Phylogenomics resolve a century-old taxonomic puzzle and reveal evolutionary history of the Acromyrmex octospinosus species complex

Accurately delimiting species boundaries is essential for discovering and protecting biodiversity. Here, we assess the taxonomy of the leaf-cutting ants in the Acromyrmex octospinosus species complex using an integrative approach incorporating morphological, population genetic, phylogenetic, and biogeographical data. We comprehensively sampled populations across the biogeographic distribution of the species complex and reconstructed the evolutionary relationships at the population level using ultraconserved elements (UCEs) as molecular markers. We evaluated morphological characters that were historically utilized to separate the putative taxa in the species complex and conducted species delimitation analyses to test for genetic divergence between evolutionary lineages. Our results support the hypothesis that the A. octospinosus species complex consists of two species: the widely distributed and socially polymorphic species A. octospinosus and its inquiline social parasite A. insinuator. We consider A. echinatior syn. nov. and A. volcanus syn. nov. as well as the subspecies A. o. cubanus syn. nov., A. o. ekchuah syn. nov., and A. o. inti 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 most likely originated during the late Miocene ~7.9 Ma ago in the Neotropical rainforest and found that A. insinuator diverged recently and in direct sympatry from its host species A. octospinosus. Our findings reshape our interpretation of the taxonomy and biology of the A. octospinosus species complex and provide a foundation for future comparative studies on Acromyrmex leaf-cutting ants.