Tropical Tree Genomics Reveals Fragmentation of the African Rainforest during the Ice Ages

The fossil record in tropical Africa suggests that dry conditions during the Ice Ages caused the contraction of rainforests in favor of savannas. This seems to explain why the African rainforest harbours less species than the other two main rainforests in South America and South Eastern Asia. Based on the rate of endemic species, the location of several refuge areas for the rainforest in Central Africa has been proposed. However, to what extent the forest was fragmented, where it survived, and the timing of demographic events remain unknown. In order to address these questions, genomic data from 500 individuals of eight rainforest tree species were generated using nuclear microsatellites and Genotyping By Sequencing (GBS). GIS tools showed remarkable congruence of the intraspecific genetic discontinuities across species in Central Africa, pointing at common past barriers to gene flow. Demographic tests revealed clear genetic signals of population expansions, possibly following bottleneck events, compatible with a history of forest fragmentation. Estimates of expansion times support a scenario of colonization after the Penultimate Glacial Maximum rather than after the Last Glacial Maximum, which may be explained by more arid climatic conditions during the former.