The disjunct distribution of Anadenanthera colubrina (Fabaceae) in Argentina: Solving the Gordian knot of its dating

The Seasonally Dry Tropical Forests (SDTF) form a special biome that extends in disjunct fragments from northwestern Mexico to northern Argentina and southwestern Brazil. In South America, the largest fragments were characterized as distribution nuclei: Caatinga, Misiones, Subandean Piedmont nucleus and Chiquitanía. Anadenanthera colubrina (Vell.) Brenan is a native forest species, due its distribution along the SDTF it’s the most paradigmatic and ecologically specialized species. This work aims to date the disjunct distribution of A. colubrina and to determine the most probable hypothesis that explains it’s genetic and geographic distribution pattern. Haplotypes were identified by analyzing two non-coding regions of chloroplast DNA and the relationship among them were represented in a MJ network. Divergence time estimates were calibrated by the Fossilized Birth-Death model incorporating external paleobotanical information. Different scenarios were proposed to test the order and time of divergence using the Approximate Bayesian Computation method. Nine haplotypes were identified, dating the divergence between the Misiones and Subandean Piedmont nuclei to the late Oligocene, while the divergence within each nucleus was dated to the Miocene. The most probable phylogeographic hypothesis indicates a pre-Pleistocene divergence between the haplotypes of the Misiones and Subandean Piedmont nuclei and a later divergence of a haplotype lineage from the Subandean Piedmont nucleus and Paraná. The divergence between chloroplastic haplotypes of A. colubrina occurred long before the Pleistocene, suggesting that A. colubrina was already widespread and fragmented during the Pleistocene in South America before the LGM and also occurred in northern Argentina before the Pleistocene. • Contradictory palaeoscenarios form a real Gordian knot for this emblematic species • New hypothesis explains the distribution of chloroplastic variability of A. colubrina • The distribution of A. colubrina shows a disjunct pattern from before the Pleistocene • The divergence coincides with the marine ingression observed in South America