Histoire évolutive du complexe Afzelia Smith (Leguminosae - Caesalpinioideae) dans les écosystèmes forestiers et savanicoles d’Afrique tropicale

The genus Afzelia Smith includes seven African tree species: two are found in the Zambezi region, one in the Sudanian region and four in the Guinea-Congo region. These taxa are commercially valuable but because they are difficult to distinguish, they are marketed under the same commercial name, either "doussié" or "afzelia", which complicates sustainable management of their populations. The aim of this PhD thesis is to characterize the evolutionary history of the Afzelia genus. More specifically, our study aims to: (i) assess the extent of morphological divergence within the Afzelia genus and describe the phylogenetic relationships in order to quantify the reproductive isolation of the different taxa by identifying the role of past climate change and / or ecological gradients in the speciation of the genus; (ii) analyze the spatial genetic diversity and structure of Afzelia spp.; (iii) identify and describe the ecological, biotic and abiotic factors that may influence population-level gene flows in an Afzelia species (A. bipindensis). Our morpho-genetic analysis of the different species confirmed the strong botanical resemblance between the taxa. The savannah species are diploid and their genome is half the size of the forest species, which are tetraploid. The phylogenies of the genes, nuclear and chloroplastic, are not the same. The differences observed could have been generated by ancient hybridization between species, which would have occurred between the forest lineages and those of A. quanzensis, a Zambezi savannah species. Polyploidy probably emerged during the evolutionary history of the genus between 7 and 9.4 million years ago. Bayesian assignment and reproductive isolation analyses also suggested interspecies crossing, but only among forest species distributed sympatrically. On a more limited spatial scale, two well differentiated genetic groups of A. bipindensis were observed to be sympatric. These show differences in their morphology and flowering phenology that could contribute to their reproductive isolation. This study made several important findings: discovery of a polyploid complex within the Afzelia genus, confirmation of the boundaries of diploid savannah species and the need to revise the taxonomy of tetraploid forest species