Molecular systematics of the subaerial green algal order Trentepohliales: an assessment based on morphological and molecular data.

The order Trentepohliales is a widespread group of terrestrial green algae. As currently circumscribed, it includes six genera (Cephaleuros, Phycopeltis, Physolinum, Printzina, Stomatochroon and Trentepohlia), the phylogenetic affinities of which are poorly understood. Sequences for the small subunit rRNA gene (18S rDNA) were obtained for several representatives of the order and phylogenetic analyses based on these sequences were compared with the traditional system of classification based on morphological characters. The results indicate that the morphological criteria used for the circumscription of genera and species of the Trentepohliales do not match phylogenetic patterns. Whereas isolates of Cephaleuros form a well-supported monophyletic clade, Trentepohlia is polyphyletic and strains of Trentepohlia, Phycopeltis, Printzina and Physolinum are mixed together in several different lineages. A reassessment of the generic boundaries and a narrower circumscription of some genera (in particular Trentepohlia) will be necessary. Subcuticular habit, heteromorphic life history and occurrence of zoosporangia in clusters are shown to be phylogenetically significant features, whereas other characters commonly used for separation of species and genera (shape and size of the cells, prostrate/erect habit, substratum colonized) have no phylogenetic relevance.