'Candidatus Moeniiplasma glomeromycotorum', an endobacterium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) are symbionts of most terrestrial plants. They commonly harbour endobacteria of a largely unknown biology, referred to as MRE (Mollicutes/mycoplasma-related endobacteria). Here, we propose to accommodate MRE in the novel genus 'Candidatus Moeniiplasma.' Phylogeny reconstructions based on the 16S rRNA gene sequences cluster 'Ca. Moeniiplasma' with representatives of the class Mollicutes, whereas phylogenies derived from amino acid sequences of 19 genes indicate that it is a discrete lineage sharing ancestry with the members of the family Mycoplasmataceae. Cells of 'Ca. Moeniiplasma' reside directly in the host cytoplasm and have not yet been cultivated. They are coccoid, ~500 nm in diameter, with an electron-dense layer outside the plasma membrane. However, the draft genomes of 'Ca. Moeniiplasma' suggest that this structure is not a Gram-positive cell wall. The evolution of 'Ca. Moeniiplasma' appears to be driven by an ultrarapid rate of mutation accumulation related to the loss of DNA repair mechanisms. Moreover, molecular evolution patterns suggest that, in addition to vertical transmission, 'Ca. Moeniiplasma' is able to transmit horizontally among distinct Glomeromycota host lineages and exchange genes. On the basis of these unique lifestyle features, the new species 'Candidatus Moeniiplasma glomeromycotorum' is proposed.