Résumé
Cycles of Quaternary climate change are assumed to be major drivers of
African rainforest dynamics and evolution. However, most hypotheses on
past vegetation dynamics relied on palaeobotanical records, an approach
lacking spatial resolution, and on current patterns of species diversity
and endemism, an approach confounding history and environmental
determinism. In this context, a comparative phylogeographic study of
rainforest species represents a complementary approach because Pleistocene
climate fluctuations may have left interpretable signatures in the
patterns of genetic diversity within species. Using 1274 plastid DNA
sequences from eight tree species (Afrostyrax kamerunensis, A.
lepidophyllus, Erythrophleum suaveolens, Greenwayodendron suaveolens,
Milicia excelsa, Santiria trimera, Scorodophloeus zenkeri, Symphonia
globulifera) sampled in 50 populations of Atlantic Central Africa (ACA),
we averaged divergence across species to produce the first map of the
region synthesizing genetic distinctiveness and standardized divergence
within and among localities. Significant congruence in divergence was
detected mostly among five of the eight species and was stronger in the
northern ACA. This pattern is compatible with a scenario of past forest
fragmentation and recolonization whereby forests from eastern Cameroon and
north-eastern Gabon would have been more affected by past climate change
than those of western Cameroon (where one or more refugia would have
occurred). By contrast, southern ACA (Gabon) displayed low congruence
among species that may reflect less drastic past forest fragmentation or a
more complex history of vegetation changes. Finally, we also highlight the
potential impact of current environmental barriers on spatial genetic
structures.