1 Kewalo Marine Laboratory, Pacific Biomedical Research Center, University of
Hawaii, 41 Ahui Street, Honolulu, HI 96813, USA
2 Department of Biology, Boston University, 5 Cummington Street, Boston, MA
02215, USA
* Author for correspondence (e-mail: mqmartin{at}hawaii.edu)
Accepted 11 February 2004
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SUMMARY |
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Key words: Germlayer, Evolution, Nematostella vectensis, Cnidaria
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Introduction |
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Cnidarians are a large and successful phylum of animals that diverged from
the Bilateria perhaps 600 million years ago. Early in the evolutionary history
of cnidarians, the phylum split into two major lineages
(Fig. 1): the class Anthozoa
(anemones and corals) and its sister group, the Medusozoa
(Bridge et al., 1995;
Bridge et al., 1992
;
Odorico and Miller, 1997
;
Schuchert, 1993
;
Collins, 2002
). The Medusozoa
comprises three classes: Hydrozoa (hydras and hydromedusae), Scyphozoa (true
jellyfishes) and Cubozoa (box jellyfishes). Most medusozoans display a
biphasic life cycle where an asexual polyp phase alternates with a sexually
reproducing medusa (jellyfish) phase. The medusa phase was subsequently lost
in certain lineages such as the freshwater hydras. Anthozoans possess only a
polyp, and the polyp can reproduce by asexual or sexual means.
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There are no definitive muscle cells in cnidarians. However, the epithelial cells of the gastrodermis (and epidermis of hydrozoans) have myoepithelial extensions on their basal surfaces. In anthozoan cnidarians (anemones and corals), longitudinal myoepithelial processes are concentrated on structures known as mesenteries. The mesenteries are lamellae consisting of two layers of gastrodermal epithelium separated by an intervening layer of mesoglea. The mesenteries radiate outward from the throat or pharynx, to the outer body wall. The pharynx is a tubular intrusion of the outer body wall that projects from the mouth into the spacious gastrovascular cavity, or coelenteron (Fig. 1). The mesenteries provide structural support for the pharynx, they increase the gastrodermal surface area, and they serve as the site of gamete production.
The starlet sea anemone, Nematostella vectensis, has recently been
developed as a model system to investigate cnidarian development.
Nematostella is a small, solitary, burrowing sea anemone found in
coastal estuaries along the Atlantic and Pacific coasts of North America and
the southeast coast of England (Hand and
Uhlinger, 1994). Nematostella has many practical
advantages as a developmental model, including a simple body plan and a simple
life history. It is a hardy species, easy to culture
(Hand and Uhlinger, 1992
) and
will spawn readily throughout the year under laboratory conditions
(Fritzenwanker and Technau,
2002
; Hand and Uhlinger,
1992
). Sexes are separate and fertilized embryos develop rapidly
to juvenile adults bearing four tentacles. After fertilization, cleavage
generates a hollow blastula (Fig.
2A). Gastrulation occurs by invagination and ingression
(Fig. 2B) (C. Byrum and M.Q.M.,
unpublished). The blastopore becomes the `mouth', the sole opening into the
gastrovascular cavity. After gastrulation, the swimming planula larva assumes
a teardrop shape (Fig. 2). At
this point, the larva consists of a ciliated epidermis surrounding a solid
core of presumptive mesendodermal cells. In the planula larva, the lumen of
the gut begins to resolve itself as the first two of eight mesenteries arise
from the thickened epidermis of the pharynx
(Fig. 2C). The planula settles
after a few days and generates a primary polyp with four tentacles
(Fig. 2D). The mature adult
polyp measures 1-6 cm in length and possesses 12-16 tentacles
(Fig. 2E).
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In Bilateria, genes associated with mesoderm specification are predominantly expressed in the mesoderm or presumptive mesoderm. By characterizing the developmental expression of cnidarian homologs, we may gain insights into the earliest stages of mesoderm developmental evolution. For each cnidarian gene: (1) expression may occur predominantly in the endoderm; (2) expression may occur predominantly in the ectoderm; or (3) expression may not exhibit a germ-layer bias. Six of the seven genes described here exhibit endodermally restricted expression during early development. Only mef2 is expressed exclusively in ectodermal derivatives. The germ layer restricted expression of these genes suggests that they may play a role in germ-layer specification. Furthermore, the overwhelming predominance of endodermal expression supports the hypothesis that both the endoderm and mesoderm of triploblasts evolved from the endoderm of diploblasts. However, a plausible evolutionary argument can be made that the diploblastic condition of cnidarians is a secondary simplification of a triploblastic ancestor.
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Materials and methods |
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Determination of gene orthology
The evolutionary relationships of the Nematostella sequences were
determined by neighbor-joining analyses based on predicted amino acid
sequences. The amino acid sequences included in each analysis were selected
through BLAST searches aimed at identifying homologous protein domains from
potential orthologs and outgroup sequences. We included sequences from other
cnidarian species where available, from the sequenced human genome (a
representative deuterostome bilaterian) and from the sequenced
Drosophila genome (a protostome bilaterian). Conserved domains were
aligned using an Internet implementation of ClustalW at the website of the
European Bioinformatics Institute
(http://www.ebi.ac.uk/clustalw).
