Rediscovery of the species of 'ancestral Volvox': morphology and phylogenetic position of Pleodorina sphaerica (Volvocales, Chlorophyceae) from Thailand.

Pleodorina sphaerica Iyengar was considered to be a phylogenetic link between Volvox and the type species Pleodorina californica Shaw because it has small somatic cells distributed from the anterior to posterior poles in 64- or 128-celled vegetative colonies. However, cultural studies and molecular and ultrastructural data are lacking in P. sphaerica, and this species has not been recorded since 1951. Here, we performed light and electron microscopy and molecular phylogeny of P. sphaerica based on newly established culture strains originating from Thailand. Morphological features of the present Thai species agreed well with those of the previous studies of the Indian material of P. sphaerica and with those of the current concept of the advanced members of the Volvocaceae. The present P. sphaerica strains exhibited homothallic sexuality; male and facultative female colonies developed within a single clonal culture. Chloroplast multigene phylogeny demonstrated that P. sphaerica was sister to two other species of Pleodorina (P. californica and Pleodorina japonica Nozaki) without posterior somatic cells, and these three species of Pleodorina formed a robust clade, which was positioned distally in the large monophyletic group including nine taxa of Volvox sect. Merrillosphaera and Volvox (sect. Janetosphaera) aureus Ehrenberg. Based on the present phylogenetic results, evolutionary losses of posterior somatic cells might have occurred in the ancestor of P. californica and P. japonica. Thus, P. sphaerica might represent an ancestral morphology of Pleodorina, rather than of Volvox.