Abstract:
The 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.
Bayesian-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.
Overall, 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.
