The model arbuscular mycorrhizal fungus Rhizophagus irregularis harbours endosymbiotic bacteria with a highly reduce genome

Abstract

Arbuscular mycorrhizal fungi (AMF; Glomeromycotina) are symbionts of most plant species that are known to possess unique intracytoplasmic endosymbiotic bacteria with an enigmatic role. Candidatus Moeniiplasma glomeromycotorum (CaMg) was shown to be widespread along the AMF phylogeny and present in most AMF species and isolates of those species. The model AMF species, Rhizophagus irregularis, that can be cultivated in vitro and for which a lot of genomic information now exists, would be the ideal model to study the true nature of the CaMg-AMF symbiosis. However, R. irregularis was never found to host endobacteria. Here we show by DNA sequencing that R. irregularis can, indeed, host CaMg (Ri-CaMg). However, this appears rare as only one R. irregularis isolate out of 58 hosted CaMg. In that isolate, the endosymbiotic bacterial population was genetically homogenous. By sequencing the complete genome of the bacteria, we found that its genome is among the smallest of all known CaMg and Mycoplasma-like genomes, with a highly reduced gene repertoire, suggesting a strong adaptation to the intracellular life. We discuss our findings in the light of previous literature on CaMg and on the same AMF isolates and suggest that these endosymbionts are more likely parasites than non-obligatory mutualists.