Keeping up with virus taxonomy: viruses that infect fungi
Posted on February 28, 2023 by Clare Baker
It’s time for the third installment of the ‘Keeping up with Virus Taxonomy’ blog series. This month we are looking at families of viruses that infect fungi – mycoviruses.
Hypoviridae
First up, we have Hypoviridae. These viruses get their name from their hypovirulence. Viruses which are hypovirulent cannot produce severe disease in their hosts. The name comes from ‘hypo’ meaning beneath or below and ‘virulence’, which refers to the severity or harmfulness. Some hypoviruses can also induce hypovirulence in the host fungi. This means that the effects of fungal infections on other organisms, such as trees, can be reduced.
An example of this is the chestnut blight fungus, Cryphonectria parasitica, a parasitic fungus of chestnut trees. This virus has almost destroyed the entirety of North America’s chestnut population. Destruction of this scale has not been seen in Europe. One reason for this is that classified members of the hypoviridae family control the effects of fungal infection in Europe. The virus weakens the effect of the fungus and helps the European trees to survive.
Hadakaviridae
Hadakaviridae are viruses that also infect fungi. Their name is derived from hadaka meaning naked in Japanese, so called for their lack of capsid. A capsid is the protein coat of a virus, which encloses its genetic material. The lack of this in Hadakaviridae means that these viruses have no true virion (complete infectious particle).
Hadakaviridae infect ascomycetous filamentous fungi, which are also referred to as molds. Studies which isolated viruses belonging to the Hadakaviridae family observed that they did not have any apparent effect on the fungi host in potato dextrose agar media. Yet it has been observed that one member of the Hadakaviridae family causes a mild growth reduction on a fungal host, Colletotrichum fructicola in China.
Yadokariviridae
Next up is the family Yadokariviridae. These viruses lack their own capsid protein, so they hijack the capsids from dsRNA viruses to stay viable. Their name comes from this mechanism, derived from the Japanese word “yadokari”, which means “room borrower”.
Yakovirids work in tandem with other viruses to co-infect host fungi, leading to different effects on the fungal hosts. For example, the yakovirid YkV1 is beneficial to its partner virus, causing harm to the host fungi. Whereas viruses YkV4a and YkV4b decrease replication of their partner virus, benefiting the host fungus.
The partnership between yakovirids and their partners is very specific. Researchers conducted ‘partner swapping’ experiments with four of these viruses and found that yadokarivid partners only bound with a specific dsRNA virus. The partner pairs can only be swapped between different strains within a single species, but not across members of different species.
Chrysoviridae
The fourth family on our list this week is chrysoviridae. Not only do chrysoviridae infect fungi, they also infect plants and possibly insects. Chrysoviridae may cause hypovirulence in their fungal hosts, just like the hypoviridae family.
They typically have four genomic segments, although it is known that some have three, five or seven. Viruses with five genomic segments are known as cinquechrysoviruses. These cinquechrysoviruses cause deleterious effects on their fungal hosts. However, most Chrysoviruses are associated with latent infections.
Partitiviridae
The final stop on our tour of virus families that infect fungi is the partitviridae family. They have two bisegmented dsRNA genomes which are individually encapsulated. Partitiviruses are associated with persistent infections and are transmitted intracellularly by hyphal anastomosis, cell division and sporogenesis in fungi. The partitviridae family infect both plants and fungi and each genus of the family has characteristic hosts. The genus Gammapartitivirus only infects fungi, whilst the genera Alphapartitivirus and Betapartitivirus infect fungi as well as plants. As Alphapartitivirus and Betapartitivirus are isolated from both fungi and plants, it is likely that there is a capacity for occasional successful transmission of these viruses between fungal and plant hosts.
The International Committee on Taxonomy of Viruses (ICTV) is responsible for developing and maintaining a universal virus taxonomy. Known viruses are categorised into a classification scheme taking into consideration their physical and biological properties in combination with their phylogenetic relationships.
These two-page summaries of each chapter of the ICTV Report (a free resource published by the ICTV which provides an up-to-date description of virus taxonomy) are freely available in the Journal of General Virology, and are supported by the Microbiology Society. These summaries are known as ICTV Virus Taxonomy Profiles and describe the structure, replication and taxonomy of each virus order and family.
