dsRNA Mycoviruses

Double-stranded RNA (dsRNA) mycoviruses represent a phylogenetically diverse lineage of fungal viral pathogens and endosymbionts. They play a considered modulating role in host physiology and are candidates for biocontrol. These mycoviruses generally lack an extracellular stage and are primarily transmitted intracellularly through hyphal fusion (anastomosis), plasmogamy, or sporogenesis.

Diversity and prevalence

In Botrytis cinerea, the Double-stranded RNA (dsRNA) mycovirus listed; Birnaviridae, Botybirnaviridae, Curvulaviridae, Endornaviridae, Fusagraviridae, Partitiviridae, Pseudototiviridae, Quadriviridae, and Totiviridae;represent the principal virus families reported. These families exhibit diverse genome segmentation patterns and capsid architectures, reflecting the structural complexity of the fungal dsRNA virome.

Families and characteristics

Family Genome Structure Key Characteristics
Totiviridae Monopartite linear dsRNA (4.6–7.0 kbp) Family of non-enveloped dsRNA viruses with a single linear genome segment. Virions are isometric and contain one major capsid protein (CP) and an RNA-dependent RNA polymerase (RdRp). The genome typically has two large, overlapping ORFs encoding CP (5′) and RdRp (3′), with the polymerase often expressed as a Gag–Pol-like fusion protein. Members are associated with latent infections of fungal or protozoan hosts.
Partitiviridae Bisegmented dsRNA genome (3–4.8 kbp total; two segments separately encapsidated) Family of small, isometric, non-enveloped dsRNA viruses with two genome segments (dsRNA1 and dsRNA2), each encoding a single protein: the RNA-dependent RNA polymerase (RdRp) and the capsid protein (CP), respectively. Members infect plants, fungi, and protozoa and are typically associated with persistent infections; transmission occurs intracellularly without known natural vectors.
Quadriviridae Quadripartite dsRNA genome (four linear segments, each 3.5–5.0 kbp; 16.8–17.1 kbp total) Family of non-enveloped, spherical dsRNA viruses infecting filamentous fungi. The genome comprises four linear, monocistronic dsRNA segments (dsRNA1–dsRNA4), each typically encoding a single protein. Genome segments are expected to be separately encapsidated, and virions have a unique capsid structure composed of two major structural proteins.
Birnaviridae Bisegmented dsRNA genome (two linear segments, total about 6 kbp) Family of non-enveloped viruses with icosahedral capsids and two double-stranded RNA genome segments. Members infect vertebrates (excluding mammals) and invertebrates, including insects, rotifers, and molluscs.
Endornaviridae Linear positive-sense ssRNA (9.7–17.6 kb) Family of viruses with a single linear positive-sense RNA genome encoding a large polyprotein and lacking true virions due to the absence of a capsid protein. Members infect plants, fungi, and oomycetes and are typically associated with persistent intracellular infections.
Botybirnaviridae Bisegmented dsRNA (2 linear segments; 11.0–12.5 kbp total) Family of non-enveloped, icosahedral fungal viruses with two linear dsRNA genome segments. Virions are 35–40 nm in diameter, and fungi are the reported hosts.
Curvulaviridae Bisegmented, spherical virions of 26 -29 nm Family in the order Durnavirales. It includes the genus Orthocurvulavirus and unclassified Curvulaviridae. Virus of this family have a genome encoding an RdRp and a second protein annotated in most species as a hypothetical protein, and that spherical virions were identified for Curvularia thermal tolerance virus
Fusariviridae Monopartite positive-sense ssRNA (5.9–10.7 kb) Family of mono-segmented positive-sense RNA viruses that typically encode a large polyprotein containing RNA-dependent RNA polymerase (RdRp) and helicase domains. Genomes are often bicistronic but may contain up to four ORFs. Members infect fungi (and occasionally oomycetes) and do not appear to form true virions, accumulating intracellular dsRNA replicative forms.
Pseudototiviridae Monopartite dsRNA (1 linear segment; 4.6–6.3 kbp) Family of non-enveloped, icosahedral dsRNA viruses with a single linear genome segment. Virions are 30–45 nm in diameter, and reported hosts include protists, fungi, and vertebrates.

Effects on fungal host

The effects of mycoviruses on their fungal hosts are variable and, in many cases, infections are persistent and asymptomatic. For example, viruses in the family Totiviridae are associated with latent infections in fungal hosts. Similarly, members of the family Endornaviridae are typically associated with persistent infections with little or no obvious effect on host phenotype, although exceptions exist, including cases of hypovirulence or physiological changes. In some instances, mycovirus infection can lead to phenotypic changes such as altered growth or virulence. For example, certain fusariviruses have been associated with hypovirulence in their fungal hosts, whereas others show no detectable symptoms.

Ecological significance and impact

Mycoviruses are widespread in fungi and are typically maintained through persistent intracellular infections. Transmission commonly occurs during cell division, sporogenesis, in Totiviridae family or hyphal anastomosis in quadriviruses. Many mycoviruses lack an extracellular phase and are transmitted intracellularly, contributing to their stable maintenance within host populations.

Evolutionary history

Double-stranded RNA (dsRNA) and positive-sense RNA viruses infecting fungi belong to diverse taxonomic groups within the realm Riboviria, reflecting substantial evolutionary diversity. Phylogenetic analyses based on RNA-dependent RNA polymerase (RdRp) sequences revealed that many of these dsRNA viruses occupying basal positions have been isolated from arthropods, and that these viruses are more closely related to known virus families infecting vertebrates and plants, indicating that arthropods harbor ancestral viral lineages. Endornaviruses' genomes include regions similar to plant, fungal, and bacterial genes, suggesting that some genetic elements, such as glycosyltransferase and S7 domains, may have been acquired through lateral gene transfer. Phylogenetic analyses show that members of the family Fusariviridae form a distinct clade related to viruses in the family Hypoviridae, indicating evolutionary divergence from common ancestors. Genome segmentation occurs in some dsRNA virus families infecting fungi, such as members of the family Quadriviridae, which possess four genomic RNA segments, contributing to genomic complexity and diversity.

Nucleotide alignment

Primary structure alignment (amino acids)

Phylogeny