Out of Africa to Madagascar—Then Back? Molecular Phylogenetics and Biogeography of Tribe Tarchonantheae (Asteraceae: Tarchonanthoideae)

Abstract

Premise of research. Molecular data have revolutionized inferences of phylogenetic relationships and historical biogeography of flowering plants. Two small genera, Brachylaena and Tarchonanthus, are the only members of Asteraceae tribe Tarchonantheae (subfamily Tarchonanthoideae). The tribe is morphologically distinct within Asteraceae and is resolved as monophyletic with molecular markers. It is distributed in southern Africa and Madagascar. The purposes of the present study were to determine whether molecular data resolve the two genera as monophyletic and to infer the origin and dispersals that produced the current distribution of the tribe. Methodology. Sequences from the nuclear ribosomal ITS and ETS and plastid rpl16 intron were analyzed using maximum likelihood and Bayesian analyses to resolve phylogenetic relationships within the tribe. An ancestral trait reconstruction assessed the likely ancestral range for Tarchonantheae using BioGeoBEARS, and BEAST was used for dating divergence. Pivotal results. We resolved Tarchonanthus as a monophyletic group nested within Brachylaena, making the latter genus paraphyletic. All Malagasy species occurred within a strongly supported clade, but also resolved within the clade was the widely distributed African species Brachylaena huillensis. This indicates two dispersal events between Africa and Madagascar—either a single dispersal to Madagascar, followed by back dispersal to Africa, or two independent dispersals from Africa. Conclusions. Tarchonanthus is a monophyletic group nested within Brachylaena, rendering the latter genus paraphyletic. An initial dispersal of Brachylaena from Africa to Madagascar with subsequent speciation, followed by back dispersal to Africa, with minimal morphological divergence between the African species and its sister species in Madagascar could explain the current distribution of Brachylaena. Alternatively, there may have been two dispersal events to Madagascar from Africa during the Miocene, but all within the same subclade. Dispersal of flowering plants back to Africa from Madagascar is very rare, if not unprecedented. These dispersal events, and most diversification within the tribe, including the divergence of Tarchonanthus and Brachylaena, took place during the Miocene.