Recent phylogenetic analyses have led to a re-evaluation of metazoan relationships, resulting in the grouping a number of invertebrates known as the spiralians. They include molluscs, and share highly conserved cleavage patterns, cell lineages and embryogenesis, but exhibit a wide diversity of adult body plans. This makes them an excellent group for studying the mechanisms by which changes to developmental programs give rise to evolution of new body plans. Spiralian cleavage results in an embryo that is divided into quadrants of tiered blastomeres designated A, B, C and D, which form the left, ventral, right and dorsal regions of the embryo respectively (Verdonk and van den Biggelaar, 1983). The establishment of the dorsoventral axis and the organization of the embryo are closely linked in molluscs and involve the specification of the dorsal, D-quadrant. In equally cleaving gastropods, one macromere at the 32-cell stage is induced by the overlying micromeres to become the D-quadrant macromere, 3D (van den Biggelaar and Guerrier 1979, Arnolds et al., 1983; Boring, 1989). The 3D macromere is required for the formation of the mesendoderm, a dorsoventral cleavage pattern and the specification and organization of the ectoderm (van den Biggelaar and Guerrier 1979, Boring 1989, Damen and Dictus 1994, 1996) This thesis investigates the molecular mechanisms of gastrulation in the gastropod mollusc Haliotis asinina. In particular it focuses on the role that the 3D macromere plays in regulating the development of the molluscan embryo. The induction of the 3D-macromere was inhibited by independent treatments: first by interrupting the MAPK signalling cascade using the inhibitor U0126; and second by preventing micromere-macromere interactions with the chemical monensin. Genetic markers for morphogenetic domains were used to assess the hypothesised roles of 3D in generating the mesendodermal lineage, inducing the surrounding ectodermal micromeres, establishing the dorsoventral axis, and regulating morphogenetic movement. Microarray analysis of treated embryos was conducted to determine the role that 3D induction plays in regulating gene expression during early development. Both U0126 and monensin treatments of H. asinina embryos yielded abnormal trochophores, similar to those of other gastropods whose normal development has been perturbed by inhibiting 3D specification (Arnolds et al., 1983; Boring, 1989; Damen and Dictus, 1996a; Kühtreiber et al., 1988; Lambert and Nagy, 2003; Martindale, 1986; Martindale et al., 1985; Raven, 1976; van den Biggelaar and Guerrier, 1979). Analysing these treatments revealed that MAPK is a crucial component of the 3D induction pathway in H. asinina, as found for Ilyanassa and Tectura (Lambert and Nagy, 2003; Lambert and Nagy, 2001), with its inhibition resulting in a loss of mesendodermal patterning. The induction of the 3D-macromere results in the establishment of a molluscan organiser, which is responsible for widespread gene activation during early development and is required for proper morphogenetic movements associated with gastrulation. There appears to be similarities in the inductive processes in gastrulation between vertebrates and gastropods. In particular the patterning of the midline and flanking neuroectoderm, which is induced to form neuronal tissue by underlying mesoderm. Inhibition of 3D induction prevented neurogenesis, supporting the hypothesis of a 3D-dependant inducer of the neuroectoderm, possibly the mesodermal bands (van den Biggelaar and Dictus, 2004). The establishment of the dorsoventral axis and patterning of the gastrulating gastropod appears more complex that previously suggested. It appears that the patterning of the gastropod is compartmentalised. The 3D macromere is required for the induction of quadrant identity and patterning of the pretrochal ectoderm, and preventing 3D induction results in a radialisation of gene expression patterns. In the posttrochal ectoderm preventing 3D induction does not prevent dorsoventral patterning in at least the vegetal ectoderm. This suggests that there is either 3D independent induction or regulatory processes involved in the axial patterning of the mollusc.
| Identifer | oai:union.ndltd.org:ADTP/252391 |
| Creators | Koop, Demian |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
Page generated in 0.0014 seconds