Default alignment parameters were used (Matrix, BLOSUM; GapOpen, 10; GapExt,
0.05; GapDist, 8). Orthology of Nv-forkhead was determined using an
81 amino acid alignment spanning part of the conserved forkhead domain.
Orthology of Nv-GATA was assessed using an 80 amino acid stretch that
encompasses a C2C2 zinc finger motif. The Nv-mef2 analysis used 130
amino acid positions spanning the conserved Mef2/MADS box motif of mef2,
serum response factor and blistered. The Nv-muscle LIM
analysis was based on 62 amino acids spanning the LIM domain. The evolutionary
relationships of NvsnailA and Nv-snailB were inferred from
105 amino acid positions spanning four C2H2 zinc-finger
motifs from snail, scratch and Kruppel genes. The
Nv-twist analysis incorporated a conserved 56-residue region of the
twist, atonal and nautilus genes. In each phylogenetic
analysis, the support for specific clades was assessed by 2000 replications of
the bootstrap (Felsenstein,
1985). Bootstrap proportions equaling or exceeding 40% are
shown.
Gene expression
Embryos from various stages were fixed in fresh ice-cold 3.7% formaldehyde
with 0.2% gluteraldehyde in 1/3x seawater for 90 seconds and then
post-fixed in 3.7% formaldehyde in 1/3x seawater at 4°C for 1 hour.
Fixed embryos were rinsed five times in Ptw (PBS buffer plus 0.1% Tween-20)
and once in deionized water, and transferred to 100% methanol for storage at
-20°C. Early embryos were removed from the jelly of the egg mass by
treating with freshly made 2% cysteine in 1/3x seawater (pH 7.4-7.6) for
10-15 minutes. Planula and polyp stages were relaxed in 7% MgCl2 in
1/3x seawater for 10 minutes prior to fixation.
In situ hybridization using 1-2 kb digoxigenin-labeled riboprobes were
performed to determine the spatial and temporal distribution of transcripts as
previously described (Finnerty et al.,
2003). Although both 5' and 3' RACE fragments were
recovered for most genes, 3' fragments tended to be longer and were used
for probe construction (MegaScript, Ambion, Austin, TX). Longer probes
generated better signal to noise ratios and shortened developing time. Probe
concentration ranged from 0.05-1.00 ng/µl and hybridizations were performed
at 65°C for 20-44 hours. Probe detection was achieved by incubation with
an anti-digoxigenin antibody conjugated to alkaline phosphatase (Roche).
Subsequently, the presence of alkaline phosphatase was detected by a
colorimetric detection reaction using the substrate NBT-BCIP. Specimens were
photographed on a Zeiss Axioplan with a Nikon Coolpix 990 digital camera.
Detailed protocols are available upon request.
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Results |
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Six unique winged-helix containing transcripts were isolated from
Nematostella. One of these transcripts, Nv-forkhead, is 1628
nucleotides long, and it encodes a predicted protein 285 amino acids long.
Within the forkhead domain, the Nematostella sequence is roughly 90%
identical to Drosophila forkhead and the FOXA2 sequence of humans
(Fig. 3A). Phylogenetic
analysis places Nv-forkhead squarely within the larger
forkhead clade, most closely related to Drosophila forkhead,
FOXA2, and the Hydra budhead genes
(Fig. 3B). Similar to
budhead (Martinez et al.,
1997), Nematostella forkhead exhibits several
phylogenetically conserved residues within transcriptional activation domain
II, and a handful of conserved residues within transcriptional activation
domain III (Fig. 3A)
(Pani et al., 1992
).
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Nv-GATA
The GATA genes constitute a family of zinc-finger transcription
factors that bind the GATA motif, a widespread cis-regulatory element found in
many promoters (Evans et al.,
1988). GATA-binding proteins can be recognized by the presence of
one or two distinctive zinc-finger motifs of the form CXNCX17CNXC
(Fig. 4A). In vertebrates, GATA
transcription factors are implicated in the development of many mesodermal
cell types including red and white blood cells, smooth muscle, cardiac muscle,
adipocytes and gonadal cells (Arceci et
al., 1993
; Ketola et al.,
2000
; Laitinen et al.,
2000
; McDevitt et al.,
1997
; Morrisey et al.,
1998
; Tong et al.,
2000
; Tsai et al.,
1994
). The Drosophila GATA transcription factor,
grain, has been shown to impact epithelial morphogenesis by affecting
cell rearrangements (Brown and
Castelli-Gair Hombria, 2000
).
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Nv-GATA expression is first detected in individual cells around the circumference of the blastula (Fig. 4C,D). Nv-GATA expression appears absent from the extreme oral and aboral poles of the blastula (Fig. 4D). In the late blastula/early gastrula, Nv-GATA-expressing cells appear to ingress into the blastocoel (Fig. 4D) and by late gastrula/early planula stages all of the gastrodermal cells lining the coelenteron appear to express Nv-GATA, while the cells of the pharynx do not express Nv-GATA (Fig. 4E-F). In the early polyp stages (Fig. 4G-H), expression is confined to a subset of gastrodermal cells distributed all along the oral-aboral axis, with the strongest expression near the oral pole. Ectodermal staining also appears at this time, in the base of the tentacles (Fig. 4G-H), but not in the tentacles themselves.
