160
F. Raible and P.R.H. Steinmetz
Siewing 1985). The comparison of cnidarian and bilaterian muscle structural genes
will allow the debated evolutionary relationship between the bilaterian smooth and
striated muscle types and the cnidarian muscle types to be clarified.
Cnidarians and bilaterians not only share a similar “mesodermal” derivative –
muscle tissue – but also many transcription factors with specific roles during bilaterian mesodermal and muscle patterning. In the hydrozoan Podocoryne carnaea,
the mesodermal specification genes twist, mef 2 (Spring et al. 2002) and msx (Galle
et al. 2005) are also expressed in the “entocodon”, an ectodermal cell mass proliferating between ecto- and endoderm during medusa formation in hydrozoan polyps.
The entocodon develops into smooth and striated muscles and has been proposed
to be a cnidarian homolog of the bilaterian mesoderm (Seipel and Schmid 2005).
However, the entocodon is adult tissue that forms during non-sexual reproduction
and therefore barely classifies as a third embryonic germ layer. Also, scyphozoan
and cubozoan medusae develop muscles without entocodon (Burton 2007).
In Nematostella, several “mesodermal” transcription factors (mef 2, twist, gata,
muscle-LIM) are differentially expressed in the endoderm of the planula larva
(Martindale et al. 2004). In anthozoans, most of the endoderm consists of myoepithelial cells, while more specialized muscles such as pharynx retractor muscles
form together with germ cells in the “mesenteries”, which are endodermal folds
reaching into the body cavity (Siewing 1985). Functional analysis of conserved
“mesodermal” transcription factors and the comparison with bilaterian orthologs is
necessary to determine the extent of conservation between the gene regulatory networks governing bilaterian and cnidarian muscle and mesoderm development. As
one hypothesis proposes that mesoderm evolved as a continuation of endodermal
folding between ectoderm and endoderm, as can still be observed in bilaterians with
enterocoelic mesoderm formation (e.g. Amphioxus, sea urchins, hemichordates,
pogonophore annelids) (Arendt 2004, Remane 1950, Sedgwick 1884, Tautz 2004),
it will be particularly interesting to compare the patterning and morphogenesis of the
mesenteries between Nematostella and bilaterians. Alternatively, mesoderm might
have evolved from endodermal cells that became mesenchymal as found in many
extant spiralians (Technau and Scholz 2003).
5.5.6 “Cryptic” Complexity in Cnidarians?
In general, cnidarians appear morphologically simple at first sight. Therefore, the
discovery that the Nematostella genome appears more complex than of insects or
nematodes was surprising. However, a closer look reveals that the morphology of
some cnidarians such as anthozoans might not be as simple after all: the presence
of regulative development, a planula larva and two asymmetric body axes are all
signs of complex development. Also, more cell types might remain to be described
by molecular techniques that were so far not morphologically distinguishable. For
example, the number of neuronal cell types might substantially increase as a result
of investigations of the differential localisation of neurotransmitter receptor and
F. Raible and P.R.H. Steinmetz
Siewing 1985). The comparison of cnidarian and bilaterian muscle structural genes
will allow the debated evolutionary relationship between the bilaterian smooth and
striated muscle types and the cnidarian muscle types to be clarified.
Cnidarians and bilaterians not only share a similar “mesodermal” derivative –
muscle tissue – but also many transcription factors with specific roles during bilaterian mesodermal and muscle patterning. In the hydrozoan Podocoryne carnaea,
the mesodermal specification genes twist, mef 2 (Spring et al. 2002) and msx (Galle
et al. 2005) are also expressed in the “entocodon”, an ectodermal cell mass proliferating between ecto- and endoderm during medusa formation in hydrozoan polyps.
The entocodon develops into smooth and striated muscles and has been proposed
to be a cnidarian homolog of the bilaterian mesoderm (Seipel and Schmid 2005).
However, the entocodon is adult tissue that forms during non-sexual reproduction
and therefore barely classifies as a third embryonic germ layer. Also, scyphozoan
and cubozoan medusae develop muscles without entocodon (Burton 2007).
In Nematostella, several “mesodermal” transcription factors (mef 2, twist, gata,
muscle-LIM) are differentially expressed in the endoderm of the planula larva
(Martindale et al. 2004). In anthozoans, most of the endoderm consists of myoepithelial cells, while more specialized muscles such as pharynx retractor muscles
form together with germ cells in the “mesenteries”, which are endodermal folds
reaching into the body cavity (Siewing 1985). Functional analysis of conserved
“mesodermal” transcription factors and the comparison with bilaterian orthologs is
necessary to determine the extent of conservation between the gene regulatory networks governing bilaterian and cnidarian muscle and mesoderm development. As
one hypothesis proposes that mesoderm evolved as a continuation of endodermal
folding between ectoderm and endoderm, as can still be observed in bilaterians with
enterocoelic mesoderm formation (e.g. Amphioxus, sea urchins, hemichordates,
pogonophore annelids) (Arendt 2004, Remane 1950, Sedgwick 1884, Tautz 2004),
it will be particularly interesting to compare the patterning and morphogenesis of the
mesenteries between Nematostella and bilaterians. Alternatively, mesoderm might
have evolved from endodermal cells that became mesenchymal as found in many
extant spiralians (Technau and Scholz 2003).
5.5.6 “Cryptic” Complexity in Cnidarians?
In general, cnidarians appear morphologically simple at first sight. Therefore, the
discovery that the Nematostella genome appears more complex than of insects or
nematodes was surprising. However, a closer look reveals that the morphology of
some cnidarians such as anthozoans might not be as simple after all: the presence
of regulative development, a planula larva and two asymmetric body axes are all
signs of complex development. Also, more cell types might remain to be described
by molecular techniques that were so far not morphologically distinguishable. For
example, the number of neuronal cell types might substantially increase as a result
of investigations of the differential localisation of neurotransmitter receptor and