Phylogenomic approach to integrative taxonomy resolves a century-old taxonomic puzzle and the evolutionary history of the Acromyrmex octospinosus species complex

Accurately delimiting species boundaries is essential for understanding biodiversity. Here, we assessed the taxonomy of the leaf-cutting ants in the Acromyrmex octospinosus (Reich) species complex using an integrative approach incorporating morphological, population genetic, phylogenetic and biogeographical data. We sampled populations across the biogeographic distribution of the species complex and reconstructed their evolutionary relationships using ultraconserved elements (UCEs) as molecular markers. We evaluated traditional morphological characters used to distinguish putative taxa and performed species delimitation analyses to investigate divergence between evolutionary lineages. Our results support the hypothesis that the A. octospinosus species complex consists of two species: the widely distributed and polymorphic species A. octospinosus and its inquiline social parasite A. insinuator Schultz et al. We consider A. echinatior (Forel) syn. nov. and A. volcanus Wheeler syn. nov. as well as the subspecies A. octospinosus cubanus Wheeler syn. nov., A. octospinosus ekchuah Wheeler syn. nov. and A. octospinosus inti Wheeler syn. nov. as junior synonyms of A. octospinosus. We also investigated the biogeographic history of the species complex and the evolutionary origin of the social parasite A. insinuator. We inferred that A. octospinosus originated during the late Miocene approximately 6.9 Ma ago in the Neotropical rainforest. Acromyrmex insinuator shared a common ancestor with A. octospinosus approximately 3.4 Ma ago, with a crown-group age of approximately 0.9 Ma. Our phylogeny supports the hypothesis that the inquiline social parasite speciated via the intra-specific route of social parasite evolution in direct sympatry from its host. Our findings reshape our understanding of the A. octospinosus species complex and provide a foundation for future studies of Acromyrmex leaf-cutting ants.