What Is a Mycophage and How Does It Affect Fungi?

A mycophage is a virus that specifically infects fungi. These viruses are widespread in nature and have been found in all major fungal groups. Most known mycophages possess genetic material made of double-stranded RNA (dsRNA), a feature that distinguishes them from many other viruses. Their discovery dates to the 1940s after a viral infection was identified as the cause of a disease in cultivated mushrooms. Unlike many viruses, mycophages often exist within their fungal hosts without causing obvious disease.

How Mycophages Infect Fungi

The way mycophages spread between fungal cells differs from many other viruses. Their genomes typically lack the genes needed to produce proteins for exiting a host to infect a new one, meaning they do not have an external infection route. Instead, transmission occurs intracellularly through the fungus’s normal life processes.

One primary transmission mechanism is hyphal anastomosis, the fusion of filamentous structures (hyphae) between two fungi. This cytoplasmic exchange allows viral particles to move from an infected fungus to a compatible, uninfected one. This process limits the virus’s host range to closely related fungi capable of fusion.

Another transmission mode is through cell division and spore production. Viral particles in the cytoplasm are passed to new cells as the fungus grows. They can also be incorporated into asexual and sexual spores, which spread the virus when they germinate into new fungal colonies. The efficiency of transmission into spores varies from 0% to 100% based on the specific virus and host.

Ecological Roles of Mycophages

In natural environments, mycophages have a range of effects on their fungal hosts, which influences the broader ecosystem. While many infections are asymptomatic, some significantly alter the host’s biology. These viruses can regulate the population size and diversity of fungal communities, similar to how bacteriophages influence bacterial populations.

A well-documented effect is hypovirulence, a state of reduced virulence in pathogenic fungi. The classic example is the mycophage Cryphonectria parasitica hypovirus 1 (CHV1), which infects the fungus responsible for chestnut blight. The virus weakens the fungus, reducing its ability to kill chestnut trees, and has been used as a biological control agent in Europe.

Mycophages can also influence competitive dynamics within fungal communities. Some induce a “killer phenotype” in their hosts, causing them to secrete proteins toxic to other fungal strains. This allows the virus-infected fungus to outcompete its rivals for resources. Through these mechanisms, mycophages act as modulators of fungal populations.

Research and Potential Uses of Mycophages

The ability of mycophages to reduce fungal virulence makes them promising biocontrol agents in agriculture. The success of using the CHV1 virus against chestnut blight serves as a model for developing strategies to combat other crop-destroying fungi.

Researchers also use mycophages as tools to study fungal biology. Since they alter gene expression and metabolic pathways in their hosts, they offer a way to investigate the function of specific fungal genes. Infecting a fungus with a mycophage can reveal information about the host’s defense mechanisms or metabolic processes.

The study of mycophages in human and animal pathogenic fungi is an emerging field. Research shows these viruses can either increase or decrease a pathogen’s virulence. For instance, a virus infecting Aspergillus fumigatus had a mild hypervirulent effect, while others weaken their hosts. Understanding this interplay is key to learning how these viruses might be used to combat fungal infections.