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Blastopore forms4/16/2023 ![]() The blastopore fate of the bilaterian ancestor plays a crucial role in understanding the transition from radial to bilateral symmetric organisms. This variability, combined with an absence of information from key taxa, hampers the reconstruction of the ancestral developmental mode of the Protostomia and the Bilateria. In deuterostomes, the blastopore forms the anus, but its fate in protostome groups is variable. The genes or proteins that are inactive are represented in light grey.The fate of the blastopore during development in the bilaterian ancestor is currently not well understood. (G) In the absence of Nodal signaling, both the ventral and the dorsal inducing signals are not produced, ciliary band genes are not repressed and unrestricted MAP kinase signaling promotes differentiation of the ventral and dorsal ectoderm into neural ectoderm and ciliary band. The ectoderm surrounding the blastopore differentiates into dorsal ectoderm likely because it receives Wnt signals that antagonize GSK3 and promote BMP signaling. The presence of Chordin and Lefty in the prospective ciliary band allows expression of ciliary band genes to be maintained in this region. A high level of MAP kinase activity resulting from FGFA signaling in the lateral ectoderm likely contributes to maintain a low level of Nodal and BMP signaling within the presumptive ciliary band region by phosphorylating Smad1/5/8 and Smad2/3 in the linker region, which inhibits their activity. In the dorsal ectoderm, BMP signaling strongly repress the ciliary band fate partly by inducing the expression of the irxA repressor. At blastula stages, protein complexes containing BMP2/4 and Chordin can diffuse towards the dorsal side to specify dorsal fates. Chordin prevents BMP signaling within the ventral ectoderm and probably within the presumptive ciliary band region. Within the ventral ectoderm, Nodal induces expression of bmp2/4 and of its antagonist chordin. Nodal induces its antagonist Lefty, which diffuses away from the ventral ectoderm up to the presumptive ciliary band territory. Nodal signaling on the ventral side promotes differentiation of the ventral ectoderm and stomodeum and represses the ciliary band fate probably through the activity of Goosecoid as well as of additional repressors. Maternal factors such as SoxB1 promote the early expression of ciliary band genes within the ectoderm. ![]() (F) Proposed model for regionalization of the ectoderm of the sea urchin embryo through restriction of the ciliary band fate by Nodal and BMP signaling. ![]() The triradiated stars represent the spicule rudiments. The animal pole domain is largely absent. An ectopic ciliary band forms in the dorsal ectoderm in addition to the normal ciliary band. A ciliary band-like ectoderm forms at the animal pole and in the ectoderm surrounding the blastopore. Most of the ectoderm differentiates into ventral ectoderm. In these embryos, the ectoderm surrounding the blastopore differentiates into dorsal ectoderm. An animal pole domain is nevertheless present in these embryos as shown by the presence of the apical tuft and at the molecular level by the expression of apical domain marker genes. Most of the ectoderm differentiates into an expanded large ciliary band. The thick ciliated epithelium of the ciliary band is restricted to a belt of cells at the interface between the ventral and dorsal ectoderm. Schemes describing the morphology of control embryos and perturbed embryos. ![]() Ancestral Regulatory Circuits Governing Ectoderm Patterning Downstream of Nodal and BMP2/4 Revealed by Gene Regulatory Network Analysis in an Echinoderm Figure 11Ĭhanges in identity of ectodermal territories following perturbations of Nodal or BMP signaling and novel model of ectoderm patterning. ![]()
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