Origin and history of the Dahomey Gap separating West and Central African rain forests: insights from the phylogeography of the legume tree Distemonanthus benthamianus

Aim The aim of this study was to understand the origin of the forest flora currently found in the Dahomey Gap (DG), a 200-km-wide forest-savanna mosaic separating the West African and Central African rain forest blocks. More specifically, using a widespread rain forest tree species, Distemonanthus benthamianus (Fabaceae), we will test the hypothesis that the DG populations are remnants of a population dating back from the African Humid Period of the Holocene, when West African and Central African rain forests were supposedly connected. Location Tropical forests of Upper Guinea (West Africa) and Lower Guinea (Atlantic Central Africa) and the forest-savanna mosaic of the DG extending from eastern Ghana to Benin. Methods Four hundred and twenty-nine D. benthamianus samples from West to Central Africa were genotyped with 11 nuclear microsatellite markers. After detecting geographically coherent gene pools, their genetic diversity and differentiation were estimated and their demographic histories were inferred using coalescent simulations and approximate Bayesian computation (ABC) tests. Results Five parapatric gene pools were identified: three in Lower Guinea, one in Upper Guinea and one in the DG. ABC tests indicate that the DG gene pool probably originates from the admixture of adjacent Upper and Lower Guinean gene pools, with a higher contribution from Upper Guinea, at a timeframe consistent with the early Holocene (around 13-7 ka). The lower genetic diversity documented in the DG could result from a founder effect and/or from a demographic decline consistent with the Holocene climatic pejoration documented around 3 ka. Main conclusions This phylogeographical study inferring the history of the DG populations of D. benthamianus is consistent with palaeovegetation data that suggest that the forest flora of the DG might be essentially relicts of the early Holocene period when the Guineo-Congolian forest reached its maximum geographical distribution.