Nv-mef2
The mef2 genes are MADS-box transcription factors. Four
mef2 genes are known from vertebrates (MEF2A-D). Drosophila
and C. elegans each possess a single mef2 gene. In both the
vertebrates and fruitfly, mef2 genes are preferentially expressed in
muscle and mesodermal tissues where they are essential for muscle development
(Black and Olson, 1998;
Lilly et al., 1994
). However,
in both vertebrates and Drosophila, mef2 can also be detected in
non-muscle and non-mesodermal tissues
(Black and Olson, 1998
;
Schulz et al., 1996
).
Surprisingly, despite a high degree of sequence conservation, the C.
elegans mef2 gene is not essential for muscle development
(Dichoso et al., 2000
). In the
hydrozoan jellyfish Podocoryne, mef2 expression is widespread and
highly dynamic (Spring et al.,
2002
). mef2 can be detected throughout the complex life
history of Podocoryne including the unfertilized egg, the blastula,
the gastrula, the planula larva, the polyp, the attached medusa and the
free-living medusa stage. In early developmental stages, the transcript does
not appear highly localized. After gastrulation, Podocoryne mef2 may
be expressed in the endoderm or the ectoderm, or even the entocodon of the
medusa, an intermediate tissue layer which is hypothesized to have homology
with mesoderm (Boero et al.,
1998
). Podocoryne mef2 is expressed in precursors of both
muscle and non-muscle cells.
The Nv-mef2 transcript is 2229 nucleotides encoding a predicted
protein of 209 amino acids. Nv-mef2 is over 90% identical to
bilaterian mef2 genes in the MADS domain
(Fig. 5A). A neighbor-joining
analysis including the MADS domain from Nv-mef2, plus other
bilaterian and cnidarian mef2 genes and putative outgroup genes
(blistered and serum response factor), places the
Nematostella sequence solidly within the mef2 radiation
(Fig. 5B). As expected,
Nv-mef2 appears most closely related to mef2 from the
hydrozoan jellyfish Podocoryne
(Spring et al., 2002).
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Nv-muscle LIM
LIM proteins are characterized by the presence of one to five LIM domains
(Stronach et al., 1996), a
double zinc-finger motif known to be involved in protein dimerization
(Feuerstein et al., 1994
).
Many LIM proteins also possess a DNA-binding homeodomain adjacent to the LIM
domains. Members of the cysteine-rich protein family of LIM proteins can
associate with the actin cytoskeleton when expressed in rat fibroblast cells
(Louis et al., 1997
;
Stronach et al., 1996
). These
proteins are known to be involved with muscle differentiation in
Drosophila (Stronach et al.,
1996
) and vertebrates (Louis
et al., 1997
).
The Nv-muscle LIM transcript is 523 nucleotides long, and it
encodes a predicted protein of 73 amino acids. This short peptide is very
similar in scale and sequence to the peptides encoded by the vertebrate
cysteine-rich protein 1 [CRIP1, 77 amino acids
(Tsui et al., 1994)] and the
Drosophila muscle LIM protein Mlp60A [92 amino acids
(Stronach et al., 1996
)].
Nv-muscle LIM encodes a single canonical LIM domain with two zinc
fingers connected by a short linker
([CX2CX17HX2C]X2-[CX2CX17CX2C]),
but it does not encode a homeodomain (Fig.
6A). The neighbor-joining analysis
(Fig. 6B) places Nv-muscle
LIM as the sister group to a clade containing Drosophila muscle
LIM proteins (Mlp60A and Mlp84A) and human cysteine
rich proteins (CSRP1-3).
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Nv-snailA and Nv-snailB
Proteins in the Snail family possess 4-6
C2H2 zinc fingers
(Hemavathy et al., 2000). In
addition, several snail family members in the fruitfly genome possess
a short conserved motif at the amino terminus called the NT box
(Hemavathy et al., 2000
).
Vertebrate snail proteins display a different conserved motif at the
N terminus known as the SNAG domain. Although the function of the NT box is
unknown, the SNAG domain is implicated in nuclear localization and
transcriptional repression (Grimes et al.,
1996
). Snail proteins play a phylogenetically widespread role in
the development of mesoderm (Hemavathy et
al., 2000
). Snail is expressed in the blastoderm at the
time of mesoderm specification in phylogenetically diverse bilaterians,
including Drosophila (Kosman et
al., 1991
; Leptin,
1991
), non-vertebrate deuterostomes
(Wada and Saiga, 1999
), and
vertebrates (Essex et al.,
1993
). Snail proteins are specifically implicated as
regulators of mesodermal invagination (reviewed by
Hemavathy et al., 2000
). Later
in development, snail plays a role in neurectodermal differentiation
in both Drosophila and vertebrates
(Essex et al., 1993
;
Kosman et al., 1991
;
Leptin, 1991
).
The Nv-snailA gene encodes a transcript 1565 nucleotides long and
the predicted protein spans 265 amino acids. The NvsnailB gene
encodes a transcript is 1729 nucleotides long and the predicted protein spans
272 amino acids. Both proteins possess four consecutive
C2H2 zinc fingers
(Fig. 7A). In the zinc-finger
region, the two Nematostella proteins are highly similar to each
other (82% identical) and other snail family members, specifically
worniu from Drosophila (82-83% identical) and
snail-2 from human (74-82% identical). Both NvsnailA and
Nv-snailB possess a conserved SNAG domain at the N terminus, sharing
eight out of nine and seven out of nine residues with vertebrate
snail-1, respectively. Neighbor-joining analysis based on the
zinc-finger region groups Nv-snailA and Nv-snailB together
in a larger clade comprising the snail gene of Podocoryne
(Spring et al., 2002),
snail-1 and snail-2 of human, and worniu and
escargot of Drosophila
(Fig. 7B).
