Characterization of the Wnt signalling system in the coral Acropora millepora

Ukolova, Svetlana (2012) Characterization of the Wnt signalling system in the coral Acropora millepora. PhD thesis, James Cook University.

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Abstract

The Wnt signalling system plays essential roles in many animal developmental processes. While in higher animals it is involved in morphogenesis and organogenesis, in lower metazoans Wnt signalling is proposed to be primarily responsible for axis formation and patterning along the body axis. The numbers of Wnt genes in triploblasts range from 4-7 among protostomes and 11-19 among deuterostomes, which cluster into 12 different subfamilies. Recent studies have shown that members of the Cnidaria have Wnt repertoires that are as complex as those of vertebrates - the genome of Nematostella vectensis (Anthozoa) encodes fourteen Wnts, with representatives of 11 vertebrate Wnt subfamilies, and that of Hydra magnipapillata (Hydrozoa) encodes eleven. My studies have shown that the complexity of the Wnt system in the coral Acropora millepora is comparable to that of Nematostella with the presence of most of its components including the antagonists of Wnt signalling system. We have identified most of the Wnt ligands known from Nematostella, as well as a range of Fzd receptors, other downstream players and antagonists. My research of Wnt expression patterns in Acropora millepora revealed that expression of all of the known Wnt genes starts after gastrulation and that the expression patterns of some Wnt genes are quite different from their sea anemone homologues. The late onset of Wnt gene expression in Acropora suggests that they play only minor roles in gastrulation, but are more significantly involved in axial patterning during larval life. This also implies that the processes of gastrulation and axis specification are less coupled in Acropora millepora than in Nematostella. Acropora therefore, potentially permits insights into the molecular basis of axial patterning without the complication of additional roles in germ-layer specification and differentiation – unlike in Nematostella vectensis, where Wnt genes participate in both processes. On the other hand, the difference in expression patterns of Wnt homologues between these two anthozoans are intriguing and may be a result of the differences in the development of coral and sea anemone. For example, Wnt16, which is involved in planar cell polarity pathway in vertebrates (Muy-Teck et al., 2007) is expressed in a different cell layer in these two species, endoderm in Nematostella and ectoderm in Acropora suggesting that different molecular mechanisms may underlie oral pore specification in the two animals.

Another major difference between coral and sea anemone is that, while Wnts are continuously expressed from the embryo or larva to the primary polyp in Nematostella, the expression of their homologues in Acropora is completely abolished during settlement and metamorphosis. The interruption of Wnt signalling seen during Acropora development may be essential to permit the much more complex metamorphosis seen in the coral.

In Hydra, the hypostome is considered to be the centre for canonical Wnt signalling with the vast majority of Hydra Wnts being expressed in this region, but this is not the case for Acropora millepora. Although the main transcription factor responding to canonical Wnt signalling, β-catenin, is expressed in the oral region of coral larvae, a growing body of data, including analysis of Wnt ligands, inhibitors and other downstream components, implies that the oral pole in Acropora is the centre for non-canonical, rather than canonical signalling. In addition to the oral region being a centre for Wnt non-canonical signalling, there is no simple relationship between Wnt genes and their antagonists in Acropora. For example, one of the Acropora Wnt antagonists, WIF (Wnt Inhibitory Factor), is co-expressed with two non-canonical Wnt genes in the oral region of Acropora larvae. Further research and functional analysis is required to better understand the regulation of Wnt signalling at both the molecular and cellular levels in Acropora.

Item ID: 27998
Item Type: Thesis (PhD)
Keywords: Acropora millepora; axial patterning; coral embryos; coral larvae; corals; embryonic development; gastrulation; gene expression; gene transcription; genetic regulation; genetics; phylogenetics; Wnt homologs; Wnt homologues; Wnt signaling pathways; Wnt signaling systems
Date Deposited: 09 Jul 2013 02:28
FoR Codes: 06 BIOLOGICAL SCIENCES > 0601 Biochemistry and Cell Biology > 060111 Signal Transduction @ 33%
06 BIOLOGICAL SCIENCES > 0601 Biochemistry and Cell Biology > 060110 Receptors and Membrane Biology @ 33%
06 BIOLOGICAL SCIENCES > 0604 Genetics > 060405 Gene Expression (incl Microarray and other genome-wide approaches) @ 34%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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