A new species of Volvox sect. Merrillosphaera from Texas
Hisayoshi Nozaki1, and Annette W. Coleman2
1) Dept. Biol. Sciences, Graduate School of Science, Univ. of Tokyo, Tokyo, Japan
2) Brown University, Providence, RI 02912, USA
Volvox is a famous genus that was originally described by Linnaeus (1758, Syst. Nat.). Smith (1944, Trans. Am. Microsc. Soc.) classified this genus into four sections, based on the difference in vegetative colonies, such as presence or absence of the cytoplasmic bridges between cells. Section Merrillosphaera is characterized by lacking cytoplasmic bridges between cells, and includes seven species (Smith 1944; Nozaki 1988, Phycologia). Since Smith (1944), no additional species have been reported in this section. In the present study, a strain ("Volvox Ovale") originating from Texas was studied by light microscopy and molecular phylogeny. Asexual colonies consisted 1,000-2,000 cells without cytoplasmic bridges and were ovoid or ellipsoidal with an expanded anterior face. Eight to 12 gonidia were distributed in posterior 2/3 of the colony. During the daughter colony formation, gonidia of the next generation were not evident due to the lack of unequal cytokinesis. In having 8-12 gonidia and lack of unequal cytokinesis during daughter colony formation, "Volvox Ovale" resembles V. tertius and V. spermatosphaera (Smith 1944, R. C. Starr personal communication). However, it differs from V. spermatosphaera in the pattern of distribution of the gonidia in the colony. Furthermore, the present light microscopical examination demonstrated differences in chloroplast morphology in gonidia between "Volvox Ovale" and V. tertius (UTEX 132 and NIES-544). ITS2 sequences resolved that "Volvox Ovale" is most closely related to V. spermatosphaera.
e-mail address of presenting author: nozaki@biol.s.u-tokyo.ac.jp