Mycoviruses in Aspergilli: A Comprehensive Review

Abstract

Fungi, similar to all species, are susceptible to viral infection. Aspergillus is arguably the most well studied fungal genus because of its medical, ecological and economical significance. Mycoviruses were initially detected in Aspergillus species almost 50 years ago and the field continues to be active today with ground-breaking discoveries. The aim of the present review is to cover the scientific progress in all aspects of mycovirology as exemplified by Aspergillus-focused research. Initially an overview of the population studies illustrating the presence of mycoviruses in numerous important Aspergillus species, such as A. niger, A. flavus, and A. fumigatus with be presented. Moreover the intricacies of mycovirus transmission, both inter- and intra-species, will be discussed together with the methodologies used to investigate viral dispersion in a laboratory setting. Subsequently, the genomic features of all molecularly characterized mycoviruses to date will be analyzed in depth. These include members of established viral families, such as Partitiviridae, Chrysoviridae and Totiviridae, but also more recent, novel discoveries that led to the proposal of new viral families, such as Polymycoviridae, Alternaviridae and, in the context of the present review, Exartaviridae. Finally, the major issue of phenotypic effects of mycoviral infection on the host is addressed, including aflatoxin production in A. flavus, together with growth and virulence in A. fumigatus. Although the molecular mechanisms behind these phenomena are yet to be elucidated, recent studies suggest that by implication, RNA silencing may be involved.

Mycoviruses mediate mycotoxin regulation in Aspergillus ochraceus

“…Additionally, several studies attempted to identify mycovirus‐mediated mechanisms involved in the qualitative or quantitative production of aflatoxins in A. flavus . Initially, a possible correlation between the presence of a Chrysovirus and the production of aflatoxins in A. flavus NRRL5565 (Schmidt et al ., ) was observed but was not confirmed in the same or different virus‐host systems (Elias and Cotty, ; Silva et al ., ; Kotta‐Loizou and Coutts, ).…”

“…Mycoviruses infecting Aspergilli have been studied extensively because of the medical, economic and ecological importance of this fungal genus. Several studies have focused on hypovirulence traits for a possible biotechnological exploitation against the human, animal and insect pathogen A. fumigatus (Kotta‐Loizou and Coutts, ) without success. Additionally, several studies attempted to identify mycovirus‐mediated mechanisms involved in the qualitative or quantitative production of aflatoxins in A. flavus .…”

A novel mycovirus evokes transcriptional rewiring in the fungusMalasseziaand stimulates interferon-β production in macrophages

“…In S. cerevisiae , exposure to the fungicide cycloheximide cures killer cells of the killer phenotype (32). This has varying rates of success in other organisms, such as Aspergillus , where a less than 50% success rate of mycovirus curing was observed after exposure to cycloheximide (33). In filamentous fungi, conidial isolation and hyphal tipping are more commonly used methods for curing (33), and polyethylene glycol was shown to cure P. destructans of PdPV-pa infection (19).…”

“…This has varying rates of success in other organisms, such as Aspergillus , where a less than 50% success rate of mycovirus curing was observed after exposure to cycloheximide (33). In filamentous fungi, conidial isolation and hyphal tipping are more commonly used methods for curing (33), and polyethylene glycol was shown to cure P. destructans of PdPV-pa infection (19). To determine if M. sympodialis could be cured of the dsRNA, the strains were passaged on modified Dixon’s (mDixon) media and subjected to various stresses.…”

A tripartite ssDNA mycovirus from a plant pathogenic fungus is infectious as cloned DNA and purified virions

“…Mycoviruses or fungal viruses are ubiquitous in all major taxa of fungi (1,2). According to the latest list of fungal viruses approved by the International Committee on Taxonomy of Viruses (ICTV), mycoviruses have double-stranded (ds) RNA linear genomes, positivesense single-stranded (+ss) RNA linear genomes including reversetranscribing RNA linear genomes, negative-sense ssRNA (−ssRNA) linear genomes, or ssDNA circular genomes (3). The discovery of fungal viruses occurred relatively late compared to the identification and characterization of viruses of animals, plants, and prokaryotes, perhaps due primarily to the cryptic or asymptomatic nature of mycoviruses (4).…”