Preventing Wnt/β-catenin signaling allows activation of presumptive ANE specification throughout the embryo.

<p>(A) Diagram showing the regulatory factors shared between the sea urchin ANE and the vertebrate forebrain/eye field. Both territories are restricted by mechanisms dependent on posterior Wnt signaling. (B, D, F, H) <i>foxq2</i> expression in glycerol control (B, D) or Axin mRNA-injected embryos (F, H) at the 32-cell stage (B, F) and late blastula stage (D, H). <i>six3</i> expression in glycerol control (C) and TCF-Eng mRNA-injected early blastulae (G). (E) A normal 3.5-d pluteus stage embryo. (I) A 3.5-d embryo misexpressing Axin mRNA. White box outlines the ANE. Serotonergic neurons (green), DAPI (nuclei, blue). (J) Schematic showing the stages of ANE restriction in 32-cell stage (6 hpf) (a), early blastula stage (7–15 hpf) (b), and mesenchyme blastula stage (24 hpf) embryos (c). ANE (blue), nβ-catenin (orange nuclei at 32-cell stage), endomesoderm territory (orange at blastula stages), posterior ectoderm (gray at blastula stages), and unknown restriction mechanism activated by posterior nβ-catenin (orange arrows).