PHYLOGENETIC-RELATIONSHIPS WITHIN THE COLONIAL VOLVOCALES (CHLOROPHYTA) INFERRED FROM RBCL GENE SEQUENCE DATA

The chloroplast-encoded large subunit of the ribulose-1, 5-bisphosphat e carboxylase/oxygenase (rbcL) gene was sequenced from 20 species of t he colonial Volvocales (the Volvacaceae, Goniaceae, and Tetrabaenaceae ) in order to elucidate phylogenetic relationships within the colonial Volvocales. Eleven hundred twenty-eight base pairs In the coding regi ons of the (rbcL) gene were analyzed by the neighbor-joining (NJ) meth od using three kinds of distance estimations, as well as by the maximu m parsimony (MP) method. A large group comprising all the anisogamous and oogamous volvocacean species was resolved in the MP tree as well a s in the NJ trees based on overall and synonymous substitutions. In al l the trees constructed, Basichlamys and Tetrabaena (Tetrabaenaceae) c onstituted a very robust phylogenetic group. Although not supported by high bootstrap values, the MP tree and the NJ tree based on nonsynony mous substitutions indicated that the Tetrabaenaceae is the sister gro up to the large group comprising the Volvocaceae and the Goniaceae. In addition, the present analysis strongly suggested that Pandorina and Astrephomene are monophyletic genera whereas Eudorina is nonmonophylet ic. These results are essentially consistent with the results of the r ecent cladistic analyses of morphological data. However, the monophyly of the Volvocaceae previously supported by four morphological synapom orphies is found only in the NJ tree based on nonsynonymous substituti ons (with very low bootstrap values). The genus Volvox was clearly res olved as a polyphyletic group with V. rousseletii Pocock separated fro m other species of volvox in the rbcL gene comparisons, although this genus represents a monophyletic group in the previous morphological an alyses. Furthermore, none of the rbcL gene trees supported the monophy ly of the Goniaceae; Astrephomene was placed in various phylogenetic p ositions.