154
F. Raible and P.R.H. Steinmetz
during the evolution of Placozoa (Jakob et al. 2004, Monteiro et al. 2006, Peterson
and Sperling 2007).
Many Trichoplax transcription factors (e.g. the T-box genes Tbx2/3 and
brachyury, or the paired box gene TriPaxB, a putative precursor gene of the sensory cell marker genes PaxA/B/C (Cnidaria), Pax2/5/8 and Pax4/6 (Bilateria)) are
expressed at the outer border of the animal, similar to the expression of RF-amide,
an abundant cnidarian neuropeptide (Martinelli and Spring 2003, Schuchert 1993).
This region includes many cells with unfamiliar morphologies that may represent ancestral neural or multipotent cell types that have diversified into different
cell types during the evolution of cnidarians and bilaterians (Jakob et al. 2004,
Martinelli and Spring 2003). The existence of putative neuronal precursor cells is
further supported by the presence of basic components for neurotransmitter synthesis, release, and uptake as well as for synapse formation, photoreception and the
electric transmission of stimuli in the Trichoplax genome.
Although placozoans are in some aspects clearly reduced, the structure of the
small Trichoplax genome (98 million base pairs) rather represents an ancestral
state. The high level of conserved linkage (synteny) between large Trichoplax and
vertebrate genomic regions, retention of ancient introns, and a high conservation
of intron-exon-boundaries oppose a secondary genomic reduction as found in the
C. elegans, Drosophila, or Oikopleura genomes. A better understanding of the
Trichoplax life cycle and cell type morphology will help elucidate whether cryptic developmental stages or so far undetected cell type diversity can account for the
relative complexity of the genome in terms of gene content and structure.
5.5 Cnidaria: A Simple Body with a Complex Genome
Cnidaria form a species-rich phylum of mainly marine animals, comprising
the classes Anthozoa (e.g. sea anemones and corals), Cubozoa (box jellyfish),
Scyphozoa (e.g. sea wasps) and Hydrozoa (e.g. Hydra) (Siewing 1985). Many
anthozoans and hydrozoans have complicated life cycles involving a ciliated planula larva (Nielsen 2001). In addition, all groups except the anthozoans have a
free-swimming medusa stage. Although the medusa is mainly considered to be a
secondary innovation, arising after the emergence of Anthozoa, secondary loss in
anthozoans is equally probable (Collins 2002). In contrast to their rather complex
life cycles, cnidarians display a simple body plan with two germ layers: ectoderm
and endoderm, separated by the acellular mesogloea consisting of extracellular
matrix. Cnidarians have a single body opening derived from the blastopore that
functions as both mouth and anus. At the opposite side of the planula resides a
ciliary apical tuft with presumptive sensory functions. Although simple radial symmetry is widespread among cnidarians, many anthozoans possess a long-known
second body axis (the “directive axis”) that runs orthogonal to the oral-aboral axis
and defines bilateral symmetry (Stephenson 1928). It is morphologically apparent
by the arrangement of muscles within endodermal folds (mesenteries) and by the
F. Raible and P.R.H. Steinmetz
during the evolution of Placozoa (Jakob et al. 2004, Monteiro et al. 2006, Peterson
and Sperling 2007).
Many Trichoplax transcription factors (e.g. the T-box genes Tbx2/3 and
brachyury, or the paired box gene TriPaxB, a putative precursor gene of the sensory cell marker genes PaxA/B/C (Cnidaria), Pax2/5/8 and Pax4/6 (Bilateria)) are
expressed at the outer border of the animal, similar to the expression of RF-amide,
an abundant cnidarian neuropeptide (Martinelli and Spring 2003, Schuchert 1993).
This region includes many cells with unfamiliar morphologies that may represent ancestral neural or multipotent cell types that have diversified into different
cell types during the evolution of cnidarians and bilaterians (Jakob et al. 2004,
Martinelli and Spring 2003). The existence of putative neuronal precursor cells is
further supported by the presence of basic components for neurotransmitter synthesis, release, and uptake as well as for synapse formation, photoreception and the
electric transmission of stimuli in the Trichoplax genome.
Although placozoans are in some aspects clearly reduced, the structure of the
small Trichoplax genome (98 million base pairs) rather represents an ancestral
state. The high level of conserved linkage (synteny) between large Trichoplax and
vertebrate genomic regions, retention of ancient introns, and a high conservation
of intron-exon-boundaries oppose a secondary genomic reduction as found in the
C. elegans, Drosophila, or Oikopleura genomes. A better understanding of the
Trichoplax life cycle and cell type morphology will help elucidate whether cryptic developmental stages or so far undetected cell type diversity can account for the
relative complexity of the genome in terms of gene content and structure.
5.5 Cnidaria: A Simple Body with a Complex Genome
Cnidaria form a species-rich phylum of mainly marine animals, comprising
the classes Anthozoa (e.g. sea anemones and corals), Cubozoa (box jellyfish),
Scyphozoa (e.g. sea wasps) and Hydrozoa (e.g. Hydra) (Siewing 1985). Many
anthozoans and hydrozoans have complicated life cycles involving a ciliated planula larva (Nielsen 2001). In addition, all groups except the anthozoans have a
free-swimming medusa stage. Although the medusa is mainly considered to be a
secondary innovation, arising after the emergence of Anthozoa, secondary loss in
anthozoans is equally probable (Collins 2002). In contrast to their rather complex
life cycles, cnidarians display a simple body plan with two germ layers: ectoderm
and endoderm, separated by the acellular mesogloea consisting of extracellular
matrix. Cnidarians have a single body opening derived from the blastopore that
functions as both mouth and anus. At the opposite side of the planula resides a
ciliary apical tuft with presumptive sensory functions. Although simple radial symmetry is widespread among cnidarians, many anthozoans possess a long-known
second body axis (the “directive axis”) that runs orthogonal to the oral-aboral axis
and defines bilateral symmetry (Stephenson 1928). It is morphologically apparent
by the arrangement of muscles within endodermal folds (mesenteries) and by the