Phylogenetic Relationships Within the Colonial Volvocales (Chlorophyta) Inferred From Cladistic Analysis Based on Morphological Data1

A cladistic analysis was used to deduce the phylogenetic relationships within the colonial Volvocales. Forty‐one pairs of characters related to gross morphology and ultrastructure of vegetative colonies as well as asexual and sexual reproduction were analyzed based on parsimony, using the PAUP 3.0 computer program, for 25 species belonging to nine volvocacean and goniacean genera of the colonial Volvocales.Chlamydomonas reinhardtiiDangeard was the outgroup. The strict consensus tree indicated the presence of two monophyletic groups, one composed of all the volvocacean species analyzed in this study and the other containing the goniacean species except for the four‐celled speciesGonium sociale(Dujardin) Warming. In addition, these two groups constitute a large monophyletic group, to whichG. socialeis a sister group. A new combinationTetrabaena socialis(Dujardin) Nozaki et Itoh and a new family Tetrabaenaceae Nozaki et Itoh are thus proposed forG. sociale.In addition, the analysis suggests that the volvocacean generaEudorinaandPleodorinaare paraphyletic groups, respectively, and that the monotypic genusYamagishiellahas no autapomorphic characters and represents primitive features of the anisogamous and oogamous genera of the Volvocaceae. Phylogenetic relationships within the Volvocaceae and the Goniaceae, as well as the various modes of sexual reproduction exhibited by these organisms, are discussed on the basis of the analysis.