The origin and spread of the southern black ant, a widely distributed leaf-cutting ant

Abstract

Aim: The existence of past connections between different regions through corridors has influenced the diversification of the biota in South America. Evidence of such connections in southern South America is scarce and poorly understood. As a model to analyse the existence of these corridors we study the evolutionary history of a widely distributed leaf-cutting ant by inferring its origin, historical demographic and dispersal processes, and explore for evidence that relates its evolutionary history to geoclimatic events. Location: Southern South America. Taxon: Southern black ant (Acromyrmex lobicornis). Methods: A total of 215 samples were obtained from Argentina, Uruguay and Bolivia. A time-calibrated phylogenetic tree, using a fossilised birth–death process model, was obtained using one mitochondrial and four nuclear molecular markers. Historical demographic processes were inferred using coalescence-based methodologies. A Bayesian continuous phylogeographical diffusion model was used to estimate the geographical origin and infer its spread routes. Results: A. lobicornis has two main occurrence areas: one goes through the Arid diagonal in Argentina up to Bolivia, and the other goes from central Uruguay up to southern Brazil. Both areas are connected by scattered populations distributed along the Espinal ecoregion in Argentina. The estimated crown age is ~1.2 Ma (early Pleistocene). The origin of the species was located between the High Monte and Dry Chaco ecoregions, from where it dispersed along the Arid diagonal and towards the east into the Uruguayan Savannas through the Espinal. Main Conclusions: The Espinal has an important role as a bridge for the dispersion and posterior maintaining of the genetic flow between populations of A. lobicornis, a corridor that could have been also used by other species. As opposed to the previous hypothesis that postulated its origin in a humid subtropical region, this study provides novel evidence placing the origin of A. lobicornis in an arid subtropical region.