Hindgut microbiota reflects different digestive strategies in dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae).

Gut microbes play an important role in the biology and evolution of insects. Australian native dung beetles (Scarabaeinae) present an opportunity to study gut microbiota in an evolutionary context as they come from two distinct phylogenetic lineages and some species in each lineage have secondarily adapted to alternative or broader diets. In this study, we characterised the hindgut bacterial communities found in 21 species of dung beetles across two lineages using 16S rRNA sequencing. We found that gut microbial diversity was more dependent on host phylogeny and gut morphology than specific dietary preferences or environment. In particular, gut microbial diversity was highest in the endemic, flightless genus that feeds on a broad range of composted organic matter. The hindgut of harbours a highly conserved core set of bacteria suggesting that the bacteria are symbiotic. Symbiosis is supported by the persistence of the core microbiota across isolated beetle populations and between species in the genus. A co-evolutionary relationship is supported by the expansion of the hindgut to form a fermentation chamber and the fermentative nature of the core microbes. In contrast, Australian species of the widespread dung beetle genus specialise on a single food resource such as dung or fungus, exhibit minimal food processing behaviour, have a short, narrow hindgut and a variable gut microbiota with relatively few core bacterial taxa. A conserved, complex gut microbiota is hypothesised to be unnecessary for this highly mobile genus. Dung beetles are a very important part of an ecosystem because of their role in the removal and decomposition of vertebrate dung. It has been suspected that symbiotic gut bacteria facilitate this role, a hypothesis that we have explored with high throughput barcoding. We found that differences in hindgut morphology had the greatest effect on the bacterial community composition. Species with a hindgut fermentation chamber harboured a distinctly different hindgut community compared to those species with a narrow, undifferentiated hindgut. Diet and phylogeny were also associated with differences in gut community. Further understanding of the relationships between dung beetles and their gut microbes will provide insights into the evolution of their behaviours and how gut communities contribute to their fitness.