56
2. ENVIRONMENTALLY DRIVEN PLASTICITY
The many factors now proven to affect coral growth partly explain the
huge variation found in corals on the reef. Phenotypic plasticity within
presumed morphological species boundaries questions the use of a morphological species concept for the Scleractinia. Accurate morphological analysis
and understanding of the underlying processes affecting a coral's shape will
be an important next step in coral research.
2.2.4 Biologically Inherent Regulation of Morphogenesis
In addition to the environmental influences on morphology discussed above,
there is always some level of inherent, biological control of morphology. This
allows us to identify species by their morphological characteristics. The
expression of genetically determined morphological variants is the subject
of modern developmental biology. At present, knowledge of the molecular
mechanisms controlling development in seaweeds, sponges, and corals is
still not available or fragmentary. In sponges some results have become
available very recently on the genetic regulation of development; these will be
discussed in this section. Although it is not confirmed that we can extrapolate
these results to scleractinians, for some model organisms, for example the
cnidarian Hydra (a fresh water, solitary polyp), there is a relatively extensive
literature available on the molecular mechanisms controlling development
(Wolpert et al.1998). Asimulation model of diffusive patterning mechanisms,
based on the observations of Hydra , has been proposed by Meinhardt (1998).
In this section the discussion will focus on the relatively well-studied stony
coral Stylophora pistillata.
SEAWEEDS: BIOLOGICAL REGULATION OF MORPHOGENESIS. Very little is
known, at the molecular level, about what controls the early development
of seaweeds beyond the establishment of polarity of the zygote (Love et al.
1997).There are at least two examples of hormone-like molecules which are
involved in the maintenance of the modular form of seaweeds.
The first example is a hormonal attractant called rhodomorphin, which
is expressed by wounded tissues of red seaweeds attracting the surrounding
cells to fuse (Waaland and Cleland 1974, Kim et al. 1995). Rhodomorphins
may also be involved in other secondary fusions of red algal cells including the connections between filaments that maintain the integrity of the
pseudoparenchymatous forms.
A second example is the presence of a molecular signal, secreted by
growing cells into the cell walls of Ulva sp., which inhibits the transformation
of sheets of cells into free-living , flagellated cells (Stratman et al. 1996).
When conditions are such that the concentration of the inhibitor drops below
a critical threshold, the individual cells of the thallus separate from each
other, become motile, and eventually form new colonies. It has long been
known that at least some green algal species develop disorganized clumps
of filaments, rather than the typical two-layered, sheet-like morphology, in
the absence of extrinsic bacteria or bacterial growth factors (Provasoli and
Pintner 1980, Nakanishi et al. 1996).
SPONGES: GENETIC REGULATION. The basic strategies that control individual cells in such a way that a highly organized pattern can emerge , can be
studied best on the molecular level in species belonging to the simplest and
evolutionary oldest still-extant metazoan phylum, the Porifera (sponges, see
also Fig. 1.8). Hence sponges can be considered as living fossils which are
2. ENVIRONMENTALLY DRIVEN PLASTICITY
The many factors now proven to affect coral growth partly explain the
huge variation found in corals on the reef. Phenotypic plasticity within
presumed morphological species boundaries questions the use of a morphological species concept for the Scleractinia. Accurate morphological analysis
and understanding of the underlying processes affecting a coral's shape will
be an important next step in coral research.
2.2.4 Biologically Inherent Regulation of Morphogenesis
In addition to the environmental influences on morphology discussed above,
there is always some level of inherent, biological control of morphology. This
allows us to identify species by their morphological characteristics. The
expression of genetically determined morphological variants is the subject
of modern developmental biology. At present, knowledge of the molecular
mechanisms controlling development in seaweeds, sponges, and corals is
still not available or fragmentary. In sponges some results have become
available very recently on the genetic regulation of development; these will be
discussed in this section. Although it is not confirmed that we can extrapolate
these results to scleractinians, for some model organisms, for example the
cnidarian Hydra (a fresh water, solitary polyp), there is a relatively extensive
literature available on the molecular mechanisms controlling development
(Wolpert et al.1998). Asimulation model of diffusive patterning mechanisms,
based on the observations of Hydra , has been proposed by Meinhardt (1998).
In this section the discussion will focus on the relatively well-studied stony
coral Stylophora pistillata.
SEAWEEDS: BIOLOGICAL REGULATION OF MORPHOGENESIS. Very little is
known, at the molecular level, about what controls the early development
of seaweeds beyond the establishment of polarity of the zygote (Love et al.
1997).There are at least two examples of hormone-like molecules which are
involved in the maintenance of the modular form of seaweeds.
The first example is a hormonal attractant called rhodomorphin, which
is expressed by wounded tissues of red seaweeds attracting the surrounding
cells to fuse (Waaland and Cleland 1974, Kim et al. 1995). Rhodomorphins
may also be involved in other secondary fusions of red algal cells including the connections between filaments that maintain the integrity of the
pseudoparenchymatous forms.
A second example is the presence of a molecular signal, secreted by
growing cells into the cell walls of Ulva sp., which inhibits the transformation
of sheets of cells into free-living , flagellated cells (Stratman et al. 1996).
When conditions are such that the concentration of the inhibitor drops below
a critical threshold, the individual cells of the thallus separate from each
other, become motile, and eventually form new colonies. It has long been
known that at least some green algal species develop disorganized clumps
of filaments, rather than the typical two-layered, sheet-like morphology, in
the absence of extrinsic bacteria or bacterial growth factors (Provasoli and
Pintner 1980, Nakanishi et al. 1996).
SPONGES: GENETIC REGULATION. The basic strategies that control individual cells in such a way that a highly organized pattern can emerge , can be
studied best on the molecular level in species belonging to the simplest and
evolutionary oldest still-extant metazoan phylum, the Porifera (sponges, see
also Fig. 1.8). Hence sponges can be considered as living fossils which are