Characterization of the early evolution of the amphibious Podostemaceae

Abstract

Podostemaceae are torrenticolous lithophytes, which are submerged during the rainy season and flower after exposure in the dry season. The terrestrial ancestor likely colonized amphibious habitats with the submergence and exposure phases. To characterize the early evolution, we did a cladistic analysis of 60 characters for the three subfamilies, including the seed-to-ovule and capsule-to-ovary ratios measured here for 24 and 27 species representing the three subfamilies. The analysis found three major patterns of character evolution. The first pattern is shown by 23 one-state apomorphic characters, 17 of which are vegetative and six are reproductive. The former characters include thin cuticular membrane and epidermal cell wall, adhesive seed coat and hair/holdfast, and epilithic organs. The prominent flavonoids/anthocyanins likely protect the exposed plant body from intense UV radiation in the habitat. Each character is adapted to the torrenticolous, epilithic life in sunny rivers. The latter reproductive characteristics, e.g., single fertilization and reduced embryo, contribute to the short-term reproduction before plant death. Probably all apomorphies accomplished the successful invasion of the amphibious habitat in the early evolution. In the second pattern, the apomorphies of 30 apomorphic and plesiomorphic characters, e.g., apical-meristemless shoot, rootless plant and spathella, produced the remarkable within-family diversification. In the third pattern, the seven plesiomorphic characters apparently little contribute to the early submergent evolution. The climatic distribution of species-rich countries suggests that Podostemaceae unchangeably preferred a monsoon/savanna climate with many habitats.