Ant agriculture, exemplified by the symbiosis between fungus-growing (attine) ants and their fungal cultivars, is intensively studied as a model system for understanding the general ecological and coevolutionary mechanisms that shape highly integrated symbioses. The attine ant agricultural symbiosis encompasses both mutualistic and parasitic interactions between (1) ant farmers, (2) fungal cultivars, (3) parasitic fungal "weeds" that infest ant gardens, and (4) antibiotic-producing bacteria that control the garden parasites and are cultured on the bodies of ants. This project specifically explores the ecological complexity of the attine ant-fungus-bacterium symbiosis and elucidates its 50-million-year coevolutionary history by testing hypotheses posed at multiple ecological and evolutionary levels, including: (1) the local ecological level within ant colonies; (2) the population and community levels; (3) the biogeographic level; and the phylogenetic level, including (4) coevolutionary processes and historical ecology; (5) symbiont-mediated speciation; and (6) the effects of symbiotic life histories on rates of evolutionary change. Because symbiosis is a major recurring theme in the history of life, this research is expected to generate general insights into the broad mechanisms that drive biological complexity and diversity. Such insights could in some cases impact issues directly relevant to human welfare, including the evolution of antibiotic resistance and the practice of sustainable agriculture.