Multiple origins of colonial green flagellates from unicells: evidence from molecular and organismal characters.

Phylogenetic hypotheses generated from cladistic analysis of organismal and molecular data are shown to be generally congruent and/or complementary for comparisons of unicellular and colonial green algae in the Chlorophyceae. Cladistic analysis of organismal character data corroborates the alliance of colonial Stephanosphaera with unicellular Haematococcus (Haematococcaceae sensu Smith), inferred from previous studies of nuclear-encoded rRNA sequence data. The organismal data also support monophyly of the colonial Volvocaceae (sensu Smith). Alliances of other unicellular taxa, including those ascribed to the "Euchlamydomonas" Hauptgruppe (sensu Ettl), are not resolved by organismal characters principally because the structure of the data is skewed to shared ancestral characters (symplesiomorphies) and unique characters (autapomorphies) which define individual taxa only. Reanalysis of rRNA sequence data, with additional sequence data for critical taxa, does not support monophyly of the colonial Volvocaceae (sensu Smith). However, these data are weak in the support of the alternate hypothesis of nonmonophyly. In contrast, relationships among most unicellular flagellates are unambiguously resolved by the molecular data. Although the failure of the sequence data to resolve relationships among colonial flagellates appears to be due to a sampling of conservative sequences, an ancient, rapid radiation event or taxon sampling bias may also be contributing to the ambiguity problem. Results from analysis of a combined data set (organismal and molecular) are generally consistent with the inferences of the organismal character data regarding the colonial flagellates and are also consistent with the inferences of the sequence data regarding the unicellular taxa.