Abstract:
Dung beetles are ecosystem engineers that provide a plethora of ecosystem services such as nutrient cycling, waste removal and parasite suppression. Recent evidence suggests that dung beetles rely on symbiotic gut bacteria, which have been linked to increased larval survival, growth and normative development. However, the eco-evolutionary factors that shape these relationships remain largely ambiguous. The native dung beetle fauna present in South Africa provide a unique opportunity to study the insect gut microbiome and how it relates to diet. There are at least five different dietary specialisations present in South African dung beetles, namely; coprophagy, necrophagy, detritophagy, fungivory and carnivory. With this in mind, the present study aimed to characterise the gut bacterial community structure within seven dung beetle species from five different genera, each genus with a distinct dietary specialisation. Additionally, this study aimed to characterise the gut bacterial community assemblage of Digitonthophagus gazella after feeding and breeding within three distinct substrates, namely; cow, horse and pig dung. The guts of each individual were dissected, bacterial DNA was extracted and finally, the 16s rRNA hypervariable region V4-V5 was sequenced utilising the Illumina NovaSeq platform. The three most common phyla observed within our samples were Firmicute, Proteobacteria and Bacteroidetes. Our results indicate that diet contributes significantly to the gut bacterial composition within different dung beetle species. Furthermore, most dietary specialisation grouped separately from one another, except for the mushroom feeding beetles which grouped within the millipede and carrion feeding groups. The millipede feeding species, Sceliages brittoni and S. hippias, both possess the most distinct and least diverse gut microbiome when compared to all other dietary specialisations in this study. Defensive secretions (hydrogen cyanide and quinones) produced by millipedes may act as a selective pressure and affect which bacterial taxa could inhabit the guts of Sceliages species. The gut microbiota of D. gazella was variable between and within the different treatment groups that were fed on different diets. However, all newly emerged individuals that were reared on cow dung, showed a highly similar gut microbiome. This could indicate the existence of a core gut microbiome shortly after emergence, possibly maternally transmitted by the production of a pedestal within the brood ball. Ultimately, this study sheds light on the effect diet has on the gut microbial composition of beetles and it is the first to characterise the gut microbiome of millipede feeding beetles. Characterisation of the gut microbiome is the first step to answering more complex questions relating to the holistic role of dung beetles in the ecosystem.
