Nozaki H

Origin and evolution of the genera Pleodorina and Volvox (Volvocales)
BIOLOGIA 58 (4): 425-431 JUL 2003

Abstract:
The previous molecular phylogenetic study using 6021 base pairs from five chloroplast genes suggested that two species of Pleodorina (P. californica, P. japonica) might have evolved from a Volvox-like alga by the decrease in colony cell number and size. However, number of species of the genus Volvox was very limited especially in the section Merrillosphaera.

In the present study, 6021 base pairs of the concatenated five chloroplast genes from 10 strains representing seven taxa of the genus Volvox were added to the previous data matrix. The sequence data resolved two anisogamous/oogamous clades within a large monophyletic group comprising five advanced genera of the Volvocaceae (Yamagishiella, Platydorina, Eudorina, Pleodorina and Volvox), one containing Volvox sect. Volvox and the anisogamous genus Platydorina (32-celled flattened colony), and the other (Eudorina group) composed of three other sections of Volvox, Pleodorina and Eudorina. The isogamous genus Yamagishiella (32-celled colony) was positioned basally to the Eudorina group. Therefore, evolution of anisogamy with sperm packets from isogamy might have occurred twice within the Volvocaceae. Based on the present molecular phylogenetic analysis, species of Volvox and Pleodorina within the Eudorina group represented three and two, respectively, separate lineages. One the three Volvox lineages [composed of V (sect. Merrillosphaera) carteri, V (sect. Merrillosphaera) obversus, V. (sect. Merrillosphaera) tertius, V. (sect. Merrillosphaera) africanus and V (sect. Copelandosphaera) dissipatrix] was sister to the monophyletic group consisting of one of the two Pleodorina lineages (P. californica and P. japonica) and V (sect. Janetosphaera) aureus. In addition, species of Eudorina were basal to the two lineages of Pleodorina and three Volvox lineages within the Eudorina group, representing the ancestral situation of Pleodorina/Volvox (excluding sect. Volvox). Thus, reverse evolution from a Volvox-like alga to Pleodorina suggested previously appears unlikely.