2.2. THE CASE STUDIES
witnesses of an evolutionary step that occurred during the maturation of
the metazoans prior to the Cambrian explosion and during which the big
bang of metazoan radiation took place (Muller 1998). This view, which classifies sponges as model organisms for the understanding of basic pattern
formation in metazoans, was strengthened by the finding that their cells are
provided with key molecules, receptors (even neuron-like receptors), and
their interacting ligands or signaling proteins, allowing the construction of
an integrated body plan. The existence of the same or similar molecules
in sponges as in higher metazoans testifies to monophyletic origin of all
(Eu)Metazoa (Muller 1995) but does not prove that these molecules have the
same functions throughout the metazoan kingdom.
Sponges, like any other metazoan have a defined body plan as has been
artistically illustrated by Haeckel (1872). In contrast to other metazoans, adult
sponges are considered to have no pronounced anterior-posterior polarity;
surely a dorsal ventral axis is absent. In higher metazoans the famous family
of homeobox genes is involved in the patterning along the anterior-posterior
axis . However, the related genes which have been identified in sponges displaya more general function as transcription factors acting in all sponge
cells (Seimiya et al. 1998); since they are also found in plants and fungi their
existence in sponges can hardly be taken as evidence for the monophyly of
metazoans.
The following groups of molecules and their corresponding genes which
are involved in the establishment and maintenance of the body plan in
sponges have been discovered: (i) molecules involved in the recognition
of self/self and self/non-self, (ii) morphogens and (iii) enzyme(s) causing
skeletal formation.
Recently it has been found that sponges are provided with molecules
which ensure protection of their individuality against non-self by rejecting
foreign tissue and recognizing self by fusion (Muller et al. 1999a). Sequence
analyses revealed in some examples a closer similarity of sponge polypeptides to mammalian (Deuterostomia) molecules than to those found in higher
invertebrates, for example, in nematode worms (Protostomia). The most
prominent molecules involved in the recognition processes are the integrins, the putative aggregation receptor with the most complex composition
of SRCR domains and the receptor tyrosine kinases with their polymorphic
immunoglobulin-like domains, as well as the cytokine-like molecules, including the allograft inflammatory factor, pre-B-cell colony-enhancing factor, or
the endothelial-monocyte-activating polypeptide. In Fig. 2.35 experiments
are shown with the demosponge Suberites domuncula. This species is found
in nature in a blue (Fig . 2.35a) and a red type (Fig. 2.3Sb). If tissue slices are
attached and the parabionts (Fig.2.3Sd) fixed with nylon fibers, autografts
(Fig. 2.3SC, tissue from the same specimen) fuse, while allografts (Fig. 2.3se,
tissue from different specimens) reject each other and form a visible cleft.
Since in allografts the rejection process is also preceded by a short fusion period of immune molecules, cell surface receptors - besides cytokines - are
likely to be involved in immune recognition.
While the mentioned immune molecules display (very likely) no role
in morphogenesis, i.e, no function which results in an arrangement of the
cells into intricate tissue assemblies, special morphogens are present which
do cause pattern formation. These are molecules present in different concentrations within the body that determine different structures. The first
57
witnesses of an evolutionary step that occurred during the maturation of
the metazoans prior to the Cambrian explosion and during which the big
bang of metazoan radiation took place (Muller 1998). This view, which classifies sponges as model organisms for the understanding of basic pattern
formation in metazoans, was strengthened by the finding that their cells are
provided with key molecules, receptors (even neuron-like receptors), and
their interacting ligands or signaling proteins, allowing the construction of
an integrated body plan. The existence of the same or similar molecules
in sponges as in higher metazoans testifies to monophyletic origin of all
(Eu)Metazoa (Muller 1995) but does not prove that these molecules have the
same functions throughout the metazoan kingdom.
Sponges, like any other metazoan have a defined body plan as has been
artistically illustrated by Haeckel (1872). In contrast to other metazoans, adult
sponges are considered to have no pronounced anterior-posterior polarity;
surely a dorsal ventral axis is absent. In higher metazoans the famous family
of homeobox genes is involved in the patterning along the anterior-posterior
axis . However, the related genes which have been identified in sponges displaya more general function as transcription factors acting in all sponge
cells (Seimiya et al. 1998); since they are also found in plants and fungi their
existence in sponges can hardly be taken as evidence for the monophyly of
metazoans.
The following groups of molecules and their corresponding genes which
are involved in the establishment and maintenance of the body plan in
sponges have been discovered: (i) molecules involved in the recognition
of self/self and self/non-self, (ii) morphogens and (iii) enzyme(s) causing
skeletal formation.
Recently it has been found that sponges are provided with molecules
which ensure protection of their individuality against non-self by rejecting
foreign tissue and recognizing self by fusion (Muller et al. 1999a). Sequence
analyses revealed in some examples a closer similarity of sponge polypeptides to mammalian (Deuterostomia) molecules than to those found in higher
invertebrates, for example, in nematode worms (Protostomia). The most
prominent molecules involved in the recognition processes are the integrins, the putative aggregation receptor with the most complex composition
of SRCR domains and the receptor tyrosine kinases with their polymorphic
immunoglobulin-like domains, as well as the cytokine-like molecules, including the allograft inflammatory factor, pre-B-cell colony-enhancing factor, or
the endothelial-monocyte-activating polypeptide. In Fig. 2.35 experiments
are shown with the demosponge Suberites domuncula. This species is found
in nature in a blue (Fig . 2.35a) and a red type (Fig. 2.3Sb). If tissue slices are
attached and the parabionts (Fig.2.3Sd) fixed with nylon fibers, autografts
(Fig. 2.3SC, tissue from the same specimen) fuse, while allografts (Fig. 2.3se,
tissue from different specimens) reject each other and form a visible cleft.
Since in allografts the rejection process is also preceded by a short fusion period of immune molecules, cell surface receptors - besides cytokines - are
likely to be involved in immune recognition.
While the mentioned immune molecules display (very likely) no role
in morphogenesis, i.e, no function which results in an arrangement of the
cells into intricate tissue assemblies, special morphogens are present which
do cause pattern formation. These are molecules present in different concentrations within the body that determine different structures. The first
57