Eben-Ezer Ewedje

Large-scale pattern of genetic differentiation within African rainforest trees: insights on the roles of ecological gradients and past climate changes on the evolution of Erythrophleum spp (Fabaceae)

BackgroundThe evolutionary events that have shaped biodiversity patterns in the African rainforests are still poorly documented. Past forest fragmentation and ecological gradients have been advocated as important drivers of genetic differentiation but their respective roles remain unclear. Using nuclear microsatellites (nSSRs) and chloroplast non-coding sequences (pDNA), we characterised the spatial genetic structure of Erythrophleum (Fabaceae) forest trees in West and Central Africa (Guinea Region, GR). This widespread genus displays a wide ecological amplitude and taxonomists recognize two forest tree species, E. ivorense and E. suaveolens, which are difficult to distinguish in the field and often confused.ResultsBayesian-clustering applied on nSSRs of a blind sample of 648 specimens identified three major gene pools showing no or very limited introgression. They present parapatric distributions correlated to rainfall gradients and forest types. One gene pool is restricted to coastal evergreen forests and corresponds to E. ivorense; a second one is found in gallery forests from the dry forest zone of West Africa and North-West Cameroon and corresponds to West-African E. suaveolens; the third gene pool occurs in semi-evergreen forests and corresponds to Central African E. suaveolens. These gene pools have mostly unique pDNA haplotypes but they do not form reciprocally monophyletic clades. Nevertheless, pDNA molecular dating indicates that the divergence between E. ivorense and Central African E. suaveolens predates the Pleistocene. Further Bayesian-clustering applied within each major gene pool identified diffuse genetic discontinuities (minor gene pools displaying substantial introgression) at a latitude between 0 and 2°N in Central Africa for both species, and at a longitude between 5° and 8°E for E. ivorense. Moreover, we detected evidence of past population declines which are consistent with historical habitat fragmentation induced by Pleistocene climate changes.ConclusionsOverall, deep genetic differentiation (major gene pools) follows ecological gradients that may be at the origin of speciation, while diffuse differentiation (minor gene pools) are tentatively interpreted as the signature of past forest fragmentation induced by past climate changes.

Morphological variability of the tallow tree, Pentadesma butyracea Sabine (Clusiaceae), in Benin

The seeds of Pentadesma butyracea (butter tree) are exploited by rural populations in West Africa to produce butter used for consumption and as an ingredient in cosmetics and medicinal preparations. To improve our knowledge of this under-studied species, the morphological variation of the trees, fruits and seeds in 12 natural populations of Benin distributed over a gradient of climate aridity is described with 14 quantitative and 13 qualitative descriptors. Most traits, except seed shape categories, show significant differences among populations. Multivariate analyses distinguish two eco-geographical groups: trees located in the more humid southern part of the species distribution are taller and have larger fruits and seeds than trees located in the north. To disentangle the relative roles of genetic variation and phenotypic plasticity in shaping, further phenotypic studies should be performed in common garden experiments.