PHYLOGENETIC ANALYSIS OF YAMAGISHIELLA AND PLATYDORINA (VOLVOCACEAE, CHLOROPHYTA) BASED ON RBCL GENE-SEQUENCES

Yamagishiella, based on Pandorina unicocca Rayburn et Starr is disting uished from Eudorina by its isogamous sexual reproduction, whereas Pla tydorina exhibits anisogamous sexual reproduction. In the present stud y, rue sequenced the large subunit of ribulose-1,5-bisphosphate carbox ylase/oxygenase (rbcL) genes from five Japanese and North American str ains of Y. unicocca (Rayburn et Stair) Nozaki, true Platydorina caudat a Kofoid strains, and two strains of Eudorina unicocca G. M. Smith, as well as eight related colonial and unicellular species. Phylogenetic trees were constructed based on these sequence data and on previously published rbcL gene sequences from 23 volvocalean species in order to deduce phylogenetic relationships within the colonial Volvocales, with particular regard to the phylogenetic positions and status of the gen era Yamagishiella and Platydorina. Two robust monophyletic groups of t he anisogamous/oogamous volvocacean species were resolved in the maxim um-parsimony tree as well as in the neighbor-joining distance tree. On e of the two groups comprises three species of Volvox section Volvox, whereas the other is composed of other sections of Volvox as well as o f all the species of Eudorina and Pleodorina. Platydorina, however, wa s positioned outside these two monophyletic groups. Therefore, derivat ion of the Platydorina lineage may be earlier than that of such anisog amous/oogamous groups, or origin of ''anisogamy with sperm, packets'' in Platydorina may De independent of sperm packet evolution in Eudorin a, Pleodorina, and Volvox. It was also resolved with high bootstrap va lues that all of the Y. unicocca strains form a monophyletic group pos itioned outside the large monophyletic group including Eudorina and Pl eodorina. These reject the possibility of the reverse evolution of iso gamy from anisogamy to give rise to Yamagishiella within the lineage o f Eudorina.