The evolution of the Green Algae

Abstract

Phylogenetic analyses of complete 18S rDNAs from the green algae reveal the monophyletic origin of three independent evolutionary lineages, the classes Chlorophyceae, Trebouxiophyceae and Ulvophyceae with a basal divergence of a heterogenous assemblage of scaled flagellates, the prasinophytes. The Trebouxiophyceae comprise mainly coccoid green algae from terrestrial habitats or lichen symbioses. The Chlorophyceae unite green algae that have flagellated cells with basal bodies oriented in the clockwise direction or with directly opposed basal bodies. 18S rDNA sequence comparisons substantiate the importance of flagellated cell morphology over vegetative cell features for tracing evolutionary relationships among the green algae. However, the counterclockwise basal body orientation which is shared among the Trebouxiophyceae and Ulvophyceae is likely a symplesiomorphy. Coccoid green algae, formerly placed in a single order (Chlorococcales), are distributed over several lineages.KeywordsChlorophytaChlorophyceaePrasinophyceaeStreptophytaTrebouxiophyceaeUlvophyceae18S rDNAcoccoid green algaelichen algaemolecular evolution