Phylogenetic status of Helicodictyon planctonicum and Desmochloris halophila gen. et comb, nov. and the definition of the class Ulvophyceae (Chlorophyta)

Electron microscopical observations of the gametes of Helicodictyon planctonicum and the zoospores of Chlorosarcinopsis halophila revealed that the basal bodies are offset in a counterclockwise orientation and are associated with a cruciate flagellar rootlet system; the rhizoplast extends along the d-rootlet. In H. planctonicum, the gametes are covered by scales of indefinite shape, two parallel striated bands connect the posterior flanks of each basal body, probasal bodies are present, and a terminal cap is associated with one microtubular triplet. In C. halophila, the zoospores are naked, the anterior flanks of the basal bodies are linked by a rectangular connecting plate in the region of overlap between the basal bodies, and the proximal end of each basal body is broadly covered by a terminal cap. Molecular phylogenetic analyses were performed using 18S rDNA sequence data for 43 taxa, including new sequences for Chlorosarcinopsis minor, Urospora penicilliformis and Acrosiphonia duriuscula var. tenuis, in addition to H. planctonicum and C. halophila. According to the molecular data, the Ulvophyceae sensu Mattox and Stewart comprises two separate lineages, viz. (1) Ulvophyceae I, including clades represented by Acetabularia and Cladophora; and (2) Ulvophyceae II, including clades of Ulva, Acrosiphonia, Ulothrix and H. planctonicum. Chlorosarcinopsis halophila is placed in the Ulva clade, whereas C minor the type species of Chlorosarcinopsis, belongs to a group within the Chlorophyceae characterized by clockwise displacement of the basal bodies. Examination of user-defined trees showed that phylogenies containing two distinct lineages within the Ulvophyceae sensu Mattox and Stewart are better supported than the monophyly of the class. Our results are consistent with the redefinition of the class Ulvophyceae proposed by van den Hoek et al. (1995: Algae. An introduction to phycology. Cambridge University Press). A new genus Desmochloris Shin Watanabe, Kuroda & Maiwa gen. nov. is proposed to accommodate C. halophila.