The virome of the arbuscular mycorrhizal fungus Gigaspora margarita

As very ancient and widespread in nature, arbuscular mycorrhizal fungi (AMF) are key com-ponents of the plant microbiota. Recent publication of genomic and transcriptomic data shed light on the biology and evolution of AMF. The genetic complexity of AMF is increased by the presence of endobacteria which live inside many AMF1. A further component of such genetic complexity is the virome associated to AMF, whose knowledge is still very limited2, 3. Here, by exploiting recently published transcriptomic data4 we describe the virome associated to Gi-gaspora margarita. Viral RNA-dependent RNA polymerases (RdRp) are markers of my-coviruses with an RNA genome. A Blast search for RdRp sequences allowed us to identify four mitoviruses (so called because they replicate in mitochondria), one narnavirus (Ourmia-like), one Giardia-like virus, and two sequences related to Fusarium mycoviruses. Northern blot and RT-PCR assays on RNA from germinating spores confirmed the authenticity of all the se-quences with the exception of the Fusarium-related ones. All the mitoviruses are replicative and functional since both positive strand and negative strand RNA are present. Interestingly, in all the four mitoviral sequences 100% of tryptophan residues are encoded by the TGG co-don: apparently a functional RdRp can be translated both in cytosol and mitochondria. In ad-dition we report, for the first time, DNA fragments corresponding to mitovirus sequences as-sociated to the presence of viral RNA: mitoviral DNA sequences are not integrated in the ge-nome, but likely exist as extrachromosomal genetic units. Phylogenetic analyses show evidence of co-evolution of mitoviruses with their AMF hosts