Are there evidences of coevolution events between endosymbiotic bacteria and arbuscular mycorrhizal fungi?

The Fungal Kingdom offers a limited number of examples of endobacteria living in the fungal cytoplasm. Arbuscular mycorrhizal (AM) fungi, which are themselves obligate plant symbionts, provide one of the best known study-cases because some AM isolates are specialised niches for rod-shaped bacteria that are vertically transmitted through the vegetative generations (Bianciotto et al. 2004). On the basis of their ribosomal sequences, the endobacteria have been identified as a new bacterial taxon, Candidatus Glomeribacter gigasporarum (Bianciotto et al. 2003). Their morphological features confirm them as Gram-microbes with a highly reduced genome evaluated as 1.4 Mbs and consisting of a chromosome and a plasmid (Jargeat et al. 2004). The status of uncultivable microbe and the reduced genome size strongly suggest that Ca. Glomeribacter gigasporarum is an obligate endosymbiont.