Historical biogeography of river-weeds (Podostemaceae)

Podostemaceae are unusual aquatic angiosperms growing as haptophytes in extreme habitats, i.e., rapids and waterfalls. They originated with an ecological shift from terrestrial life, followed by dispersal and diversification. It has been argued that the Tristichoideae originated on the Gondwana and diversified on the drifting continents. We aimed to test this hypothesis, focusing on the place of origin and the frequency and direction of dispersal. We used a Maximum Likelihood (ML) analysis of phylogenetic relationships and comparative morphology for Podostemaceae. The results of the ML analysis showed that the family originated and then diverged to subfamilies Podostemoideae and Weddellinoideae in America, while the Tristichoideae most likely originated in Asia. Comparison with the sister family Hypericaceae indicated that Tristichoideae exhibit plesiomorphic states for all the morphological characters but pollen aperture, whereas Podostemoideae have apomorpic states. Under the island biogeography model, an Asian origin of Podostemaceae may be likely. Those American- and Asian-origin scenarios are contradictory and need further study. The biogeography of the major clades was shaped by several intercontinental dispersals followed by diversification in the colonized continents. A few dispersals happened on the continents of Africa, America and Asia, resulting in the dual biogeography of the continents. Repetition of diversification-dispersal-diversification is suggested by paraphyletic parental groups at higher and lower (species) taxonomic levels.