Shaping the tripartite symbiosis: are termite microbiome functions directed by the environmentally acquired fungal cultivar?

Microbiome assembly critically impacts the ability of hosts to access beneficial symbiont functions. Fungus-farming termites have co-evolved with a fungal cultivar as a primary food source and complex gut microbiomes, which collectively perform complementary degradation of plant biomass. A large subset of the bacterial community residing within termite guts are inherited (vertically transmitted) from parental colonies, while the fungal symbiont is, in most termite species, acquired from the environment (horizontally transmitted). It has remained unknown how the gut microbiota sustains incipient colonies prior to the acquisition of the fungal cultivar, and how, if at all, bacterial contributions are modulated by fungus garden establishment. Here, we test the latter by determining the composition and predicted functions of the gut microbiome using metabarcoding and shotgun metagenomics, respectively. We focus our functional predictions on bacterial carbohydrate-active enzyme and nitrogen cycling genes and verify compositional patterns of the former through enzyme activity assays. Our findings reveal that the vast majority of microbial functions are encoded in the inherited microbiome, and that the establishment of fungal gardens incurs only minor modulations of predicted bacterial capacities for carbohydrate and nitrogen metabolism. While we cannot rule out that other symbiont functions are gained post-fungus garden establishment, our findings suggest that fungus-farming termite hosts are equipped with a near-complete set of gut microbiome functions at the earliest stages of colony life. This inherited, incipient bacterial microbiome likely contributes to the high extent of functional specificity and coevolution observed between termite hosts, gut microbiomes, and the fungal cultivar.