Phylogeny of the colonial Volvocales

Abstract

Recent phylogenetic studies of the colonial Volvocales are reviewed. They are based on morphological data of vegetativ eultrastructure and life cycle or nucleotide sequence data. Comaparative morphological studies of the sexual reproduction of the colonial Volvocales suggested that the four-celled species Gonium sociale is placed in a phylogenetic position separated from other members of the colonial Volvocales(Nozaki 1986). A cladistic analysis based on morphological data also indicated the peculiar phylogentic position of G. sociale(thus transferred to another genus Tetrabaena)and suggested that the evolution from isogamy to anisogamy had occurred once within the colonial Volvocales(Nozaki & Itoh 1994). However, molecular phylogeny based on rRNA sequence data resolved multiple evolution of anisogamy as well as reverse evolution from anisogamy to isogamy within such organisms(Larson et al. 1992). More recent molecular phylogenetic research based on rbcL sequence data of the colonial Volvocales(Nozaki et al. 1995)supports the morphological study(Nozaki & Itoh 1994), with regard to the evolution of isogamy/anisogamy.