Résumé
Several studies have suggested that climatic changes during the Pleistocene may have resulted in a reduction in tropical rainforest cover. During coot drier periods, populations of some tropical rainforest taxa may have been restricted to small and isolated areas where precipitation was sufficient to maintain this ecosystem (the refuge hypothesis). This hypothesis is supported by data sets on the spatial distribution of numerous taxa (insects, birds and trees) in South America and Africa. However, little data exist from the tropics to measure the impact of past climatic changes on genetic diversity. The refuge hypothesis makes two predictions for genetic diversity. First, a reduction in population sizes results in a decrease of genetic diversity within population, i.e. a bottleneck effect. Second, the geographic isolation results in an increase of genetic differentiation among populations. We studied the genetic diversity of Vouacapoua americana, a tree species of mature rainforests from the Guiana shield. Within French Guiana, we examined 22 populations for chloroplast diversity using a PCR-RFLP method, and 17 populations at nine microsatellite loci. A high genetic differentiation among populations exists for chloroplast diversity. Moreover, geographic clustering of chlorotypes strongly suggests the role of historical events such as population contractions, on the spatial organization of genetic diversity. In contrast, spatial genetic structure of the nuclear genome exhibited a pattern more consistent with the "isolation by distance model". In the light of these results, we discuss the possibility of post-Pleistocene recolonizations from several refuges in French Guiana and we suggest that reduction of population sizes in V. americana were probably ancient and/or slight. (Résumé d'auteur)