Molecular phylogenetics, biogeography and character evolution in Sesamum (Pedaliaceae)

As recently circumscribed, Sesamum was rendered monophyletic by including the three segregate genera Ceratotheca, Dicerocaryum and Josephinia. Here, the monophyly of Sesamum and the currently accepted infrageneric taxa was tested using expanded phylogenetic analyses. In addition, nine taxonomically useful morphological characters were optimized on the resulting phylogenetic tree to assess their evolutionary pattern. The phylogenetic analyses support the expanded circumscription of Sesamum and the monophyly of Sesamum section Sesamum relative to Sesamum sections Aptera, Ceratotheca, Chamaesesamum, Dicerocaryum, Josephinia and Sesamoptera. Most sections were retrieved as monophyletic, except Sesamum section Ceratotheca which was shown to be paraphyletic. Character reconstruction infers the ancestor of Sesamum to have had an erect growth habit, simple leaves with entire margins, ovaries with incompletely divided locules, indehiscent capsules that are oblong-quadrangular in lateral view with a beaked apex and winged seeds. Relative to the ancestral states, Sesamum has dehiscent fruits and fully divided locules as synapomorphies, with reversals to the ancestral state (indehiscence, undivided locules) in Sesamum sections Josephinia and Dicerocaryum. The absence of trichomes on mature vegetative parts and the development of lobed leaves are synapomorphies for Sesamum section Sesamopteris. Characters such as dentate and lobulate leaf margins and discoid fruit with horn-like protrusions are homoplastic in Sesamum. Dated phylogeny and ancestral range estimation suggest a southern African origin for the genus with subsequent diversification to tropical Africa, India and Australia. Sesamum has an estimated crown age of 38.1 +/- 5.2 Myr with subsequent dispersal scenarios into tropical Africa, Asia and Australia during the Miocene from 24.5 Mya. Dispersal, rather than continental vicariance, is the probable explanation for the pantropical distribution of Sesamum.

Automated summary

The monophyly of Sesamum genus and its infrageneric taxa was confirmed through expanded phylogenetic analyses and morphological character optimization. The analyses supported the expanded circumscription of the genus and the monophyly of its sections. Character reconstruction revealed the ancestral traits of Sesamum and the synapomorphies of different sections. Additionally, the origin and dispersal of the genus were estimated.