Mycoviruses.

Viruses infect virtually all forms of cellular life, and fungi are no exception. Knowledge regarding the diverse fungal viruses, or mycoviruses, including their genome structures, host ranges, and phenotypic effects, is growing at a fast pace. Mycovirus research has been stimulated by the idea that they could be an effective tool for biocontrol of fungal pathogens. In many cases, mycoviruses are known to reduce the growth rate of their host and/or reduce their virulence. This observation, however, creates a paradox as most mycoviruses are predominately transmitted vertically, which, according to theoretical predictions, should select for more mutualistic interactions. It is possible, therefore, that widespread mutualism between mycoviruses and their hosts has been overlooked. To properly weaponize mycoviruses as biocontrol agents, a better understanding of their basic biology, including transmission modes and molecular mechanisms of parasitism, is needed. In this primer we highlight what is known about the types of viruses that have been detected in fungi and their phenotypic effects. We pay special attention to three well-studied models - the hypovirulence-causing viruses (hypoviruses or Hypoviridae) of the chestnut blight fungus, the Sclerotinia sclerotiorum ssDNA virus SsHADV-1, and the killer viruses of Saccharomyces cerevisiae - and highlight avenues for further exploration.