5 Metazoan Complexity
159
to the opposite, apical pole, gives rise directly to the oral-aboral axis (Keller et al.
2000, Nielsen 2001). This is not the case in bilaterians, where the early apicalblastoporal (AB) axis is not directly comparable to the bilaterian antero-posterior
(AP) or dorso-ventral (DV) axes. The single bilaterian AB axis often transforms
into the AP and DV axis by gastrulation movements such as convergent extension
(Fioroni 1992, Keller et al. 2000, Steinmetz et al. 2007). For example, the neuroectoderm adjacent to the early “organizer” regions at the blastopore margin of fishes
or frogs gives rise to both anterior brain and posterior spinal chord (Hirose et al.
2004, Keller 1975, Woo and Fraser 1995). Also in protostome spiralians, the 2d
blastomere is localized at one end of the early blastopore rim, and gives rise to
the entire trunk ectoderm (Ackermann 2002, Nielsen 2004, Shankland and Seaver
2000). The fate of the blastopore, although considered to be conserved in bilaterians
and cnidarians, differs and gives rise to the anterior mouth in deuterostomes, the posterior anus in protostomes, or both mouth and anus in “amphistome” animals, e.g.
some annelids or nematodes (Arendt and Nübler-Jung 1997, Holland 2000, Nielsen
2001). Therefore, the apical-blastoporal axis of Bilateria is in fact as difficult to
compare to the definite AP and DV axes as the apical-blastoporal (=aboral-oral)
axis of Cnidaria (see Fig. 5.4).
In turn, this implies that it is the apical-blastoporal axes of cnidarians and bilaterians that might be truly homologous. Indeed, such a homology is supported by
the conserved expression of brachyury (Technau 2001), forkhead (Fritzenwanker
et al. 2004) and several Wnts (Kusserow et al. 2005) at the blastopore, the nuclear
localization of β-catenin at the endoderm invagination site, and the capacity of
the blastopore rim to induce a second apical-blastoporal axis (Kraus et al. 2007).
The conflicting positions of the blastoporus at the cnidarian animal pole and the
bilaterian vegetal pole is easily explained by the independent repositioning of the
pronucleus, defining the animal-vegetal axis, in cnidarians or bilaterians (Lee et al.
2007, Martindale 2005). The hypothesis of axial evolution can further be tested
by comparing the early cnidarian oral-aboral patterning with apical-blastoporal
patterning in pre-gastrula stages of bilaterians.
5.5.5 Cnidarians and the Evolution of Mesoderm
Another example of how the ancestral components of newly acquired features can
be detected by comparing cnidarian and bilaterian development is the origin of
mesoderm, the third germ layer in bilaterians, from a bilayered cnidarian-bilaterian
ancestor. Although a mesoderm proper is absent in cnidarians, they possess muscle cells, a main derivative of the mesoderm in Bilateria (Siewing 1985). Several
general cnidarian muscle types can be discerned: the “myo-epithelial” type that is
ubiquitous in cnidarians and combines contraction with additional sensory, secretory or digestive functions; and the more specialized muscle cell types in hydrozoan
medusae and anthozoan polyps that often appear “striated” due to intracellular serial
repetition of contractile units (Amerongen and Peteya 1980, Schuchert et al. 1993,
159
to the opposite, apical pole, gives rise directly to the oral-aboral axis (Keller et al.
2000, Nielsen 2001). This is not the case in bilaterians, where the early apicalblastoporal (AB) axis is not directly comparable to the bilaterian antero-posterior
(AP) or dorso-ventral (DV) axes. The single bilaterian AB axis often transforms
into the AP and DV axis by gastrulation movements such as convergent extension
(Fioroni 1992, Keller et al. 2000, Steinmetz et al. 2007). For example, the neuroectoderm adjacent to the early “organizer” regions at the blastopore margin of fishes
or frogs gives rise to both anterior brain and posterior spinal chord (Hirose et al.
2004, Keller 1975, Woo and Fraser 1995). Also in protostome spiralians, the 2d
blastomere is localized at one end of the early blastopore rim, and gives rise to
the entire trunk ectoderm (Ackermann 2002, Nielsen 2004, Shankland and Seaver
2000). The fate of the blastopore, although considered to be conserved in bilaterians
and cnidarians, differs and gives rise to the anterior mouth in deuterostomes, the posterior anus in protostomes, or both mouth and anus in “amphistome” animals, e.g.
some annelids or nematodes (Arendt and Nübler-Jung 1997, Holland 2000, Nielsen
2001). Therefore, the apical-blastoporal axis of Bilateria is in fact as difficult to
compare to the definite AP and DV axes as the apical-blastoporal (=aboral-oral)
axis of Cnidaria (see Fig. 5.4).
In turn, this implies that it is the apical-blastoporal axes of cnidarians and bilaterians that might be truly homologous. Indeed, such a homology is supported by
the conserved expression of brachyury (Technau 2001), forkhead (Fritzenwanker
et al. 2004) and several Wnts (Kusserow et al. 2005) at the blastopore, the nuclear
localization of β-catenin at the endoderm invagination site, and the capacity of
the blastopore rim to induce a second apical-blastoporal axis (Kraus et al. 2007).
The conflicting positions of the blastoporus at the cnidarian animal pole and the
bilaterian vegetal pole is easily explained by the independent repositioning of the
pronucleus, defining the animal-vegetal axis, in cnidarians or bilaterians (Lee et al.
2007, Martindale 2005). The hypothesis of axial evolution can further be tested
by comparing the early cnidarian oral-aboral patterning with apical-blastoporal
patterning in pre-gastrula stages of bilaterians.
5.5.5 Cnidarians and the Evolution of Mesoderm
Another example of how the ancestral components of newly acquired features can
be detected by comparing cnidarian and bilaterian development is the origin of
mesoderm, the third germ layer in bilaterians, from a bilayered cnidarian-bilaterian
ancestor. Although a mesoderm proper is absent in cnidarians, they possess muscle cells, a main derivative of the mesoderm in Bilateria (Siewing 1985). Several
general cnidarian muscle types can be discerned: the “myo-epithelial” type that is
ubiquitous in cnidarians and combines contraction with additional sensory, secretory or digestive functions; and the more specialized muscle cell types in hydrozoan
medusae and anthozoan polyps that often appear “striated” due to intracellular serial
repetition of contractile units (Amerongen and Peteya 1980, Schuchert et al. 1993,