Phylogénie et biogéographie du genre Bauhinia s.l. (Leguminosae)

Bauhinia s.l. is the largest genus of the tribe Cercideae (Ceasalpinioideae, Leguminoseae), with over 300 species. It has a pantropical distribution and high morphological variability. These two features have resulted in few studies that focus on the entire genus, resulting in several regional studies or studies of certain subgroups only. In 1987, Wunderlin et al. presented a broad taxonomic revision of the tribe Cercideae, based on morphological data, and divided the genus Bauhinia into four subgenera. In 2005, Lewis and Forest published a new preliminary classification based on molecular data, but for a limited taxonomic sampling. Their findings question the monophyly of the genus Bauhinia and suggest instead the recognition of eight genera in the Bauhinia s.l. grade. To test the hypotheses of Lewis and Forest, and to obtain a clearer view of the history of Bauhinia s.l., we sequenced two chloroplast regions (trnL-trnF and matK-trnK) and two nuclear regions (Leafy and Legcyc) for a large representative sampling of the Cercideae. A primary phylogeny of the tribe was first generated based on trnL-trnF sequences only and confirmed the non-monophyly of Bauhinia s.l., with the inclusion of the genus Brenierea, traditionally recognized as sister group of Bauhinia s.l. In order to obtain a deaper view of the evolutionary history of the Cercideae, a new series of analysis was performed, including all nuclear and chloroplast sequences. Individual phylogenies were reconstructed for each region of the genome, and both a species and a supermatrix trees were reconstructed. Although certain conflicting relationships appear between phylogenies, the outline of the history of the Cercideae has been resolved. Bauhinia s.l. is divided into two lineages: Phanera and Bauhinia groups. The Bauhinia group includes Bauhinia s.s., Piliostigma and Brenierea. The Phanera group is composed of Gigasiphon, Tylosema, Lysiphyllum, Barklya, Phanera and Schnella. Cercis, Griffonia and Adenolobus are sister groups of Bauhinia s.l. At least one duplication event of Legcyc has been highlighted for the entire tribe Cercideae, excluding Cercis. Several other duplication events are also suggested by both Legcyc and Leafy . Finally, a divergence time analysis and a reconstruction of ancestral areas were conducted. The root of the tribe is evaluated to be 49.7 Mya old, and to originate from  temperate regions in the northern hemisphere, mostly around the Tethys Sea. The tribe then dispersed into drier biomes in Africa, where the separation of the Bauhinia and the Phanera groups occurred. These two lineages then dispersed following parallel routes to Southeast Asia in the early Miocene. At the same time, a dispersal of the African Bauhinia s.s. to South America permitted the diversification of the American species of this genus, and Schnella (the only American genus within the  Phanera group) dispersed to the American continent from Australia. This dispersal to Australia is also at the origin of Lysiphyllum and Barklya.

Keyphrases: Bauhinia, Cercideae, Caesalpinioideae, pantropical, Leafy, Legcyc, trnL-trnF, matK-trnK, phylogeny, biogeography