NOZAKI H, KUROIWA T

ULTRASTRUCTURE OF THE EXTRACELLULAR-MATRIX AND TAXONOMY OF EUDORINA, PLEODORINA AND YAMAGISHIELLA GEN-NOV (VOLVOCACEAE, CHLOROPHYTA)
PHYCOLOGIA 31 (6): 529-541 NOV 1992

Abstract:
Vegetative colonies of Pandorina unicocca Rayburn et Starr, four taxa of Eudorina [E. elegans Ehrenberg (type species), E. illinoisensis (Kofoid) Pascher, E. unicocca G.M. Smith var. unicocca and E. unicocca var. peripheralis Goldstein] and two species of Pleodorina [P. californica Shaw (type species) and P. indica (Iyengar) Nozaki] were examined with electron microscopy in order to characterize the structure of the extracellular matrix. Each cell of the colonies of all the taxa examined was tightly enclosed by a dense fibrillar zone of the extracellular matrix (cellular envelope) with sparse fibrillar material filling the space outside the cellular envelopes within the tripartite colonial boundary of the matrix. This arrangement is essentially different from that of Pandorina morum (O.F. Muller) Bory (type species) and P. colemaniae Nozaki. As Eudorina and Pleodorina both have anisogamous sexual reproduction with sperm packets (bundles of male gametes), a new genus, Yamagishiella Nozaki, is proposed for encompassing the isogamous species Yamagishiella unicocca (Rayburn et Starr) Nozaki comb. nov. [Pandorina unicocca].

LARSON A, KIRK MM, KIRK DL

MOLECULAR PHYLOGENY OF THE VOLVOCINE FLAGELLATES
MOL BIOL EVOL 9 (1): 85-105 JAN 1992

Abstract:
Phylogenetic studies of approximately 2,000 bases of sequence from the large and small nuclear-encoded ribosomal RNAs are used to investigate the origins of the genus Volvox. The colonial and multicellular genera currently placed in the family Volvocaceae form a monophyletic group that is significantly closer phylogenetically to Chlamydomonas reinhardtii than it is to the other unicellular green flagellates that were tested, including Chlamydomonas eugametos, Chlorella pyrenoidosa, and Haematococcus lacustris. Statistical analysis of 251 phylogenetically informative nucleotide positions rejects the "volvocine lineage" hypothesis, which postulates a monophyletic evolutionary progression from unicellular organisms (such as Chlamydomonas), through colonial organisms (e.g., Gonium, Pandorina, Eudorina, and Pleodorina) demonstrating increasing size, cell number, and tendency toward cellular differentiation, to multicellular organisms having fully differentiated somatic and reproductive cells (in the genus Volvox). The genus Volvox appears not to be monophyletic. Volvox capensis falls outside a lineage containing other representatives of Volvox(V. aureus, V. carteri, and V. obversus), and both of these Volvox lineages are more closely related to certain colonial genera than they are to each other. This implies either a diphyletic origin of Volvox from different colonial volvocacean ancestors, a phylogenetic derivation of some of the colonial genera from a multicellular (i.e., Volvox) ancestor, or both. Considered together with previously published observations, these results suggest that the different levels of organizational and developmental complexity found in the Volvocaceae represent alternative stable states, among which evolutionary transitions have occurred several times during the phylogenetic history of this group.