Phylogenetic and biogeographical history of the Afro-Madagascan genera Delonix, Colvillea and Lemuropisum (Fabaceae: Caesalpinioideae)

Abstract

Our understanding of the taxonomic limits and biogeographical evolution of the Afro-Madagascan genus Delonix and the closely related monospecific Madagascan endemic genera Colvillea and Lemuropisum have been hampered by unresolved intergeneric and interspecific relationships. Here we study the phylogenetics of the group using nucleotide sequences from ribosomal ITS, four low-copy nuclear genes and four plastid regions. We find poorly resolved gene trees typical of closely related species, but most species are resolved as monophyletic, and we report conflicting phylogenetic placements only for D. brachycarpa and C. racemosa. A time-calibrated biogeographical analysis of the concatenated matrix of all nine loci suggests that the ancestral area of the Delonix, Lemuropisum and Colvillea clade was in south-western Madagascar c. 24–18 Ma. An out-of-Madagascar dispersal to Africa took place during the Miocene, most likely by vegetative rafting. Delonix is not monophyletic due to the robustly supported placement of L. edule in Delonix, warranting treatment of Lemuropisum as congeneric with Delonix and establishment of the new name combination Delonix edule, in line with the close morphological similarities between these two genera. In contrast, the phylogenetic position of C. racemosa is unstable occurring in the Delonix–Lemuropisum clade in the nuclear gene trees, but sister to this clade in the plastid gene tree. The status of Colvillea as a distinct genus, therefore, remains questionable, but it is here maintained based on a suite of morphological distinctions, pending further insights into the relationships of this elusive genus.