Endosymbionts and the evolution of herbivory in ants and the diversification of the ants

Part I. Ants are a dominant feature of terrestrial ecosystems, and recent phylogenetic analyses have shed light on the evolution of their varied life histories. Measurements using stable isotopes have demonstrated that ant diets can range from herbivorous to predaceous, with "herbivores" feeding primarily on nitrogen-poor exudates from plants and sap-feeding insects. Bacteria have been observed in the guts of some herbivorous ant species, leading to the hypothesis that microbes play beneficial nutritional roles for their ant hosts. In a survey of 283 species from 18 of the 21 ant subfamilies, we have uncovered a wealth of specialized bacterial gut symbionts. Herbivorous ants from the tribe Cephalotini (turtle ants) commonly harbor gut symbionts from ant-specific clades. Related microbes from the Rhizobiales are also distributed beyond the Cephalotini, showing a significant pattern of association with unrelated, herbivorous ants. We infer that Rhizobiales symbionts have independently evolved associations with herbivorous ants on at least five occasions. We therefore propose that symbiotic bacteria have facilitated the convergent evolution of herbivory across this ecologically dominant insect family. Part II. Teasing apart the factors that have shaped the diversification of the ants has been greatly facilitated by our growing knowledge of ant phylogeny from the subfamily level to species relationships. With a clearer understanding of the evolutionary history of the ants we have been able to shed light on the timing of the origin of this ecologically important group of organisms and the elements that may be correlated with their diversification. Many factors have shaped and promoted the speciation of the ants from the rise of the flowering plants to biogeography and historical processes to ecological and morphological adaptations. By investigating these patterns in the light of phylogeny, we can begin to understand the evolutionary processes that have lead to the astonishing diversity of the ants.