Convergent evolution of complex structures for ant–bacterial defensive symbiosis in fungus-farming ants
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abstract
Evolutionary adaptations for maintaining beneficial microbes are hallmarks of mutualistic evolution. Fungus-farming “attine” ant species have complex cuticular modifications and specialized glands that house and nourish antibiotic-producing Actinobacteria symbionts, which in turn protect their hosts’ fungus gardens from pathogens. Here we reconstruct ant–Actinobacteria evolutionary history across the full range of variation within subtribe Attina by combining dated phylogenomic and ultramorphological analyses. Ancestral-state analyses indicate the ant–Actinobacteria symbiosis arose early in attine-ant evolution, a conclusion consistent with direct observations of Actinobacteria on fossil ants in Oligo-Miocene amber. qPCR indicates that the dominant ant-associated Actinobacteria belong to the genusPseudonocardia. Tracing the evolutionary trajectories ofPseudonocardia-maintaining mechanisms across attine ants reveals a continuum of adaptations. InMyrmicocryptaspecies, which retain many ancestral morphological and behavioral traits,Pseudonocardiaoccur in specific locations on the legs and antennae, unassociated with any specialized structures. In contrast, specialized cuticular structures, including crypts and tubercles, evolved at least three times in derived attine-ant lineages. Conspicuous caste differences inPseudonocardia-maintaining structures, in which specialized structures are present in worker ants and queens but reduced or lost in males, are consistent with verticalPseudonocardiatransmission. Although the majority of attine ants are associated withPseudonocardia, there have been multiple losses of bacterial symbionts and bacteria-maintaining structures in different lineages over evolutionary time. The early origin of ant–Pseudonocardiamutualism and the multiple evolutionary convergences on strikingly similar anatomical adaptations for maintaining bacterial symbionts indicate thatPseudonocardiahave played a critical role in the evolution of ant fungiculture.
