Discovery of a New Tristicha Species (Podostemaceae, Tristichoideae) from Cameroon Broadens the Morphological Limits of the Genus

Abstract

Podostemaceae, a family of strictly aquatic plants found in rapids and waterfalls, include the single African species belonging to subfamily Tristichoideae, Tristicha trifaria (Bory ex Willd.) Spreng. While con­ducting an environmental impact assessment for the Kikot hydroelectric dam project on the Sanaga River in southern Cameroon, 208 Tristicha Thouars collec­tions were made, 93 of which exhibited peculiar mor­phological features that clearly distinguished them from T. trifaria and were found to represent a new species, T. spinulosa E. Bidault & Rutish., which is described. It differs from T. trifaria by having ovaries with only two locules, each with a single ovule (vs. many), two stigmas (vs. three), a capsule with a very thin wall that decays early (rather than a 3-valved dry capsule), and three tepals that surround the fruit and become scleri­fied during seed maturation to form spines. These mor­phological features, not previously observed in Tristicha, require an expansion of the morphological delimitation of the genus. The new species is endemic to the Sanaga River system; its conservation status is preliminarily assessed as Vulnerable because of threats resulting from hydroelectric and regulating dams, which have signifi­cantly increased submersion of most colonies in the Sanaga and Mbam rivers. The inclusion of a historical collection of T. spinulosa in a previous phylogeny reinforces the hypothesis that T. trifaria in Africa rep­resents multiple species. Further phylogenetic and mor­phological studies at the scale of tropical Africa are currently being undertaken by the authors.