Symbiont digestive arsenals can support palynivory in turtle ants (Cephalotes sp.)

Bacterial symbionts can promote digestion of plant complex carbohydrates and lignins (i.e. “fibers”) for the development and survival of their animal-hosts. The symbiosis between turtle ants (genus Cephalotes) and their gut bacteria provides an ideal model to study this phenomenon. Cephalotes adult workers host abundant and diverse gut bacteria, that are conserved across Cephalotes species and recycle nitrogen. Cephalotes ants feed substantially on pollen, but unlike honeybees, bacterial degradation of pollen-derived fibers has never been demonstrated in Cephalotes ants. Moreover, roles for gut bacteria in Cephalotes larvae are currently undefined, and recent discoveries of distinct bacterial species in larvae vs. adults suggest a potential for separate, hypothetically complementary, digestive functions. To explore bacterial contributions toward Cephalotes-host digestion, we used shotgun metagenomics and metatranscriptomics on Cephalotes guts. We then assessed the presence and expression level of bacterial genes encoding enzymes to degrade pollen-derived fibers. Among larval gut bacteria, Enterobacteriales bacteria can catabolize predominant pollen fibers such as cellulose, hemicelluloses, lignins, starches and pectins, including homogalacturonan and rhamnogalacturonan-I. Across Cephalotes species, several adult bacterial symbionts encode enzymes to initiate the degradation of fibers including hemicelluloses, lignins, starches and rhamnogalacturonan-I. Our metatranscriptomic analyses reveal that in adult Cephalotes varians feeding on a pollen-rich diet, members of the Sphingobacteriales and Xanthomonadales express genes coding for arabinofuranosidase, an enzyme degrading side-chains of hemicelluloses and rhamnogalacturonan-I. This partially unique arsenal of digestive enzymes between Cephalotes life stages suggests conserved, symbiont-enhanced degradation of several fibers from pollen in both larvae and adults.