Mycoviruses are widespread in almost all fungi [1,2]. With the development of sequencing technology, more and more mycoviruses have been discovered and identified, and most of them have genome types of positive-sense single-stranded RNA (+ssRNA) and double-stranded RNA (dsRNA), with a small number of negative-sense singlestranded RNA (-ssRNA) and single-stranded DNA (ssDNA) [3,4]. Most mycoviruses are neutral and have no effect on their hosts, but some may alter colony morphology, promote or inhibit fungal growth, sporulation, pigmentation, and pathogenicity [5,10-12]. Notably, mycoviruses can reduce or even eliminate pathogenic fungal pathogenicity or increase plant resistance to pathogenic fungi and they are also thought to have the potential to help humans control fungal-induced plant diseases with reported success cases and important research results. For example, Cryphonectria hypovirus 1 (CHV1) has been successfully used to control chestnut blight in Europe, Pestalotiopsis theae chrysovirus 1 (PtCV1) could cause high resistance in host plants to hypervirulence strains of Pestalotiopsis theae and Sclerotinia sclerotiorum hypovirulence-associated DNA virus 1 (SsHADV-1) could convert pathogenic fungi into beneficial endophytes and improve Brassica yields [4,6,7].
According to the International Committee on Taxonomy of Viruses (ICTV), members of the family Narnaviridae consist of a +ssRNA genome of 2.3-2.9 kb length and they are not encapsulated by a shell protein envelope (https://ictv.global/report_9th/RNApos/Narnaviridae). This class of viruses usually encodes only one protein with a characteristic RNA-dependent RNA polymerase (RdRp) motifs and the protein size of 80-104 kDa. However, researchers have found that some Narnavirus viruses also contain reversible frame ORFs (rORFs) which are almost the same length as RdRp on the other side of the genome [15,16]. Previously, the family Narnaviridae consisted of two genera, Narnavirus and Mitovirus, but recently Mitovirus has been placed in the new family Mitoviridae, therefore the family Narnaviridae now contains only one genus Narnavirus. Currently, only two members of the genus Narnavirus have been approved by ICTV which are Saccharomyces 20S RNA narnavirus (ScNV20S) and Saccharomyces 23S RNA narnavirus (ScNV23S). High throughput sequencing is increasingly being used to explore the diversity of mycoviruses, and a large number of genomic sequences associated with mycoviruses have been identified and deposited in the National Center for Biotechnology Information (NCBI) database [9-11]. Of these large numbers of identified viral sequences were found to cluster closely with representative members of the genus Narnavirus after phylogenetic analysis, and these clustered sequences remained independent of members of neighbouring families Mitoviridae and Botourmiaviridae [3]. Currently, 330 genome sequences of +ssRNA viruses associated with Narnavirus have been recorded in the NCBI database (https://www.ncbi.nlm.nih.gov/data-hub/taxonomy/186767/). Recent studies have supported the division of the family Narnaviridae into at least three genera including Alphanarnavirus, Betanarnavirus and Mycoambinarnavirus [3,8,13].
Botryosphaeria dothidea is a common pathogen on woody plants with a wide host range and worldwide distribution, and it’s also the most important causative agent of apple ring rot [15-17]. To date, a total of four mycoviruses have been reported to cause hypovirulence in B. dothidea and these are potential tools for biological control of B. dothidea [18-21]. In this study, we identified a novel narnavirus from B. dothidea strain ZM210167-1 which we designated "Botryosphaeria dothidea narnavirus 5" (BdNV5). Although BdNV5 was most closely related to the family Narnaviridae, it didn’t belong to the recently proposed Alphanarnavirus, Betanarnavirus and Mycoambinarnavirus, it was a separate branch.