Molecular Biology and Evolution, Vol 9, 85-105, Copyright © 1992 by Society for Molecular Biology and Evolution
A Larson, MM Kirk and DL Kirk
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.
ORIGINAL ARTICLE
Molecular phylogeny of the volvocine flagellates
Department of Biology, Washington University, St. Louis, Missouri 63130.
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