Phylogenetic Systematics and Biogeography of the Pantropical Genus Sesbania (Leguminosae)

The pantropical genus Sesbania (Leguminosae) comprises approximately 85 species and 34 infraspecific taxa. This genus is unusual ecologically in its predilection for wetlands, and morphologically distinctive among legumes, yet has never been comprehensively investigated for the phylogenetic integrity of its subgeneric classification. We subjected a broad sampling of Sesbania species to phylogenetic analysis using both nuclear and plastid DNA sequence data. The resulting phylogenies support the monophyly of Sesbania and two constituent subclades, one represented by the New World endemic Sesbania sections Daubentonia, Daubentoniopsis and Glottidium, and the second comprising the pantropically distributed S. section Sesbania and former S. subgenera Pterosesbania and Agati. The species-rich S. section Sesbania is paraphyletic with respect to S. subg. Agati and Pterosesbania, while S. sect. Daubentonia is paraphyletic with respect to S. sect. Daubentoniopsis. Based on these results we revise the sectional classification of Sesbania. The genus is consistently thoughweakly supported as sister to tribe Loteae. Rate and age estimates suggest that the Sesbania crown clade has an estimated mean age of approximately 18.9 Ma. This implicates long-distance dispersal as causing the pantropical distribution of the genus. Regardless, the New World is optimized as the ancestral area for Sesbania. The prevalence of Sesbania in riparian, wetland, and coastal systems combined with low levels of geographic phylogenetic structure suggested that hydrochory and local extinction potentially explain present-day distributions.