A molecular phylogeny of Caesalpinia sensu lato: Increased sampling reveals new insights and more genera than expected

Caesalpinia sensu lato, in its broadest circumscription, is a pantropical group of c.150 species of trees, shrubs and lianas many of which grow in arid habitats of the Succulent Biome (sensu Schrire et al., 2005), and especially in the seasonally dry tropical forests of Central and South America and the Caribbean. As traditionally circumscribed, Caesalpinia s.l. was one of the largest genera in tribe Caesalpinieae, but seven generic segregates, namely Coulteria, Erythrostemon, Guilandina, Libidibia, Mezoneuron, Poincianella and Tara were reinstated by Lewis (2005), greatly reducing the number of species remaining in Caesalpinia sensu stricto. Nevertheless, doubts remain regarding the monophyly and delimitation of some of these segregate genera, which have not been thoroughly tested using molecular data, and this has hindered the establishment of a comprehensive generic classification of the broader Caesalpinia Group as a whole. Here we present a new phylogeny of the Caesalpinia Group, based on plastid rps16 sequences and dense taxon sampling including 18 of the 21 genera of the Caesalpinia Group and 98 of the c.150 species of Caesalpinia s.l. Our results support the monophyly of five of the genera reinstated by Lewis, but the three other genera (including Caesalpinia s.s.) are non-monophyletic and need to be re-evaluated. Furthermore, three robustly supported newly discovered clades within Caesalpinia s.l. potentially merit recognition as distinct genera pending complete investigation of diagnostic morphological characters. Uncertainties concerning the delimitation of some clades are discussed especially in relation to the extensive morphological variation found within Caesalpinia s.l.