1Department of Biology; Vanderbilt University,
Nashville, Tennessee 37235, USA
2Station Biologique, BP?74, 29682 Roscoff
Cedex FRANCE
3Department of Biological Sciences; University
of Alberta; Edmonton, Alberta T6G 2E9,
Canada; ccameron@gpu.srv.ualberta.ca
4Department of Entomology, University of Wisconsin,
Madison, Wisconsin 53706, USA;
corley@entomology.wisc.edu
5Department of Biological Sciences, University
of South Florida, 4202 East Fowler
Avenue SCA 110, Tampa, Florida 33620-5150,
USA; garey@chuma.cas.usf.edu
6Present address: Department of Zoology, Box
351800, University of Washington,
Seattle, WA 98195-1800, USA; BJS18@psu.edu
Abstract. - Understanding the phylogenetic relationships of the
three major urochordate
groups within the deuterostomes is central to understanding the evolution
of the
chordates. We have prepared a detailed phylogenetic analysis of urochordates
based on
comparisons of 10 new urochordate 18S ribosomal DNA sequences with
other
urochordate sequences in GenBank. Maximum parsimony, Neighbor-Joining,
Minimum
Evolution and Maximum Likelihood analyses of this large urochordate
dataset are
consistent with a topology in which the urochordates are monophyletic
within the
deuterostomes and there are four separate clades of urochordates. These
four distinct
clades - styelid + pyurid ascidians, molgulid ascidians, phlebobranch
ascidians +
thaliaceans, and larvaceans - are mostly consistent with traditional
morphological
hypotheses and classifications. However, we find that the ascidians
may not be a
monophyletic group, as they have been considered traditionally, but
instead appear
polyphyletic. Another disparity with traditional classification
is that the thaliaceans do
not form a separate urochordate clade, rather they cluster with the
phlebobranch
ascidians. Larvaceans have long branch lengths, which can be
problematic for molecular
phylogenetic methods, and their position within the urochordates cannot
be unequivocally
determined with 18S rDNA. This is important because the tadpole morphology
of
larvacean and ascidian larvae is the key trait of interest that distinguishes
urochordates as
chordates. Nevertheless, the present data set resolves at least
three clades of urochordates
and suggests strongly that urochordates form a monophyletic clade within
the
deuterostomes.
Figure 1. The neighbor-joining tree was calculated using
Kimura two-parameter
evolutionary distances with branches drawn to scale. The
major urochordate groups are
labeled as Clades I, II, III, and IV and do not completely agree with
morphology-based
urochordate phylogenetic hypotheses. Bootstrap values are
shown as percentages of
1000 replicates at each node only if they are 50% or greater.
Urochordate families are
marked with square brackets and urochordate order names are underlined.
NJ trees
calculated with a number of other evolutionary distance methods were
consistent with the
tree shown above.
Figure 2. Majority rule consensus of 100 bootstrap replicates
of minimum evolution trees
recovered using Kimura two-parameter evolutionary distances.
Nodes with less than
50% bootstrap support are shown collapsed. The distribution of
species with a colonial
life-cycle are marked as shown to demonstrate the polyphyly of colonialism
among
urochordates. Clades I, II, III, and IV include the same taxa
as in Figure 1 and are
marked with braces. ME trees recovered using a number of other
evolutionary distance
methods were consistent with the tree shown above (see Table 2).
Figure 3. Majority rule consensus of 100 bootstrap replicates
of maximum parsimony
trees recovered using PAUP*. Two equally parsimonius trees of
1135 steps were
recovered in the initial analysis. Nodes with less than 50% bootstrap
support are shown
collapsed. Taxa within Clades I, II, and III+IV as defined in Figures
1 and 2 are marked
with braces.
Figure 4. Majority rule consensus of 100 bootstrap replicates
of maximum likelihood
trees recovered using DNAML and associated programs from PHYLIP.
A subset of the
data (24 of 48 taxa) used for Figures 1-3 was analyzed. Nodes
with less than 50%
bootstrap support are shown collapsed. Taxa within Clades I,
II, and III+IV as defined in
Figures 1 and 2 are marked with braces.
Figure 5. One possible tree showing the evolution of the
chordates and urochordates
based on molecular and morphological analyses. Possible morphological
characters
defining the nodes are shown. Characters are represented as black
bars at each node and
represent the following traits:
1. Chordata: notochord, pharyngeal gill slits, muscular post-anal tail,
dorsal nerve cord
2. Vertebrata + Cephalochordata: presence of somites
3. Urochordata: endostyle, tunic
4. Larvacea: planktonic adults which secrete a special mucus house
5. Urochordates which lose their tail after metamorphosis
6. Branchial sac without longitudinal folds
7. Branchial sac with longitudinal vessels with a discrete number of
folds
8. Solitary adults, single gonads, presence of symbiotic cyanobacteria