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.

Keywords: Aspergillus; hypervirulence; hypovirulence; mycovirus; mycovirus classification; mycovirus transmission; population study.

Figure 1

Schematic representation of the 13 established mycovirus families; 6 families containing dsRNA as genetic material, 5 families containing ssRNA as genetic material and 2 families containing ssRNA that is reverse transcribed. Depicted is the virion structure and number of genomic segments for each family. The information was derived from Virus Taxonomy: The Classification and Nomenclature of Viruses—The Online (10th) Report of the ICTV; https://talk.ictvonline.org/ictv-reports/ictv_online_report/.

Figure 2

Schematic representation of the genomic organization of all fully sequenced Aspergillus mycoviruses, including the A. foetidus mycovirus complex, the A. ochreacus and A. fumigatus partitiviruses and the A. fumigatus chrysovirus and polymycovirus. For each genomic segment, the ORF(s) (colored boxes) are flanked by 5′- and 3′-UTRs (black boxes) and the function of the encoded protein(s) is indicated.

Figure 3

Phylogenetic analysis of the proposed families (A) Exartaviridae and (B) Alternaviridae. Putative exartaviruses (RdRP_1, PF00680; Finn et al., 2014) include Aspergillus foetidus slow virus 2 (CCD33025), Penicillium aurantiogriseum foetidus-like virus (YP_009182156), Rosellinia necatrix mycovirus 2-W1032/S6 (BAT50982), Rhizoctonia solani mycovirus 1 (ANR02697), and Fusarium poae mycovirus 2 (YP_009272910). A bat calicivirus (AIF74264), the most closely related virus (PSI-BLAST; Altschul et al., 1997), was used as an outgroup. Putative alternaviruses (RdRP_4, PF02123; Finn et al., 2014) include Aspergillus foetidus dsRNA mycovirus (YP_007353985), Aspergillus mycovirus 341 (ABX79997), Fusarium poae alternavirus 1 (YP_009272952) and Alternaria alternata alternavirus-1 (YP_001976142). Hubei toti-like virus 7 (APG76025), the most closely related virus (PSI-BLAST; Altschul et al., 1997), was used as an outgroup. The RdRP sequences of the viruses were aligned with MUSCLE as implemented by MEGA 6 (Tamura et al., 2013) the alignment was improved manually and all positions with less than 30% site coverage were eliminated. Maximum likelihood phylogenetic trees were constructed with MEGA 6 using the WAG+G+I+F and LG+G substitution models for exartaviruses and alternaviruses respectively. At the end of the branches blue and green squares indicate that the virus infects ascomycetes and basidiomycetes respectively; red and purple circles indicate that the virus infects mammals and invertebrates respectively.

Figure 4

(A) Agarose gel electrophoresis of dsRNA molecules from A. fumigatus strain Exeter 10.2 harboring a partitivirus. The dsRNAs 1 and 2 encode the RdRP and CP, respectively, while dsRNA 3 is an additional genomic component (adapted from Bhatti, 2011). (B) Agarose gel electrophoresis of dsRNA molecules from A. fumigatus strain A78, harboring a polymycovirus. A fifth dsRNA molecule is visible under the four dsRNAs constituting the viral genome (adapted from Readman, 2011). (C,D) Three virus-free isogenic colonies derived by curing the virus-infected A. fumigatus strain A78 exhibit reduced radial growth and radial growth rate on solid minimal medium. The p-values indicate the statistical significance of the difference in growth rate between the virus-infected and the three cured virus-free strains (Student's t-test; adapted from Readman, 2011).