Systematics of the green algal family Hydrodictyaceae (Sphaeropleales, Chlorophyceae), with special reference to the genus Pediastrum Meyen 1829

Date of Completion

January 2007


Biology, Botany




Accurate recognition of green algal species is essential for ecological studies, estimates of biodiversity, reconstructing past climates, and biotechnology. This study combined molecular and morphological data to resolve evolutionary relationships among the genera of the freshwater family Hydrodictyaceae (Chlorophyta), and tested species boundaries within historic Pediastrum Meyen 1829. The first phylogenetic analyses, incorporating 34 ingroup isolates and the nuclear large subunit (26S rDNA) and internal transcribed spacer-2 (ITS-2 rDNA) sequence data, revealed the two-dimensional genus Pediastrum was not monophyletic, and the three-dimensional genera Hydrodictyon A. Roth 1797 and Sorastrum Kuetzing 1845 were nested among Pediastrum spp. This analysis also revealed the family was more diverse phylogenetically than previously hypothesized. Additional molecular phylogenetic analyses using 26S rDNA and chloroplast RuBisCo (rbcL) sequence data were performed on 103 ingroup isolates, 44 of which were from culture collections representing North America and Europe, and the remaining 59 were wild isolates collected from North America, Europe and Australia, all unique to this study. The results supported the previous indications in the first phylogenetic analyses that the P. duplex morphotype was polyphyletic, recovering the morphotype in three distinct lineages, and allowed testing of the recent taxonomic revisions of the family that split Pediastrum into five genera. Some revisions were supported (erection of Stauridium and Monactinus), while others were not (Pediastrum, Pseudopediastrum, Parapediastrum). A landmark-based morphometric analysis was performed to determine if the three lineages possessing Pediastrum duplex morphology were distinguishable. Two of the groups were not morphologically distinguishable, while the significant separation of the third group supported its distinct phylogenetic placement. The erection of a new genus is recommended to recognize the third group as a distinct taxonomic unit.^ This study supports the conclusions of other recently published investigations of microscopic eukaryotes, that the morphospecies concept is inadequate for accurately recognizing species lineages of eukaryotic microorganisms because the diversity of these organisms is poorly captured and vastly underestimated by phenotype alone. The inclusion of additional forms of data is therefore integral for accurate assessments of species boundaries and biodiversity. ^