Evidence of high genetic differentiation driven by limited gene flow in a lower canopy African tropical rainforest tree species, Coula edulis Baill. (Coulaceae)

Background The distribution of intra-specific genetic diversity in tropical African forest tree species often reveals discontinuities in the form of genetic clusters distributed in parapatry or allopatry. To date, most population genetic studies have focused on canopy trees with potentially high gene dispersal capacities. In contrast, few studies have examined lower canopy tree species, whose more limited gene dispersal may exacerbate genetic discontinuities. In this study, we used nuclear microsatellites (SSRs) to characterize the genetic structure of populations of Coula edulis Baill., a lower canopy species commonly known as "African walnut", exploited for its edible seeds. Results Across its distribution range, we detected one genetic cluster in Upper Guinea (UG) and six in Lower Guinea (LG). High levels of genetic differentiation ( F _ST = 0.39 to 0.59) were found between UG and LG, while differentiation within LG varied widely ( F _ST = 0.08 to 0.50). Genetic discontinuities associated to high differentiation and a phylogeographic signal ( R _ST > F _ST ) suggest ancient divergence, possibly driven by population fragmentation during historical climatic fluctuations, while those associated with limited differentiation may reflect more recent divergence and/or genetic homogenization effect due to ongoing gene flow. Genetic diversity within LG clusters was highly variable ( H _e = 0.40–0.71), with evidence of a founder or bottleneck effect observed in the southeastern Cameroon cluster, resulting in reduced diversity of a relict population. The UG population also showed low diversity ( H _e = 0.38), likely attributable to a historical bottleneck. Morphometric analyses of herbarium specimens revealed some differentiation between LG and UG populations, questioning the taxonomic status of the taxon. Conclusions Our results suggest that, given the limited gene dispersal observed in C. edulis , the observed genetic discontinuities are expected to persist over extended timescales and provide baseline information for the conservation and potential domestication of the species’ genetic resources.