Floral character evolution in response to an aquatic environment in Podostemaceae: A phylogenetic approach

Abstract

Habitat transition is a common driving force for change in morphological characters. One of the most dramatic habitat transitions is that between terrestrial and aquatic ecosystems, which has occurred numerous times in both directions throughout the evolutionary history of flowering plants. Podostemaceae, more commonly known as the riverweeds, evolved from terrestrial to freshwater ecosystems and subsequently experienced an overall reduction in number of floral characters. Taking a phylogenetic approach, this study served as a preliminary investigation into evolutionary trends of four floral characters: 1) stamen number, 2) tepal number, 3) stigma number and 4) locule number. Mapped on a phylogeny based upon Maximum Likelihood, all four characters show overall reduction but characters became reduced at different rates. Stamen and tepal numbers showed a rapid initial decrease followed by a gradual increase, stigma number showed a rapid initial decrease then stabilization, and locule number showed a gradual persistent decrease. The difference in trends among the four floral characters is likely due to differences in habitat, but further research is needed.