Alphapartitiviruses of Heterobasidion Wood Decay Fungi Affect Each Other's Transmission and Host Growth

ORIGINAL RESEARCH published: 26 March 2019 doi: 10.3389/fcimb.2019.00064 Alphapartitiviruses of Heterobasidion Wood Decay Fungi Affect Each Other’s Transmission and Host Growth Muhammad Kashif 1*, Jaana Jurvansuu 2 , Eeva J. Vainio 1 and Jarkko Hantula 1 1 Forest Health and Biodiversity, Natural Resources Institute Finland, Helsinki, Finland, 2 Department of Biology, University of Oulu, Oulu, Finland Edited by: Nobuhiro Suzuki, Okayama University, Japan Reviewed by: Sotaro Chiba, Asian Satellite Campuses Institute, Nagoya University, Japan Maria A. Ayllón, Polytechnic University of Madrid, Spain *Correspondence: Muhammad Kashif Specialty section: This article was submitted to Fungal Pathogenesis, a section of the journal Frontiers in Cellular and Infection Microbiology Received: 28 November 2018 Accepted: 01 March 2019 Published: 26 March 2019 Citation: Kashif M, Jurvansuu J, Vainio EJ and Hantula J (2019) Alphapartitiviruses of Heterobasidion Wood Decay Fungi Affect Each Other’s Transmission and Host Growth. Front. Cell. Infect. Microbiol. 9:64. doi: 10.3389/fcimb.2019.00064 Heterobasidion spp. root rot fungi are highly destructive forest pathogens of the northern boreal forests, and are known to host a diverse community of partitiviruses. The transmission of these mycoviruses occurs horizontally among host strains via mycelial anastomoses. We revealed using dual cultures that virus transmission rates are affected by pre-existing virus infections among two strains of H. annosum. The transmission efficacy of mycovirus HetPV15-pa1 to a pre-infected host was elevated from zero to 50% by the presence of HetPV13-an1, and a double infection of these viruses in the donor resulted in an overall transmission rate of 90% to a partitivirus-free recipient. On contrary, pre-existing virus infections of two closely related strains of HetPV11 hindered each other’s transmission, but had unexpectedly dissimilar effects on the transmission of more distantly related viruses. The co-infection of HetPV13-an1 and HetPV15-pa1 significantly reduced host growth, whereas double infections including HetPV11 strains had variable effects. Moreover, the results showed that RdRp transcripts are generally more abundant than capsid protein (CP) transcripts and the four different virus strains express unique transcripts ratios of RdRp and CP. Taken together, the results show that the interplay between co-infecting viruses and their host is extremely complex and highly unpredictable. Keywords: mycovirus, partitivirus, transmission, transcripts, Heterobasidion annosum, growth rate INTRODUCTION Fungi are known to host a wide variety of mycoviruses (Vainio and Hantula, 2018). Unlike other viruses, fungal RNA viruses lack extracellular infective particles, and are transmitted only via intramycelial anastomosis contacts and sexual or asexual spores (Ghabrial and Suzuki, 2009; Son et al., 2015; Vainio et al., 2015b). These viruses replicate within their host’s cytoplasm or mitochondria and usually cause no visible phenotypic changes, although both adverse and mutualistic effects have been described (Huang and Ghabrial, 1996; Lakshman et al., 1998; Preisig et al., 2000; Ahn and Lee, 2001; Márquez et al., 2007; Yu et al., 2010; Hyder et al., 2013; Xiao et al., 2014; Vainio et al., 2018b). Some mycoviruses are used as biocontrol agents as demonstrated by the highly successful control of the chestnut blight fungus, Cryphonectria parasitica, by hypoviruses in Europe (Milgroom and Cortesi, 2004). Frontiers in Cellular and Infection Microbiology | www.frontiersin.org 1 March 2019 | Volume 9 | Article 64 Kashif et al. Heterobasidion Virus Transmission of Rosellinia necatrix mycoreovirus 3 (RnMyRV3) in recipient mycelia restricts horizontal transmission of Rosellinia necatrix partitivirus 1 (RnPV1) and Rosellinia necatrix megabirnavirus 1 (RnMBV1) (Yaegashi et al., 2011). We tested whether the presence of other viruses in the donor or recipient affects the probability of viral transmission, and whether this effect depends on the taxonomic similarity of the coinfecting viruses in Heterobasidion annosum. We also determined the effects of viral co-infections on the host’s growth rate and on the ratio of viral RdRp and CP transcripts in the mycelium, and compared the results to those measured from single infections. Heterobasidion annosum s.lat. species complex includes some of the most devastating infectious agents of conifer forests in the Northern Hemisphere (Garbelotto and Gonthier, 2013). About 15–17% of Heterobasidion strains are infected by one or more viruses (Vainio et al., 2013; Kashif et al., 2015; Vainio and Hantula, 2016). The most common species is the taxonomically unclassified Heterobasidion RNA virus 6 (HetRV6) which accounts for 70% of double stranded RNA (dsRNA) infections in Heterobasidion isolates of European origin, but also viruses of the families Partitiviridae and Narnaviridae are known to inhabit Heterobasidion mycelia (Ihrmark, 2001; Vainio et al., 2011b, 2012, 2015a). Both partitiviruses and HetRV6 may transmit across vegetatively incompatible or distantly related host isolates of Heterobasidion in laboratory and natural forest environments (Ihrmark, 2001; Vainio et al., 2010, 2011a,b, 2013, 2015b). In addition, uncharacterized dsRNA elements have been shown to be present in both basidiospores and conidia (Ihrmark et al., 2002, 2004), but only HetRV6 has been identified in basidiospores (Vainio et al., 2015b). Most of the mycovirus species in Heterobasidion spp. belong to the family Partitiviridae with more than 20 species observed in these fungi. Partitiviruses have genomes composed of two segments of dsRNA packed in separate protein capsids and encoding for a putative RNA-dependent RNA polymerase (RdRp) and a capsid protein (CP) (Ihrmark, 2001; Vainio et al., 2010, 2011a,b, 2013; Kashif et al., 2015). The ratio of the RdRp and CP segments and their transcripts in infected mycelia usually deviates from 1:1, and is virus species specific but responds to environmental conditions (Jurvansuu et al., 2014). Partitiviruses of Heterobasidion spp. are mostly cryptic or have only slight effects on their hosts (Vainio et al., 2010, 2012), but Heterobasidion partitivirus 13 strain an1 (HetPV13an1) originally observed in H. annosum causes serious growth debilitation in both H. annosum and H. parviporum (Vainio et al., 2018b). Also in other fungi, partitiviruses have been shown to cause variable phenotypical changes or hypovirulence (Magae and Sunagawa, 2010; Bhatti et al., 2011; Xiao et al., 2014; Zheng et al., 2014; Zhong et al., 2014; Sasaki et al., 2016). Multiple virus infections are common among Heterobasidion strains (Vainio et al., 2012, 2013, 2015a,b; Kashif et al., 2015; Hyder et al., 2018) as well as other fungi such as Gremmeniella abietina (Tuomivirta and Hantula, 2005; Botella et al., 2013), Rhizoctonia solani (Lakshman et al., 1998), Helminthosporium victoriae (Ghabrial et al (...truncated)