PHYLOGENETIC RELATIONSHIPS WITHIN CLADOPHORALES (ULVOPHYCEAE, CHLOROPHYTA) INFERRED FROM 18S rRNA GENE SEQUENCES, WITH SPECIAL REFERENCE TOAEGAGROPILA LINNAEI 1

The phylogenetic position of a freshwater green alga,Aegagropila linnaei(Cladophorales, Ulvophyceae), was investigated using nuclear 18S rRNA gene sequences. This alga has usually been calledCladophora aegagropila(L.) Rabenhorst orCladophora sauteri(Nees ex Kütz.) Kütz. Based on morphology, it was formerly classified into the sectionAegagropilaor into the subgenusAegagropila, together with several marine species of the genusCladophora.This classification is not supported by the present phylogenetic analyses in which two very distinct Cladophorales clades are recognized.Aegagropila linnaeigroups together in a well‐supported clade withCladophorasp.,Pithophorasp.,Chaetomorpha okamurae,Arnoldiella conchophila,Wittrockiella lyallii, andCladophora conchopheria. Aegagropila linnaeiand its closely related species share some ultrastructural and biochemical characteristics, like pyrenoid structure, carotenoid composition, and cell wall composition. Freshwater species, included in the analysis, were located in two distantly related lineages, indicating that adaptation from a marine to a freshwater habitat has happened at least twice independently in the Cladophorales.