Reexamination of phylogenetic relationships within the colonial Volvocales(Chlorophyta): An analysis of atpB and rbcL gene sequences

The chloroplast-encoded atpB gene was sequenced from 33 strains representin g 28 species of the colonial Volvocales (the Volvocaceae and its relatives) to reexamine phylogenetic relationships as previously deduced by morpholog ical data and rbcL gene sequence data.1128 base pairs in the coding regions of the atpB gene were analyzed by MP, NJ, and ML analyses, Although suppor ted with relatively low bootstrap values (75% and 65% in the NJ and ML anal yses, respectively), three anisogamous/oogamous volvocacean genera-Eudorina , Pleodorina, and Volvox, excluding the section Volvox(= Euvolvox, illegiti mate name), constituted a large monophyletic group (Eudorina group), Outsid e the Eudorina group, a robust Lineage composed of three species of Volvox sect, Volvox was resolved as in the rbcL gene trees, rejecting the hypothes is of the previous cladistic analysis based on morphological data that the genus Volvox is monophyletic, In addition, the NJ and ML trees suggested th at Eudorina is a non-monophyletic genus as inferred from the morphological data and rbcL gene sequences. Although phylogenetic status of the genus Gon ium is ambiguous in the rbcL gene trees and the paraphyly of this genus is resolved in the cladistic analysis based on morphological data, the atpB ge ne sequence data suggest monophyly of Gonium with relatively low bootstrap values (56-61%) in the NJ and ML trees. On the basis of the combined sequen ce data (2256 base pairs) from atpB and rbcL genes, Gonium was resolved as a robust monophyletic genus in the NJ and ML trees (with 68-86% bootstrap v alues), and Eudorina elegans Ehrenberg represented a paraphyletic species p ositioned most basally within the Eudorina group, However, phylogenetic sta tus and relationships of the families of the colonial Volvocales were still almost ambiguous even in the combined analysis.