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Nv-twist
Twist is a basic helix-loop-helix (bHLH) transcription factor that is
required for mesoderm specification in Drosophila and vertebrates
(Castanon and Baylies, 2002;
Thisse et al., 1988
;
Wolf et al., 1991
). The
Nv-twist transcript is 1182 nucleotides long, and it encodes a
predicted protein of 129 amino acids. Nv-twist displays two domains that are
highly conserved relative to other twist proteins. In the 53-residue bHLH
domain (positions 36 through 87), Nv-twist is identical to human
TWIST1 at 44 positions and Drosophila twist at 43 positions
(Fig. 8A). Nv-twist is
also identical to human TWIST1 and Podocoryne twist at all
14 residues of a conserved C-terminal motif known as the WR motif
(ERLSYAFSVWRMEG) (Spring et al.,
2000
). Phylogenetic analysis strongly supports the orthology of
Nv-twist to twist genes of Podocoryne, human, and
fruitfly (Fig. 8B).
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Discussion |
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The predominantly endodermal expression of six `mesodermal' genes studied
here suggests that these genes are playing a role in endoderm specification.
Therefore, it is most parsimonious to infer that these genes were involved in
germ layer specification in the cnidarian-bilaterian ancestor, and thus we can
rule out scenario one in favor of scenario two or three. We currently favor
scenario two, where these genes were involved specification of the endoderm in
diploblasts, and that as mesoderm evolved from the primordial endoderm, their
expression became associated with the presumptive mesoderm. A growing body of
evidence, both developmental (e.g. Henry
et al., 2000; Martindale and
Henry, 1999
) and molecular
(Maduro and Rothman, 2002
;
Rodaway and Patient, 2001
;
Stainier, 2002
) supports the
conclusion that mesoderm evolved from endoderm. Cell lineage studies in two
basal metazoans, the non-bilaterian ctenophore Mnemiopsis leidyi
(Martindale and Henry, 1999
)
and the acoel flatworm Neochildia fusca
(Henry et al., 2000
), a
putative basal bilaterian (Ruiz-Trillo et
al., 2002
; Ruiz-Trillo et al.,
1999
; Telford et al.,
2003
), reveal that mesodermal tissues arise exclusively from
endodermal precursors. In addition to the gene expression patterns reported
here, other genes implicated in mesodermal patterning and differentiation in
bilaterians are localized to the endoderm of Nematostella. These
include mox, bagpipe, tinman and muscle-specific tropomyosin
(J.R.F., M.Q.M. and K.P., unpublished). The widespread expression of
mesodermal genes in the gastrodermis of Nematostella is certainly
consistent with an endodermal origin for mesoderm.
However, the data presented here do not rule out the hypothesis that
triploblasty predated the divergence of Bilateria and Cnidaria (scenario
three). Cnidarian hydromedusae appear triploblastic as they possess a third
tissue layer that is independent of either the endoderm or the ectoderm - the
entocodon. The entocodon arises from polyp ectoderm, not endoderm, and it has
been hypothesized that the entocodon is homologous to the mesoderm of
bilaterians (Boero et al.,
1998; Muller et al.,
2003
; Spring et al.,
2002
; Spring et al.,
2000
). If so, then the cnidarian-bilaterian ancestor was a
triploblast, and diploblasty evolved within the Cnidaria by loss of the
mesoderm. The observed expression of mesodermal genes in Podocoryne,
however, is not generally supportive of this hypothesis. If the entocodon is
homologous to the mesoderm, then we should expect mesodermal genes to be
expressed in this tissue layer. However, none of the mesodermal genes whose
expression has been studied in Podocoryne are predominantly expressed
in the entocodon. For example, in Podocoryne, twist is barely
detectable by RT-PCR during embryonic, planula or polyp stages, and appears
only in the `striated muscle' (an epithelial sheet lining the subumbrellar
plate) of the medusa (Boero et al.,
1998
; Muller et al.,
2003
; Spring et al.,
2002
; Spring et al.,
2000
). Podocoryne snail
(Spring et al., 2002
) is
expressed weakly in the endoderm of early planula larvae and in the oral
ectoderm and tentacle endoderm of polyps. mef2
(Boero et al., 1998
;
Muller et al., 2003
;
Spring et al., 2002
;
Spring et al., 2000
) is
expressed at low levels at all life stages in the endoderm but never in any
localized fashion until the late planula stage where it is expressed in aboral
ectoderm. mef2 is also expressed in the entocodon of the
Podocoryne medusa, but as it is expressed in both endoderm and
ectoderm, it cannot be said to exhibit a preference for the entocodon.
Overall, the expression of `mesodermal genes' in Podocoryne appears
less rigidly restricted by germ layer, so the gene expression patterns do not
provide an unambiguous signal that the entocodon is homologous to the mesoderm
of bilaterians. Even at equivalent developmental stages, the expression
patterns of mesodermal genes in Nematostella are quite distinct from
those of Podocoryne.
