Giant Viruses Are Infecting Fungi, and Now They Have a Name

Giant viruses are actively infecting fungi, a kingdom of life in which the paper says they had not previously been definitively found, according to a study published Aug. 25 in the open-access journal PLOS Biology.

The viruses belong to the phylum Nucleocytoviricota, whose members carry genomes far larger than those of ordinary viruses and have been detected in hosts across the eukaryotic tree of life. The paper's abstract states that "such viruses have not been definitively found in Kingdom Fungi," though it notes that earlier genomic and metagenomic work had suggested possible associations.

The 11 authors, led by Jillian M. Myers, report both "viral fossils", viral sequences left behind inside host genomes, and active infection. Both were found in zoosporic fungi, whose spores swim with a whip-like tail, and specifically in two of their phyla, Blastocladiomycota and Chytridiomycota.

The recovered viral assemblies span up to 350 kilobases and encode more than 300 genes, the paper reports, and they form a single family-level branch within Nucleocytoviricota, related to the orders Imitervirales and Algavirales. The authors name that branch Mycodnaviridae.

Infection was not uniform across the fungal isolates the team examined. Some appeared actively infected; in others the viral genes were present but transcriptionally suppressed, meaning they were not being read out into RNA. The authors describe that pattern as suggesting viral activation "may be constrained to certain life stages of the host."

The team writes that the cultures add to the small number of natural virus-host systems available in the laboratory for studying giant viruses.