A unique midgut-associated bacterial community hosted by the cave beetle Cansiliella servadeii (Coleoptera: Leptodirini) reveals parallel phylogenetic divergences from universal gut-specific ancestors

Background: Cansiliella servadeii (Coleoptera) is an endemic troglobite living in deep carbonate caves in North- Eastern Italy. The beetle constantly moves and browses in its preferred habitat (consisting in flowing water and moonmilk, a soft speleothem colonized by microorganisms) self-preens to convey material from elytra, legs, and antennae towards the mouth. We investigated its inner and outer microbiota using microscopy and DNA-based approaches. Results: Abundant microbial cell masses were observed on the external appendages. Cansiliella's midgut is fully colonized by live microbes and culture-independent analyses yielded nearly 30 different 16S phylotypes that have no overlap with the community composition of the moonmilk. Many of the lineages, dominated by Gram positive groups, share very low similarity to database sequences. However for most cases, notwithstanding their very limited relatedness with existing records, phylotypes could be assigned to bacterial clades that had been retrieved from insect or other animals' digestive traits. Conclusions: Results suggest a history of remote separation from a common ancestor that harboured a set of gutspecific bacteria whose functions are supposedly critical for host physiology. The phylogenetic and coevolutionary implications of the parallel occurrences of these prokaryotic guilds appear to apply throughout a broad spectrum of animal diversity. Their persistence and conservation underlies a possibly critical role of precise bacterial assemblages in animal-bacteria interactions.