UCE phylogenomics and the arboreal ant genus Myrmelachista (Formicidae: Myrmelachistini): A case study on the evolution of obligatory ant-plant symbioses

Obligatory ant-plant symbiotic interactions have evolved multiple times, with plants providing ants with housing and often food resources in exchange for protection from herbivores or competitors. Those associations are thought to have originated as a parasitic interaction when ants start to occupy live plant cavities. Few ant genera provide adequate behavioral repertoires to allow the study of transitions between dead and live-stem nesting in ants, limiting a thorough understanding of the evolutionary changes in those associations. The ant genus Myrmelachista (Formicidae: Myrmelachistini) comprises ~60 species distributed throughout the New World tropics and subtropics. Within the genus, species can be divided into dead stem nesters or generalist inhabitants of live stems, and those engaged in tight associations with a set of plant species. The latter includes specialist inhabitants of understory Lauraceae and Meliaceae in Central America, and "devil's garden" ants inhabiting Melastomataceae and Rubiaceae in South America. We apply a phylogenomic approach (enrichment of ultraconserved elements [UCEs]) to reconstruct the evolutionary history of Myrmelachista. We present a UCE phylogeny for 140 specimens covering the genus range and much of its morphological and behavioral variability. We use the phylogeny to investigate the evolution of nesting behavior within Myrmelachista. The topology, coupled with ancestral trait reconstructions, suggests that the evolution of obligatory symbiotic relationships occurred several times independently within the genus. Moreover, our divergence timing analyses estimated the timing for the obligatory associations within the Centroamerican fauna to be highly similar, suggesting a relatively recent (Pliocene) of convergent evolution.