Despite these differences between Nematostella and
Podocoryne, some genes have similar expression patterns in hydrozoans
and anthozoans. The expression of brachyury has been studied in both
Nematostella (Scholz and Technau, 2002) and Podocoryne
(Spring et al., 2002). The
early expression of brachyury in Nematostella and
Podocoryne appears to be similar (around the blastopore), although
expression diverges dramatically at later stages of development. The
forkhead gene hnf3/budhead has been studied in a freshwater
Hydra (Martinez et al.,
1997
). It is expressed in a band around lower half of the
hypostome, the ring of tissue lying between the tentacles and the prospective
mouth. In Nematostella, forkhead is expressed early in gastrulation
and is a robust marker for the pharynx and pharyngeal mesenteries. Such
similarities in gene expression between the hypostome of Hydra and
the pharynx of Nematostella could argue for the homology of these
structures, although there appear to be differences in the germ layer of
expression. It is simple to envision how the cone of tissue that protrudes
above the tentacle ring of Hydra intrudes into the gut cavity of
Nematostella.
Considering the antiquity of the last common ancestor of Anthozoa and Hydrozoa, and the pronounced differences in life history between Nematostella and Podocoryne, it is not entirely surprising to find differences in gene expression. Hydrozoans and anthozoans probably diverged well over 500 million years ago and the ancestral hydrozoan life history is much more complex than the ancestral anthozoan life history. In most Hydrozoa, the benthic polyp (which looks like an adult anthozoan) gives rise to one or more pelagic medusae (`jellyfish') by fission or budding. The medusa is the sexually reproducing phase of the life cycle. Hydrozoans are widely regarded as the most derived group of Cnidaria with complex life histories, colony specialization and morphological novelty. Therefore, we may expect that the co-option of developmental genes for novel functions is likely to be more common in Hydrozoa than in Anthozoa. However, although the Hydrozoa may be uniquely derived with respect to some features, every evolutionary lineage is a mosaic of primitive and derived traits. For this reason, it will be necessary to obtain data from both anthozoans and medusozoans if we hope to have any confidence in reconstructing the ancestral cnidarian condition, which, in turn, is crucial for reconstructing the ancestral bilaterian condition and understanding the origin of key bilaterian innovations such as mesoderm.
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ACKNOWLEDGMENTS |
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REFERENCES |
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---|
Arceci, R. J., King, A. A., Simon, M. C., Orkin, S. H. and Wilson, D. B. (1993). Mouse GATA-4: a retinoic acid-inducible GATA-binding transcription factor expressed in endodermally derived tissues and heart. Mol. Cell Biol. 13,2235 -2246.[Abstract]
Barnes, R. S. K., Calow, P., Olive, P. J. W., Golding, D. W. and Spicer, J. I. (2001). The Invertebrates. A Synthesis. Oxford, UK: Blackwell Science.
Baylies, M. K. and Michelson, A. M. (2001). Invertebrate myogenesis: looking back to the future of muscle development. Curr. Opin. Genet. Dev. 11,431 -439.[CrossRef][Medline]
Black, B. L. and Olson, E. N. (1998). Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins. Annu. Rev. Cell Dev. Biol. 14,167 -196.[CrossRef][Medline]
Boero, F., Gravili, C., Pagliara, P., Piraino, S., Bouillon, J. and Schmid, V. (1998). The cnidarian premises of metazoan evolution: from triploblasty, to coelom formation, to metamery. Ital. J. Zool. 65,5 -9.
Bridge, D., Cunningham, C. W., Schierwater, B., DeSalle, R. and Buss, L. W. (1992). Class-level relationships in the phylum Cnidaria: evidence from mitochondrial genome structure. Proc. Natl. Acad. Sci. USA 89,8750 -8753.[Abstract]
Bridge, D., Cunningham, C. W., DeSalle, R. and Buss, L. W. (1995). Class-level relationships in the phylum Cnidaria: molecular and morphological evidence. Mol. Biol. Evol. 12,679 -689.[Abstract]
Brown, S. and Castelli-Gair Hombria, J. (2000).
Drosophila grain encodes a GATA transcription factor required for cell
rearrangement during morphogenesis. Development
127,4867
-4876.
Brusca, R. C. and Brusca, G. J. (2003). Invertebrates. Sunderland, MA: Sinauer Associates.
Castanon, I. and Baylies, M. K. (2002). A Twist in fate: evolutionary comparison of Twist structure and function. Gene 287,11 -22.[CrossRef][Medline]
Collins, A. G. (2002). Phylogeny of the Medusozoa and the evolution of cnidarian life cycles. J. Evol. Biol. 15,4811 -4432.
