Symbiont-mediated degradation of dietary carbon sources in social herbivorous insects

In this chapter we present the current state of research on the contribution of the gut microbiome to the degradation of dietary sources of carbon in three social insects: termites, honeybees and herbivorous ants. Insects from these groups can face similar dietary challenges, and an emerging body of research illustrates that their gut microbes have provided symbiotic solutions for the degradation of a variety of carbon sources. Termites, particularly ‘lower’ termites have a diet enriched in perhaps the most recalcitrant molecule found in the diet of social insects—lignocellulose. Lower termites have a diverse (> 400 species/gut) community of protists and bacteria that work together with endogenous host cellulases to provision acetate to the host. Honeybees, feeding on pollen, pollen-rich beebread and nectar, have a relatively simple gut microbiome (8–10 bacterial species). Recently it has been demonstrated that some of these bacteria possess pectinases and an assortment of other sugar- and flavonoid-degrading enzymes and may provision the host with several molecules, including butyrate. In leaf-cutter ants, feeding on their fungal cultivar, bacteria may be involved in the depolymerization of chitin and fermentation of citrate to acetate. In Cephalotes turtle ants, metagenomic studies have shown that some gut bacteria encode genes for the degradation of recalcitrant polysaccharides found in pollen and, possibly, other foods from their generally cryptic diets. Across these social insect hosts, faithful symbioses with specialized microbes were enabled by modifications to social behaviour that facilitated faithful spread of microbes within colonies and across generations.