152
F. Raible and P.R.H. Steinmetz
Amq11/max or the Suberites brachyury) (Larroux et al. 2008, Simionato et al. 2007),
indicating that the respective gene families had already diversified in the poriferaneumetazoan ancestor. In cases where these families group monophyletically with
families that lack a clear Amphimedon orthologue, gene loss in the sponge can be
assumed. Similarly, analysis of the NK homebox genes indicated loss of several
NK gene families in demosponges (Peterson and Sperling 2007). Direct evidence
for these gene losses comes from comparisons with other sponge species where the
respective genes are present, examples including brachyury (present in Suberites,
missing in Amphimedon) (Larroux et al. 2008) and mnt/mad (present in Oscarella,
missing in Amphimedon; Fig. 5.3b) (Simionato et al. 2007). As sequence data is so
far scarce for other sponge genomes, the real number of transcription factors in the
poriferan-eumetazoan ancestor is probably higher than currently assumed.
The presence of representatives from almost all major eumetazoan transcription
factor families is surprising, considering the relatively low number of adult cell
types and the simplicity of the sponge body plan. This paradox might be resolved
by considering that the life cycle of most sponges includes a ciliated larval stage
followed by a complex metamorphosis (Leys and Ereskovsky 2006). The high variability of early embryonic development has led to currently opposing views on the
presence and definition of germ layers and gastrulation in sponges (Ereskovsky and
Dondua 2006, Leys 2004). However, in contrast to adults, many larvae are clearly
bilayered and exhibit a single body axis (Ereskovsky and Dondua 2006, Leys and
Ereskovsky 2006). Expression analysis suggests that molecular patterning along the
axis in the Amphimedon larva is mediated by Wnt and TGF-β members (Adamska
et al. 2007a). The axis is directly related to swimming direction and can be influenced by phototactic cues (Leys and Degnan 2001). Although lacking neurons and
ciliary or rhabdomeric photoreceptors, some photoresponsive larvae possess pigmented cells with a long cilium that can simultaneously act as receptor and effector
and could therefore represent the proto-photoreceptor cell before the specialisation into distinct pigment shading and photoreceptor cells (Arendt 2008, Leys and
Degnan 2001). An almost complete set of bilaterian post-synaptic proteins in the
Amphimedon genome also indicates that despite the lack of neurons, synapses or
post-synaptic densities, a simple system of sensory stimuli transduction might be
in place (Sakaraya 2007). Further functional, physiological and expression analysis
will elucidate if the number of sponge cell types is higher than currently assumed. In
fact, what is classified as one single cell type by morphological criteria might consist
of several, functionally different cell types distinguishable only by their differential
molecular composition.
5.4 The Placozoan Trichoplax: A Primitively Simple or Highly
Reduced Metazoan?
Until recently, the phylum Placozoa consisted only of Trichoplax adhaerens and
Treptoplax reptans (only described once) but molecular analysis have discovered
additional cryptic (morphologically indistinguishable) species (Grell 1971b, Voigt
F. Raible and P.R.H. Steinmetz
Amq11/max or the Suberites brachyury) (Larroux et al. 2008, Simionato et al. 2007),
indicating that the respective gene families had already diversified in the poriferaneumetazoan ancestor. In cases where these families group monophyletically with
families that lack a clear Amphimedon orthologue, gene loss in the sponge can be
assumed. Similarly, analysis of the NK homebox genes indicated loss of several
NK gene families in demosponges (Peterson and Sperling 2007). Direct evidence
for these gene losses comes from comparisons with other sponge species where the
respective genes are present, examples including brachyury (present in Suberites,
missing in Amphimedon) (Larroux et al. 2008) and mnt/mad (present in Oscarella,
missing in Amphimedon; Fig. 5.3b) (Simionato et al. 2007). As sequence data is so
far scarce for other sponge genomes, the real number of transcription factors in the
poriferan-eumetazoan ancestor is probably higher than currently assumed.
The presence of representatives from almost all major eumetazoan transcription
factor families is surprising, considering the relatively low number of adult cell
types and the simplicity of the sponge body plan. This paradox might be resolved
by considering that the life cycle of most sponges includes a ciliated larval stage
followed by a complex metamorphosis (Leys and Ereskovsky 2006). The high variability of early embryonic development has led to currently opposing views on the
presence and definition of germ layers and gastrulation in sponges (Ereskovsky and
Dondua 2006, Leys 2004). However, in contrast to adults, many larvae are clearly
bilayered and exhibit a single body axis (Ereskovsky and Dondua 2006, Leys and
Ereskovsky 2006). Expression analysis suggests that molecular patterning along the
axis in the Amphimedon larva is mediated by Wnt and TGF-β members (Adamska
et al. 2007a). The axis is directly related to swimming direction and can be influenced by phototactic cues (Leys and Degnan 2001). Although lacking neurons and
ciliary or rhabdomeric photoreceptors, some photoresponsive larvae possess pigmented cells with a long cilium that can simultaneously act as receptor and effector
and could therefore represent the proto-photoreceptor cell before the specialisation into distinct pigment shading and photoreceptor cells (Arendt 2008, Leys and
Degnan 2001). An almost complete set of bilaterian post-synaptic proteins in the
Amphimedon genome also indicates that despite the lack of neurons, synapses or
post-synaptic densities, a simple system of sensory stimuli transduction might be
in place (Sakaraya 2007). Further functional, physiological and expression analysis
will elucidate if the number of sponge cell types is higher than currently assumed. In
fact, what is classified as one single cell type by morphological criteria might consist
of several, functionally different cell types distinguishable only by their differential
molecular composition.
5.4 The Placozoan Trichoplax: A Primitively Simple or Highly
Reduced Metazoan?
Until recently, the phylum Placozoa consisted only of Trichoplax adhaerens and
Treptoplax reptans (only described once) but molecular analysis have discovered
additional cryptic (morphologically indistinguishable) species (Grell 1971b, Voigt