Contrasting dates of rainforest fragmentation in Africa inferred from trees with different dispersal abilities

Abstract

The rainforests of Tropical Africa have fluctuated over time. Although today the forest cover is continuous in Central Africa this may have not always been the case, as the scarce fossil record in this region suggests that more arid conditions might have significantly reduced the density of trees during the Ice Ages. Our aim was to investigate whether the dry ice-age periods left a genetic signature on tree species that can be used to date the past fragmentation of the rainforest. We sequenced reduced representation libraries of 182 samples representing five Legume tree species that are widespread in African rainforests and seven outgroups. Phylogenetic analyses identified an early divergent lineage for all species in West Africa (Upper Guinea), and two clades in Central Africa: Lower Guinea-North and Lower Guinea-South. As the structure separating the Northern and Southern clades cannot be explained by geographic barriers, we tested other hypotheses using demographic model testing. The best estimates recovered using {partial}a{partial}I indicate that the two clades split between the Upper Pliocene and the Pleistocene, a date compatible with forest fragmentation driven by ice-age climatic oscillations. Furthermore, we found remarkably older split dates for the shade-tolerant tree species with non-assisted seed dispersal than for light-demanding long-distance wind-dispersed trees. We also show that the genetic diversity significantly declines with the distance from ice-age refugia in the two long-distance dispersed species only. Different recolonisation abilities after recurrent cycles of forest fragmentation seem to explain why we observe congruent genetic spatial structures across species with contrasted timescales.\n\nSIGNIFICANCE STATEMENTAlthough today the rainforest cover is continuous in Central Africa, the scarce fossil record suggests that arid conditions during the Ice Ages might have reduced the density of trees during the Ice Ages. However, the vast majority of the fossil pollen records preserved in Tropical Africa is too young to inform about this period. Investigating whether the past climate change left a genetic signature on trees can thus be useful to date past forest fragmentation. However, most genetic studies available to date lack resolution as they use limited numbers of loci. In this study we use modern DNA technology to study five Legume trees. Our results show significant differentiation of the populations of each species at a date compatible with forest fragmentation driven by ice-age climatic oscillations. Contrasted timescales were obtained for each species, which probably reflects their different recolonisation abilities after forest fragmentation.