5 Metazoan Complexity
153
et al. 2004). So far, only asexual reproduction by fission or budding of spherical
“swarmers” has been observed (Siewing 1985). The discovery of putative oocytes,
cleaving stages up to the 64-cell stage (Grell 1971a, 1972, Grell and Ruthman 1991)
and molecular signatures for recombination and sex (Signorovitch et al. 2005) are
indications for the existence of sexual reproduction. Hence, the description of the
Trichoplax life cycle is most likely incomplete and an intermediate parasitic stage in
a so far unidentified host or a larval stage cannot be excluded (Miller and Ball 2005).
Trichoplax exhibits a unique mode of algal feeding (“transepithelial cytophagy”)
through gaps of the upper, monocliliated epithelium and subsequent phagocytosis by
inner fiber cells (Wenderoth 1986). In addition, the ventral, non-ciliated epithelium
probably secretes digestive enzymes but is not able to perform phagocytosis.
Presenting four somatic cell types within three cell layers, a single, “top-bottom”
axis and lacking a basement membrane, extracellular matrix, mouth, gut, and nervous system, the body plan of Trichoplax is often considered as simplest throughout
metazoans (Syed and Schierwater 2002). The cell layers consist of an upper and
lower epithelium separated by inner, contractile “fibre cells” probably controlling
the amoeboid-like locomotion (Syed and Schierwater 2002). In the context of this
simple axial organization, it is surprising to find a complete set of components
required for functional Wnt or TGF-β signaling (conserved during bilaterian body
axis patterning) in the Trichoplax genome (Srivastava et al. 2008).
Phylogenetic analysis based on a large dataset from the sequenced genomes of
Trichoplax and other metazoans (Srivastava et al. 2008) favours the emergence
of Placozoa after the poriferan split (Borchiellini et al. 2001, Collins 1998, da
Silva et al. 2007) over a scenario based on mitochondrial sequences that groups
Placozoa with sponges and cnidarians as a monophyletic sister phylum to bilaterians (Dellaporta et al. 2006, Haen et al. 2007, Signorovitch et al. 2007). At present,
the position of placozoans as a sister group to all other metazoans is not supported
by any molecular phylogeny. Therefore, the apparent morphological simplicity of
placozoans evolved probably by secondary reduction. As an extracellular matrix
and basement membranes are present in homoscleromorph sponges, these features
were secondarily lost in placozoans.
The ANTP homeobox gene repertoire also indicates secondary losses in placozoans that might underlie morphological simplification. Genome sequencing
(Schierwater et al. 2008, Srivastava et al. 2008) identified some ANTP homeobox genes in addition to the previously described members of the Hox/ParaHox
(Trox-2) (Jakob et al. 2004), NK-like (Dlx, Hmx, Not) (Martinelli and Spring 2004,
Monteiro et al. 2006) and Extended Hox (Mnx) (Monteiro et al. 2006) families.
Altogether, however, the number of ANTP genes remains relatively low. Evidence
for ANTP gene losses includes the absence of some NK genes (msx, barH ), which
are present in sponges (Schierwater et al. 2008), and the assignment of single
members of large ANTP sublasses to distinct bilaterian/cnidarian gene families
(e.g. trox2 as a cnox/gsx ortholog within Hox/ParaHox subclass) (Monteiro et al.
2006, Schierwater et al. 2008). Either the distinct family is a founder family of
the entire subclass (improbable in the case of cnox/gsx), or, as already proposed
for sponges (see previous section), an extensive Hox/ParaHox genes loss occurred
153
et al. 2004). So far, only asexual reproduction by fission or budding of spherical
“swarmers” has been observed (Siewing 1985). The discovery of putative oocytes,
cleaving stages up to the 64-cell stage (Grell 1971a, 1972, Grell and Ruthman 1991)
and molecular signatures for recombination and sex (Signorovitch et al. 2005) are
indications for the existence of sexual reproduction. Hence, the description of the
Trichoplax life cycle is most likely incomplete and an intermediate parasitic stage in
a so far unidentified host or a larval stage cannot be excluded (Miller and Ball 2005).
Trichoplax exhibits a unique mode of algal feeding (“transepithelial cytophagy”)
through gaps of the upper, monocliliated epithelium and subsequent phagocytosis by
inner fiber cells (Wenderoth 1986). In addition, the ventral, non-ciliated epithelium
probably secretes digestive enzymes but is not able to perform phagocytosis.
Presenting four somatic cell types within three cell layers, a single, “top-bottom”
axis and lacking a basement membrane, extracellular matrix, mouth, gut, and nervous system, the body plan of Trichoplax is often considered as simplest throughout
metazoans (Syed and Schierwater 2002). The cell layers consist of an upper and
lower epithelium separated by inner, contractile “fibre cells” probably controlling
the amoeboid-like locomotion (Syed and Schierwater 2002). In the context of this
simple axial organization, it is surprising to find a complete set of components
required for functional Wnt or TGF-β signaling (conserved during bilaterian body
axis patterning) in the Trichoplax genome (Srivastava et al. 2008).
Phylogenetic analysis based on a large dataset from the sequenced genomes of
Trichoplax and other metazoans (Srivastava et al. 2008) favours the emergence
of Placozoa after the poriferan split (Borchiellini et al. 2001, Collins 1998, da
Silva et al. 2007) over a scenario based on mitochondrial sequences that groups
Placozoa with sponges and cnidarians as a monophyletic sister phylum to bilaterians (Dellaporta et al. 2006, Haen et al. 2007, Signorovitch et al. 2007). At present,
the position of placozoans as a sister group to all other metazoans is not supported
by any molecular phylogeny. Therefore, the apparent morphological simplicity of
placozoans evolved probably by secondary reduction. As an extracellular matrix
and basement membranes are present in homoscleromorph sponges, these features
were secondarily lost in placozoans.
The ANTP homeobox gene repertoire also indicates secondary losses in placozoans that might underlie morphological simplification. Genome sequencing
(Schierwater et al. 2008, Srivastava et al. 2008) identified some ANTP homeobox genes in addition to the previously described members of the Hox/ParaHox
(Trox-2) (Jakob et al. 2004), NK-like (Dlx, Hmx, Not) (Martinelli and Spring 2004,
Monteiro et al. 2006) and Extended Hox (Mnx) (Monteiro et al. 2006) families.
Altogether, however, the number of ANTP genes remains relatively low. Evidence
for ANTP gene losses includes the absence of some NK genes (msx, barH ), which
are present in sponges (Schierwater et al. 2008), and the assignment of single
members of large ANTP sublasses to distinct bilaterian/cnidarian gene families
(e.g. trox2 as a cnox/gsx ortholog within Hox/ParaHox subclass) (Monteiro et al.
2006, Schierwater et al. 2008). Either the distinct family is a founder family of
the entire subclass (improbable in the case of cnox/gsx), or, as already proposed
for sponges (see previous section), an extensive Hox/ParaHox genes loss occurred