Davidson, E. H., Rast, J. P., Oliveri, P., Ransick, A., Calestani, C., Yuh, C. H., Minokawa, T., Amore, G., Hinman, V., Arenas-Mena, C. et al. (2002). A provisional regulatory gene network for specification of endomesoderm in the sea urchin embryo. Dev. Biol. 246,162 -190.[CrossRef][Medline]
Dichoso, D., Brodigan, T., Chwoe, K. Y., Lee, J. S., Llacer, R., Park, M., Corsi, A. K., Kostas, S. A., Fire, A., Ahnn, J. et al. (2000). The MADSBox factor CeMEF2 is not essential for Caenorhabditis elegans myogenesis and development. Dev. Biol. 223,431 -440.[CrossRef][Medline]
Essex, L. J., Mayor, R. and Sargent, M. G. (1993). Expression of Xenopus snail in mesoderm and prospective neural fold ectoderm. Dev. Dyn. 198,108 -122.[Medline]
Evans, T., Reitman, M. and Felsenfeld, G. (1988). An erythrocyte-specific DNA-binding factor recognizes a regulatory sequence common to all chicken globin genes. Proc. Natl. Acad. Sci. USA 85,5976 -5980.[Abstract]
Felsenstein, J. (1985). Confidence limits on phylogenies: an approach using the bootstrap. Evolution 39,783 -791.
Feuerstein, R., Wang, X., Song, D., Cooke, N. E. and Liebhaber,
S. A. (1994). The LIM/double zinc-finger motif functions as a
protein dimerization domain. Proc. Natl. Acad. Sci.
USA 91,10655
-10659.
Finnerty, J. R., Paulson, D., Burton, P., Pang, K. and Martindale, M. Q. (2003). Early evolution of a homeobox gene: the paraHox gene Gsx in the Cnidaria and the Bilateria. Evol. Dev. 5,331 -345.[Medline]
Fritzenwanker, J. H. and Technau, U. (2002). Induction of gametogenesis in the basal cnidarian Nematostella vectensis (Anthozoa). Dev. Genes Evol. 212,99 -103.[CrossRef][Medline]
Grimes, H. L., Chan, T. O., Zweidler-McKay, P. A., Tong, B. and Tsichlis, P. N. (1996). The Gfi-1 proto-oncoprotein contains a novel transcriptional repressor domain, SNAG, and inhibits G1 arrest induced by interleukin-2 withdrawal. Mol. Cell Biol. 16,6263 -6272.[Abstract]
Groger, H., Callaerts, P., Gehring, W. J. and Schmid, V. (1999). Gene duplication and recruitment of a specific tropomyosin into striated muscle cells in the jellyfish Podocoryne carnea. J. Exp. Zool. 285,378 -386.[CrossRef][Medline]
Hand, C. and Uhlinger, K. (1992). The culture,
sexual and asexual reproduction, and growth of the sea anemone
Nematostella vectensis. Biol. Bull.
182,169
-176.
Hand, C. and Uhlinger, K. (1994). The unique, widely distributed sea anemone, Nematostella vectensis Stephenson: a review, new facts, and questions. Estuaries 17,501 -508.
Harada, Y., Akasaka, K., Shimada, H., Peterson, K. J., Davidson, E. H. and Satoh, N. (1996). Spatial expression of a forkhead homologue in the sea urchin embryo. Mech. Dev. 60,163 -173.[CrossRef][Medline]
Hemavathy, K., Ashraf, S. I. and Ip, Y. T. (2000). Snail/slug family of repressors: slowly going into the fast lane of development and cancer. Gene 257, 1-12.[CrossRef][Medline]
Henry, J. Q., Martindale, M. Q. and Boyer, B. C. (2000). The unique developmental program of the acoel flatworm, Neochildia fusca. Dev. Biol. 220,285 -295.[CrossRef][Medline]
Hukriede, N. A., Tsang, T. E., Habas, R., Khoo, P. L., Steiner, K., Weeks, D. L., Tam, P. P. and Dawid, I. B. (2003). Conserved requirement of Lim1 function for cell movements during gastrulation. Dev. Cell 4,83 -94.[Medline]
Kaufmann, E. and Knochel, W. (1996). Five years on the wings of fork head. Mech. Dev. 57, 3-20.[CrossRef][Medline]
Ketola, I., Pentikainen, V., Vaskivuo, T., Ilvesmaki, V., Herva,
R., Dunkel, L., Tapanainen, J. S., Toppari, J. and Heikinheimo, M.
(2000). Expression of transcription factor GATA-4 during human
testicular development and disease. J. Clin. Endocrinol.
Metab. 85,3925
-3931.
Kidson, S. H., Kume, T., Deng, K., Winfrey, V. and Hogan, B. L. (1999). The forkhead/winged-helix gene, Mf1, is necessary for the normal development of the cornea and formation of the anterior chamber in the mouse eye. Dev. Biol. 211,306 -322.[CrossRef][Medline]
Kosman, D., Ip, Y. T., Levine, M. and Arora, K. (1991). Establishment of the mesoderm-neuroectoderm boundary in the Drosophila embryo. Science 254,118 -122.[Medline]
Kusch, T. and Reuter, R. (1999). Functions for
Drosophila brachyenteron and forkhead in mesoderm specification and cell
signalling. Development
126,3991
-4003.
Laitinen, M. P., Anttonen, M., Ketola, I., Wilson, D. B.,
Ritvos, O., Butzow, R. and Heikinheimo, M. (2000).
Transcription factors GATA-4 and GATA-6 and a GATA family cofactor, FOG-2, are
expressed in human ovary and sex cord-derived ovarian tumors. J.
Clin. Endocrinol. Metab. 85,3476
-3483.
