Spores of arbuscular mycorrhizal fungi host surprisingly diverse communities of endobacteria

Summary Arbuscular mycorrhizal fungi (AMF) are ubiquitous plant root symbionts, which can house two endobacteria: Ca. Moeniiplasma glomeromycotorum ( Ca Mg) and Ca . Glomeribacter gigasporarum ( Ca Gg). However, little is known about their distribution and population structure in natural AMF populations and whether AMF can harbour other endobacteria. We isolated AMF from two environments and conducted detailed analyses of endobacterial communities associated with surface‐sterilised AMF spores. Consistent with the previous reports, we found that Ca Mg were extremely abundant (80%) and Ca Gg were extremely rare (2%) in both environments. Unexpectedly, we discovered an additional and previously unknown level of bacterial diversity within AMF spores, which extended beyond the known endosymbionts, with bacteria belonging to 10 other phyla detected across our spore data set. Detailed analysis revealed that: Ca Gg were not limited in distribution to the Gigasporaceae family of AMF, as previously thought; Ca Mg population structure was driven by AMF host genotype; and a significant inverse correlation existed between the diversity of Ca Mg and diversity of all other endobacteria. Based on these data, we generate novel testable hypotheses regarding the function of Ca Mg in AMF biology by proposing that they might act as conditional mutualists of AMF.