Here, we discuss an evolutionary prospective for the organizer, focusing on (1) the formation of the organizer by combinatorial signaling pathways such as Wnt, Nodal, and Bmp and (2) the GRN regulating organizer formation and activity by TFs such as Lim1/Lhx1, Otx, Goosecoid, Brachyury, and FoxA. These analyses provide a platform for genome-wide evolutionary study of organizer-equivalent tissues in other organisms, including cnidarians. Recent genome-wide investigations have provided a comprehensive overview of transcription factor (TF) binding sites and regulatory principles of the gene regulatory network (GRN) in the Xenopus organizer. Here we review the molecular basis of vertebrate and cnidarian organizers, which have been widely studied using Xenopus, zebrafish, and mice for more than 20 years, and have relatively recently been studied using Nematostella. In sea anemone embryos, the organizer has recently been recognized in the blastoporal lip, implying its ancient origin among eumetazoans. This organizer is capable of inducing a secondary body axis when transplanted into the ventral region of a blastula embryo. In amphibians, the organizer-also known as the Spemann–Mangold organizer-is located in the dorsal blastopore lip of the gastrula embryo. The gastrula organizer, an embryonic tissue, has a central role in early development of all eumetazoans, from cnidarians to vertebrates.
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