In sea urchin embryos, Wnt and Nodal signalling centres initiate patterning along the primary and secondary axes, respectively. Now, on , Lynne Angerer and colleagues characterise a third, neurogenic patterning centre, the animal pole domain (APD). By investigating the gene regulatory network acting in the sea urchin APD, the researchers discover that the transcription factor Six3 is required for the expression of most of the regulatory genes expressed early in this domain. Six3 is necessary, they report, for the development of the APD and all neurons and is sufficient to suppress Nodal and Wnt signals and to respecify nearly all the cells in the embryo to form an enlarged but appropriately patterned APD. Thus, the APD is a Six3-dependent neurogenic patterning centre in sea urchin embryos and, the researchers suggest, because many Six3-dependent regulatory genes are orthologous to genes expressed in the developing vertebrate forebrain, certain components of the gene regulatory network that regulates neurogenesis may have originated in the common ancestor of echinoderms and vertebrates.
