• 18S rRNA gene;
  • adaptation;
  • Aegagropila- clade;
  • Aegagropila linnaei;
  • Cladophora;
  • Cladophorales;
  • freshwater species

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 called Cladophora aegagropila (L.) Rabenhorst or Cladophora sauteri (Nees ex Kütz.) Kütz. Based on morphology, it was formerly classified into the section Aegagropila or into the subgenus Aegagropila, together with several marine species of the genus Cladophora. This classification is not supported by the present phylogenetic analyses in which two very distinct Cladophorales clades are recognized. Aegagropila linnaei groups together in a well-supported clade with Cladophora sp., Pithophora sp., Chaetomorpha okamurae, Arnoldiella conchophila, Wittrockiella lyallii, and Cladophora conchopheria. Aegagropila linnaei and 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.