Wnt and TGF-β Expression in the Sponge Amphimedon queenslandica and the Origin of Metazoan Embryonic Patterning

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Abstract
The origin of metazoan development and differentiation was contingent upon the evolution of cell adhesion, communication and cooperation mechanisms. While components of many of the major cell signalling pathways have been identified in a range of sponges (phylum Porifera), their roles in development have not been investigated and remain largely unknown. Here, we take the first steps toward reconstructing the developmental signalling systems used in the last common ancestor to living sponges and eumetazoans by studying the expression of genes encoding Wnt and TGF-β signalling ligands during the embryonic development of a sponge. Using resources generated in the recent sponge Amphimedon queenslandica (Demospongiae) genome project, we have recovered genes encoding Wnt and TGF-β signalling ligands that are critical in patterning metazoan embryos. Both genes are expressed from the earliest stages of Amphimedon embryonic development in highly dynamic patterns. At the time when the Amphimedon embryos begin to display anterior-posterior polarity, Wnt expression becomes localised to the posterior pole and this expression continues until the swimming larva stage. In contrast, TGF-β expression is highest at the anterior pole. As in complex animals, sponge Wnt and TGF-β expression patterns intersect later in development during the patterning of a sub-community of cells that form a simple tissue-like structure, the pigment ring. Throughout development, Wnt and TGF-β are expressed radially along the anterior-posterior axis. We infer from the expression of Wnt and TGF-β in Amphimedon that the ancestor that gave rise to sponges, cnidarians and bilaterians had already evolved the capacity to direct the formation of relatively sophisticated body plans, with axes and tissues. The radially symmetrical expression patterns of Wnt and TGF-β along the anterior-posterior axis of sponge embryos and larvae suggest that these signalling pathways contributed to establishing axial polarity in the very first metazoans.