5 Metazoan Complexity
155
positioning of longer cilia at either end of the slit-like pharynx (Siewing 1985,
Stephenson 1935). The homology of cnidarian and bilaterian axes is discussed in
a later section.
Overall, the cnidarian nervous system is considered to be simple and diffuse
(Bullock and Horridge 1965, Siewing 1985). However, a higher axonal concentration is found in the nerve ring of many medusae, and the rhopalia, a sensory structure
containing lens eyes and statocysts in the medusae of Cubozoa and Scyphozoa
(Nilsson et al. 2005, Piatigorsky and Kozmik 2004, Skogh et al. 2006). Molecular
studies on neurogenesis and the diversity and specification of neural cell types in
Cnidaria were mostly restricted to the freshwater polyp Hydra until recent work on
Nematostella started to shed light on nervous system development in anthozoan larvae (Marlow et al. 2009). More specific analyses of neural structure development
focused mainly on photosensory systems (Kozmik et al. 2003, Stierwald et al. 2004,
Suga et al. 2008) and the patterning of the apical organ (Matus et al. 2007, Pang
et al. 2004, Rentzsch et al. 2008). We believe that a better molecular understanding
of the development of the nervous system and the diversity of neural cell types in
cnidarians is the key to understanding the evolutionary innovations that led to the
complex nervous systems of many bilaterians.
Interest in the use of cnidarians for non-bilaterian developmental and genomic
studies is mainly due to the success of the freshwater hydrozoan Hydra vulgaris
as a model organism for axial patterning and stem cell research. In addition, the
last decade has seen the emergence of anthozoans (the brackish-water sea anemone
Nematostella vectensis, and the marine coral Acropora millipora), cubozoans
(Tripedalia cystophora) and hydrozoans (the marine Hydractinia echinata, Clytia
hemisphaerica, Podocoryne carnea) for comparative genomic and developmental
studies.
5.5.1 The Nematostella Genome
The sequencing of the Nematostella genome has confirmed previous assumptions
based on Nematostella and Acropora EST analyses that the anthozoan genome is
more complex than some bilaterian genomes (Kortschak et al. 2003, Miller and
Ball 2008, Miller et al. 2005, Putnam et al. 2007, Technau et al. 2005). For example, Nematostella has more genes and a larger genome than insects and nematodes
(Miller and Ball 2008). Furthermore, its genome encodes representatives of all bilaterian signalling pathways and of almost all transcription factor families (Larroux
et al. 2008, Putnam et al. 2007, Ryan et al. 2006, Technau et al. 2005). In some
cases (Wnt ligands and antagonists or BMP antagonists), Nematostella shares more
orthologues with vertebrates than with insects or nematodes (Kusserow et al. 2005,
Matus et al. 2006a, Rentzsch et al. 2006). On the other hand, several Fox genes
appear secondarily lost in Nematostella (Larroux et al. 2008). Also, the cnidarian
genome seems to be as complex as those of many bilaterians: in conserved genes,
about 80% of human gene introns are conserved in Nematostella genes (considerably more than in C. elegans or Drosophila) (Putnam et al. 2007). The high
155
positioning of longer cilia at either end of the slit-like pharynx (Siewing 1985,
Stephenson 1935). The homology of cnidarian and bilaterian axes is discussed in
a later section.
Overall, the cnidarian nervous system is considered to be simple and diffuse
(Bullock and Horridge 1965, Siewing 1985). However, a higher axonal concentration is found in the nerve ring of many medusae, and the rhopalia, a sensory structure
containing lens eyes and statocysts in the medusae of Cubozoa and Scyphozoa
(Nilsson et al. 2005, Piatigorsky and Kozmik 2004, Skogh et al. 2006). Molecular
studies on neurogenesis and the diversity and specification of neural cell types in
Cnidaria were mostly restricted to the freshwater polyp Hydra until recent work on
Nematostella started to shed light on nervous system development in anthozoan larvae (Marlow et al. 2009). More specific analyses of neural structure development
focused mainly on photosensory systems (Kozmik et al. 2003, Stierwald et al. 2004,
Suga et al. 2008) and the patterning of the apical organ (Matus et al. 2007, Pang
et al. 2004, Rentzsch et al. 2008). We believe that a better molecular understanding
of the development of the nervous system and the diversity of neural cell types in
cnidarians is the key to understanding the evolutionary innovations that led to the
complex nervous systems of many bilaterians.
Interest in the use of cnidarians for non-bilaterian developmental and genomic
studies is mainly due to the success of the freshwater hydrozoan Hydra vulgaris
as a model organism for axial patterning and stem cell research. In addition, the
last decade has seen the emergence of anthozoans (the brackish-water sea anemone
Nematostella vectensis, and the marine coral Acropora millipora), cubozoans
(Tripedalia cystophora) and hydrozoans (the marine Hydractinia echinata, Clytia
hemisphaerica, Podocoryne carnea) for comparative genomic and developmental
studies.
5.5.1 The Nematostella Genome
The sequencing of the Nematostella genome has confirmed previous assumptions
based on Nematostella and Acropora EST analyses that the anthozoan genome is
more complex than some bilaterian genomes (Kortschak et al. 2003, Miller and
Ball 2008, Miller et al. 2005, Putnam et al. 2007, Technau et al. 2005). For example, Nematostella has more genes and a larger genome than insects and nematodes
(Miller and Ball 2008). Furthermore, its genome encodes representatives of all bilaterian signalling pathways and of almost all transcription factor families (Larroux
et al. 2008, Putnam et al. 2007, Ryan et al. 2006, Technau et al. 2005). In some
cases (Wnt ligands and antagonists or BMP antagonists), Nematostella shares more
orthologues with vertebrates than with insects or nematodes (Kusserow et al. 2005,
Matus et al. 2006a, Rentzsch et al. 2006). On the other hand, several Fox genes
appear secondarily lost in Nematostella (Larroux et al. 2008). Also, the cnidarian
genome seems to be as complex as those of many bilaterians: in conserved genes,
about 80% of human gene introns are conserved in Nematostella genes (considerably more than in C. elegans or Drosophila) (Putnam et al. 2007). The high