Leptin, M. (1991). twist and snail as positive and negative regulators during Drosophila mesoderm development. Genes Dev. 5,1568 -1576.[Abstract]
Lespinet, O., Nederbragt, A. J., Cassan, M., Dictus, W. J., van Loon, A. E. and Adoutte, A. (2002). Characterisation of two snail genes in the gastropod mollusc Patella vulgata. Implications for understanding the ancestral function of the snail-related genes in Bilateria. Dev. Genes Evol. 212,186 -195.[CrossRef][Medline]
Lilly, B., Galewsky, S., Firulli, A. B., Schulz, R. A. and Olson, E. N. (1994). D-MEF2: a MADS box transcription factor expressed in differentiating mesoderm and muscle cell lineages during Drosophila embryogenesis. Proc. Natl. Acad. Sci. USA 91,5662 -5666.[Abstract]
Louis, H. A., Pino, J. D., Schmeichel, K. L., Pomies, P. and
Beckerle, M. C. (1997). Comparison of three members of the
cysteine-rich protein family reveals functional conservation and divergent
patterns of gene expression. J. Biol. Chem.
272,27484
-27491.
Maduro, M. F. and Rothman, J. H. (2002). Making worm guts: the gene regulatory network of the Caenorhabditis elegans endoderm. Dev. Biol. 246,68 -85.[CrossRef][Medline]
Martindale, M. Q. and Henry, J. Q. (1999). Intracellular fate mapping in a basal metazoan, the ctenophore Mnemiopsis leidyi, reveals the origins of mesoderm and the existence of indeterminate cell lineages. Dev. Biol. 214,243 -257.[CrossRef][Medline]
Martinez, D. E., Dirksen, M. L., Bode, P. M., Jamrich, M., Steele, R. E. and Bode, H. R. (1997). Budhead, a fork head/HNF-3 homologue, is expressed during axis formation and head specification in hydra. Dev. Biol. 192,523 -536.[CrossRef][Medline]
McDevitt, M. A., Shivdasani, R. A., Fujiwara, Y., Yang, H. and
Orkin, S. H. (1997). A `knockdown' mutation created by
cis-element gene targeting reveals the dependence of erythroid cell maturation
on the level of transcription factor GATA-1. Proc. Natl. Acad. Sci.
USA 94,6781
-6785.
Medina, M., Collins, A. G., Silberman, J. D. and Sogin, M.
L. (2001). Evaluating hypotheses of basal animal phylogeny
using complete sequences of large and small subunit rRNA. Proc.
Natl. Acad. Sci. USA 98,9707
-9712.
Morrisey, E. E., Tang, Z., Sigrist, K., Lu, M. M., Jiang, F.,
Ip, H. S. and Parmacek, M. S. (1998). GATA6 regulates HNF4
and is required for differentiation of visceral endoderm in the mouse embryo.
Genes Dev. 12,3579
-3590.
Muller, P., Yanze, N., Schmid, V. and Spring, J. (1999). The homeobox gene Otx of the jellyfish Podocoryne carnea: role of a head gene in striated muscle and evolution. Dev. Biol. 216,582 -594.[CrossRef][Medline]
Muller, P., Seipel, K., Yanze, N., Reber-Muller, S., Streitwolf-Engel, R., Stierwald, M., Spring, J. and Schmid, V. (2003). Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish. Dev. Biol. 255,216 -229.[CrossRef][Medline]
Odorico, D. M. and Miller, D. J. (1997). Internal and external relationships of the Cnidaria: implications of primary and predicted secondary structure of the 5'-end of the 23S-like rDNA. In Proc. R. Soc. Lond. B 264, 77-82.[CrossRef][Medline]
Olsen, C. L. and Jeffery, W. R. (1997). A
forkhead gene related to HNF-3beta is required for gastrulation and axis
formation in the ascidian embryo. Development
124,3609
-3619.
Olsen, C. L., Natzle, J. E. and Jeffery, W. R. (1999). The forkhead gene FH1 is involved in evolutionary modification of the ascidian tadpole larva. Mech. Dev. 85, 49-58.[CrossRef][Medline]
Pani, L., Overdier, D. G., Porcella, A., Qian, X., Lai, E. and Costa, R. H. (1992). Hepatocyte nuclear factor 3 beta contains two transcriptional activation domains, one of which is novel and conserved with the Drosophila fork head protein. Mol. Cell Biol. 12,3723 -3732.[Abstract]
Pechenik, J. A. (2000). Biology of the Invertebrates. New York: McGraw-Hill Higher Education.
Perez Sanchez, C., Casas-Tinto, S., Sanchez, L., Rey-Campos, J. and Granadino, B. (2002). DmFoxF, a novel Drosophila fork head factor expressed in visceral mesoderm. Mech. Dev. 111,163 -166.[CrossRef][Medline]
Perez-Pomares, J. M. and Munoz-Chapuli, R. (2002). Epithelial-mesenchymal transitions: a mesodermal cell strategy for evolutive innovation in Metazoans. Anat. Rec. 268,343 -351.[CrossRef][Medline]
Ransick, A., Rast, J. P., Minokawa, T., Calestani, C. and Davidson, E. H. (2002). New early zygotic regulators expressed in endomesoderm of sea urchin embryos discovered by differential array hybridization. Dev. Biol. 246,132 -147.[CrossRef][Medline]
Rodaway, A. and Patient, R. (2001). Mesendoderm. an ancient germ layer? Cell 105,169 -172.[CrossRef][Medline]
Ruiz i Altaba, A., Placzek, M., Baldassare, M., Dodd, J. and Jessell, T. M. (1995). Early stages of notochord and floor plate development in the chick embryo defined by normal and induced expression of HNF-3 beta. Dev. Biol. 170,299 -313.[CrossRef][Medline]
Ruiz-Trillo, I., Riutort, M., Littlewood, D. T., Herniou, E. A.
and Baguna, J. (1999). Acoel flatworms: earliest extant
bilaterian Metazoans, not members of Platyhelminthes.
