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F. Raible and P.R.H. Steinmetz
the colony-forming Proterospongia-like species and metazoans (e.g. Fat-type cadherin proteins) could be established, functional analysis has helped to discriminate
between a “primitive metazoan”-type function in cell–cell recognition and/or adhesion during colony formation, or a “unicellular”-type function prior to the evolution
of multicellularity (e.g. predation and phagocytosis of bacteria, detection of environmental influences). In the former case, those domains evolved divergent functions in
Monosiga after the loss of colony formation. In the latter case, the evolution of protein domains absent in choanoflagellates (e.g. integrin β) and the evolution of novel
proteins by rearranging pre-existing domains (into e.g. “classical”-type cadherins)
were crucial steps on the way to multicellularity.
As far as cell type diversity is concerned, the major absence of metazoan-specific
transcription factor families correlates with the low potential for cell differentiation in choanoflagellates. However, the secondary loss of homeobox genes shows
that in the course of choanoflagellate evolution, transcription factor complexity and
cell type diversity could have undergone secondary reduction. Extending molecular analyses to a larger number of choanoflagellate species, and also to filasterean
and ichtyosporean choanozoans (e.g. Ministeria, Capsaspora or Sphaeroforma) that
form outgroups to the Choanoflagellate + Metazoa group, will clarify the pattern of
gene gains and losses during the evolution of choanoflagellates and metazoans.
5.3 Sponges: The Evolution of Animal Development, Body Axis,
Cell Types and Epithelia
Sponges are mostly marine, sessile and filter-feeding animals that are widely
regarded as the most “simple” animal phylum. Their body plan consists only of two
main epithelial layers: the single- or double-layered pinacoderm forming a protective outer and partially inner layer, and the choanoderm, built of ciliated collar cells
(choanocytes), that produce a water flow entering through pores (ostia) and exiting
via an larger opening (osculum). In between the pinaco- and the choanoderm lies
the mesohyl, connective tissue filled with amoeboid cells and extracellular matrix.
The choanocytes either cover most of the inner body layer or lie within a system
of internal chambers. The apparent simplicity of sponges is also reflected on the
level of cell types: demosponges are thought to have only a dozen histologically
distinguishable adult cell types among which are skeleton-forming sclerocytes or
contractile myocytes (Siewing 1985). Additional cell types may be larval-specific
(e.g. primitive photoreceptor cells, see below) or further distinguishable by specific
vesicle forms and contents. Sponges lack true muscle cells and neurons.
Traditionally, the phylum Porifera is subdivided into glass sponges
(Hexactinellida), demosponges (Demospongiae) and calcareous sponges (Calcarea)
all having evolved from a common ancestor (monophyly) (Philippe et al. 2009).
Alternative models suggest that homoscleromorphs (traditionally demosponges),
Calcaraea and/or Hexaxtinellida might be paraphyletic sponge groups more closely
related to other metazoans than to the other sponges (Borchiellini et al. 2004, 2001,
Haen et al. 2007, Sperling and Peterson 2007) (for a more extensive discussion, see
Chapter 4). In any case, sponges are regarded as the oldest extant metazoan group
F. Raible and P.R.H. Steinmetz
the colony-forming Proterospongia-like species and metazoans (e.g. Fat-type cadherin proteins) could be established, functional analysis has helped to discriminate
between a “primitive metazoan”-type function in cell–cell recognition and/or adhesion during colony formation, or a “unicellular”-type function prior to the evolution
of multicellularity (e.g. predation and phagocytosis of bacteria, detection of environmental influences). In the former case, those domains evolved divergent functions in
Monosiga after the loss of colony formation. In the latter case, the evolution of protein domains absent in choanoflagellates (e.g. integrin β) and the evolution of novel
proteins by rearranging pre-existing domains (into e.g. “classical”-type cadherins)
were crucial steps on the way to multicellularity.
As far as cell type diversity is concerned, the major absence of metazoan-specific
transcription factor families correlates with the low potential for cell differentiation in choanoflagellates. However, the secondary loss of homeobox genes shows
that in the course of choanoflagellate evolution, transcription factor complexity and
cell type diversity could have undergone secondary reduction. Extending molecular analyses to a larger number of choanoflagellate species, and also to filasterean
and ichtyosporean choanozoans (e.g. Ministeria, Capsaspora or Sphaeroforma) that
form outgroups to the Choanoflagellate + Metazoa group, will clarify the pattern of
gene gains and losses during the evolution of choanoflagellates and metazoans.
5.3 Sponges: The Evolution of Animal Development, Body Axis,
Cell Types and Epithelia
Sponges are mostly marine, sessile and filter-feeding animals that are widely
regarded as the most “simple” animal phylum. Their body plan consists only of two
main epithelial layers: the single- or double-layered pinacoderm forming a protective outer and partially inner layer, and the choanoderm, built of ciliated collar cells
(choanocytes), that produce a water flow entering through pores (ostia) and exiting
via an larger opening (osculum). In between the pinaco- and the choanoderm lies
the mesohyl, connective tissue filled with amoeboid cells and extracellular matrix.
The choanocytes either cover most of the inner body layer or lie within a system
of internal chambers. The apparent simplicity of sponges is also reflected on the
level of cell types: demosponges are thought to have only a dozen histologically
distinguishable adult cell types among which are skeleton-forming sclerocytes or
contractile myocytes (Siewing 1985). Additional cell types may be larval-specific
(e.g. primitive photoreceptor cells, see below) or further distinguishable by specific
vesicle forms and contents. Sponges lack true muscle cells and neurons.
Traditionally, the phylum Porifera is subdivided into glass sponges
(Hexactinellida), demosponges (Demospongiae) and calcareous sponges (Calcarea)
all having evolved from a common ancestor (monophyly) (Philippe et al. 2009).
Alternative models suggest that homoscleromorphs (traditionally demosponges),
Calcaraea and/or Hexaxtinellida might be paraphyletic sponge groups more closely
related to other metazoans than to the other sponges (Borchiellini et al. 2004, 2001,
Haen et al. 2007, Sperling and Peterson 2007) (for a more extensive discussion, see
Chapter 4). In any case, sponges are regarded as the oldest extant metazoan group