Gradients and Regulatory Networks of Wnt Signalling in Hydra Pattern Formation
The Wnt/β-catenin pathway plays an important role in axis formation and axial patterning during metazoan development. Wnt genes are expressed around the blastopore of prebilaterian and bilaterian embryos and create a gradient of ligands governing the prim
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Abstract The Wnt/b-catenin pathway plays an important role in axis formation and axial patterning during metazoan development. Wnt genes are expressed around the blastopore of prebilaterian and bilaterian embryos and create a gradient of ligands governing the primary oral-aboral body axis. Although this polarised Wnt signalling along a primary body axis is a conserved property of metazoan development, the mechanisms governing localised Wnt expression and secretion are poorly understood. We study these questions in the freshwater polyp Hydra, a classical model system for the analysis of morphogenetic gradients and modelling of de novo pattern formation processes. New data emphasise the importance of gene regulatory networks for the formation of Wnt secreting signalling centres. It is proposed that a twist of transcriptional control and gradient formation was essential for the formation of Wnt signalling centres in metazoan axis formation.
1 Introduction Wnt genes are specific for metazoans. No Wnt genes have been described so far from any unicellular eukaryotes, only certain modules of the Wnt signalling pathway have been identified in several protozoans including the cellular slime mould Dictyostelium [1]. This suggests that the emergence of Wnt genes was tightly coupled with the transition from unicellular to multicellular organisms and the evolutionary origin of metazoans [1, 2]. Orthologs of the main components of Wnt signalling pathway and a basic set of two to four Wnt ligands was identified in sponges and other prebilaterian animals [1]. In bilaterians, a major loss of Wnt gene
T.W. Holstein (*) Department for Molecular Evolution and Genomics, Centre for Organismal Studies (COS), Heidelberg University, Heidelberg, Germany e-mail: [email protected] V. Capasso et al. (eds.), Pattern Formation in Morphogenesis, Springer Proceedings in Mathematics 15, DOI 10.1007/978-3-642-20164-6_3, # Springer-Verlag Berlin Heidelberg 2013
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Fig. 1 Phylogenetic relationship of the phylum Cnidaria. The phylogenetic tree is based on the results of Collins et al. [5]. Sequenced genomes are available for Nematostella and Hydra [3, 4]
subfamilies occurred in various protostome lineages, while in deuterostomes this gene loss was minor [1, 2]. The first animals exhibiting the complete set of bilaterian Wnt genes are cnidarians. Cnidarians are more than 600 Myr old prebilaterians that exhibit a gene repertoire with 18,000–20,000 bona fide protein-coding genes [3, 4]. Cnidarians are the sister group to bilaterians (Fig. 1). The high genomic complexity of cnidarians is based on the presence of all major bilaterian gene families including those that encode for the major signalling pathways and transcription factor families. All bilaterian Wnt gene subfamilies are present in the genomes of two major cnidarian model systems (Fig. 2), i.e. the sea anemone Nematostella vectensis [6, 7] and the fresh water polyp Hydra magnipapillata [8]. Most cnidarian Wnt genes are expressed at the site of the blastopore, and they ar
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