Science 283,1919
-1923.
Ruiz-Trillo, I., Paps, J., Loukota, M., Ribera, C., Jondelius,
U., Baguna, J. and Riutort, M. (2002). A phylogenetic
analysis of myosin heavy chain type II sequences corroborates that Acoela and
Nemertodermatida are basal bilaterians. Proc. Natl. Acad. Sci.
USA 99,11246
-11251.
Scholz, C. B. and Technau, U. (2003). The ancestral role of Brachyury: expression of NemBra1 in the basal cnidarian Nematostella vectensis (Anthozoa). Dev. Genes Evol. 212,563 -570.[Medline]
Schuchert, P. (1993). Phylogenetic analysis of the Cnidaria. Z. zool. syst. Evolut.-forsch 31,161 -173.
Schulz, R. A., Chromey, C., Lu, M. F., Zhao, B. and Olson, E. N. (1996). Expression of the D-MEF2 transcription in the Drosophila brain suggests a role in neuronal cell differentiation. Oncogene 12,1827 -1831.[Medline]
Spring, J., Yanze, N., Middel, A. M., Stierwald, M., Groger, H. and Schmid, V. (2000). The mesoderm specification factor twist in the life cycle of jellyfish. Dev. Biol. 228,363 -375.[CrossRef][Medline]
Spring, J., Yanze, N., Josch, C., Middel, A. M., Winninger, B. and Schmid, V. (2002). Conservation of Brachyury, Mef2, and Snail in the myogenic lineage of jellyfish: a connection to the mesoderm of bilateria. Dev. Biol. 244,372 -384.[CrossRef][Medline]
Stainier, D. Y. (2002). A glimpse into the
molecular entrails of endoderm formation. Genes Dev.
16,893
-907.
Stronach, B. E., Siegrist, S. E. and Beckerle, M. C. (1996). Two muscle-specific LIM proteins in Drosophila. J. Cell Biol. 134,1179 -1195.[Abstract]
Technau, U. and Bode, H. R. (1999). HyBra1, a
Brachyury homologue, acts during head formation in Hydra.
Development 126,999
-1010.
Telford, M. J., Lockyer, A. E., Cartwright-Finch, C. and Littlewood, D. T. (2003). Combined large and small subunit ribosomal RNA phylogenies support a basal position of the acoelomorph flatworms. Proc. R. Soc. Lond. B Biol. Sci. 270,1077 -1083.[CrossRef][Medline]
Thisse, B., Stoetzel, C., Gorostiza-Thisse, C. and Perrin-Schmitt, F. (1988). Sequence of the twist gene and nuclear localization of its protein in endomesodermal cells of early Drosophila embryos. EMBO J. 7,2175 -2183.[Abstract]
Tong, Q., Dalgin, G., Xu, H., Ting, C. N., Leiden, J. M. and
Hotamisligil, G. S. (2000). Function of GATA transcription
factors in preadipocyte-adipocyte transition. Science
290,134
-138.
Tsai, F. Y., Keller, G., Kuo, F. C., Weiss, M., Chen, J., Rosenblatt, M., Alt, F. W. and Orkin, S. H. (1994). An early haematopoietic defect in mice lacking the transcription factor GATA-2. Nature 371,221 -226.[CrossRef][Medline]
Tsui, S. K., Yam, N. Y., Lee, C. Y. and Waye, M. M. (1994). Isolation and characterization of a cDNA that codes for a LIM-containing protein which is developmentally regulated in heart. Biochem. Biophys. Res. Commun. 205,497 -505.[CrossRef][Medline]
Wada, S. and Saiga, H. (1999). Cloning and embryonic expression of Hrsna, a snail family gene of the ascidian Halocynthia roretzi: implication in the origins of mechanisms for mesoderm specification and body axis formation in chordates. Dev. Growth Differ. 41,9 -18.[CrossRef][Medline]
Wainright, P. O., Hinkle, G., Sogin, M. L. and Stickel, S. K. (1993). Monophyletic origins of the metazoa: an evolutionary link with fungi. Science 260,340 -342.[Medline]
Weigel, D., Jurgens, G., Kuttner, F., Seifert, E. and Jackle, H. (1989). The homeotic gene fork head encodes a nuclear protein and is expressed in the terminal regions of the Drosophila embryo. Cell 57,645 -658.[Medline]
Whitsett, J. A. and Tichelaar, J. W. (1999).
Forkhead transcription factor HFH-4 and respiratory epithelial cell
differentiation. Am. J. Respir. Cell Mol. Biol.
21,153
-154.
Wolf, C., Thisse, C., Stoetzel, C., Thisse, B., Gerlinger, P. and Perrin-Schmitt, F. (1991). The M-twist gene of Mus is expressed in subsets of mesodermal cells and is closely related to the Xenopus X-twi and the Drosophila twist genes. Dev. Biol. 143,363 -373.[Medline]