Yamagishiella, based onPandorina unicoccaRayburn et Starr, is distinguished fromEudorinaby its isogamous sexual reproduction, whereasPlatydorinaexhibits anisogamous sexual reproduction. In the present study, we sequenced the large subunit of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (rbcL) genes from five Japanese and North American strains of Y. unicocca (Rayburn et Starr) Nozaki, twoPlatydorina caudataKofoid strains, and two strains ofEudorina unicoccaG. 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 generaYamagishiellaandPlatydorina.Two robust monophyletic groups of the anisogamous/oogamous volvocacean species were resolved in the maximum‐parsimony tree as well as in the neighbor‐joining distance tree. One of the two groups comprises three species ofVolvoxsectionVolvox,whereas the other is composed of other sections ofVolvoxas well as of all the species ofEudorinaandPleodorina. Platydorina,however, was positioned outside these two monopliyletic groups. Therefore, derivation of thePlatydorinalineage may be earlier than that of such anisogamous/oogamous groups, or orgin of “anisogamy with sperm packets” inPlatydorinamay be independent of sperm packet evolution inEudorina, Pleodorina,andVolvox.It was also resolved with high bootstrap values that all of theY. unicoccastrains form a monophyletic group positioned outside the large monophyletic group includingEudorinaandPleodorina.These reject the possibility of the reverse evolution of isogamy from anisogamy to give rise toYamagishiellawithin the lineage of Eudorina.
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Nozaki et al. (1997) studied this question.