Convergent evolution of complex structures for ant-bacterial defensive symbiosis in fungus-farming ants

Animal-microbial mutualistic symbioses are important examples of evolutionary adaptation in which symbionts shape diverse traits of their hosts, from physiology to morphology; yet few examples are known of multiple convergences on identical phenotypes within a clade of closely related hosts. Fungus-farming ants possess complex cuticular structures for maintaining Pseudonocardia symbionts. By reconstructing evolutionary history, we find ant-Pseudonocardia associations originated early in evolution of fungus-farming ants and multiple subsequent losses occurred over evolutionary timescales. Complex, strikingly similar structures for maintaining Pseudonocardia have arisen at least three times in fungus-farming ants. The anatomical convergence for maintaining symbionts reveals repeated selection for mutualistic adaptation, likely driven by the necessity of controlling a coevolved fungus-garden parasite.