ranges are 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 fungal viruses 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 coat protein and an RNA-dependent RNA polymerase. The mycoviruses with unencapsidated RNA genomes (hypoviruses) and those with bacilliform (+)-strand RNA genomes (barnaviruses) have more complex genomes and appear to have common ancestry with plant (+)-strand 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 biocontrol strategies.