nr2f1a expression. A,B: Dorsal view during early segmentation stages co-stained with r3 and r5 marker krox20 (red). C: Optical section of 12 hours post fertilization (hpf) embryo at anteroposterior level indicated by dashed line in B. Arrow indicates endoderm expression. D–I: Dorsal view (D–F) and lateral view (G–I) during segmentation stages. J,K,M: Dorsal view. L,N: Lateral view. am, anterior midbrain; at, anterior tegmentum; c, cerebellum; d, diencephalon; LPM, lateral plate mesoderm; MHB, midbrain–hindbrain boundary; ov, otic vesicle; pa, pharyngeal arch; pt, prethalamus; r, retina; s, somite; sc, spinal cord; t, telencephalon. Arrowheads indicate r3 and r5. Bracket indicates hindbrain.

nr2f1b expression. A–C: Dorsal view during segmentation stages, anterior to left. D,G,I: Dorsal view. E,H,J: Lateral view, anterior to left. F: Transverse section through r5. am, anterior midbrain; at, anterior tegmentum; c, cerebellum; d, diencephalon; MHB, midbrain–hindbrain boundary; n, notochord; ov, otic vesicle; pa, pharyngeal arch; pt, prethalamus; r4, rhombomere 4. Arrowheads indicate r3 and r5. Bracket indicates hindbrain.

nr2f2 expression. A,C: Dorsal view, co-stained with krox20 (red). Arrow indicates gap between presumptive diencephalon and anterior midbrain. B: Dorsal view of trunk, anterior to left, co-stained with aldh1a2 (red). D: Dorsal view, co-stained with pea3 (red). E–G,K,N,P: Dorsal view. H–J,L,O,Q: Lateral view. M: Transverse section through r5. am, anterior midbrain; at, anterior tegmentum; c, cerebellum; d, diencephalon; LPM, lateral plate mesoderm; MHB, midbrain–hindbrain boundary; n, notochord; ov, otic vesicle; pa, pharyngeal arch; pf, pectoral fin; pt, prethalamus; r, retina; s, somite; sc, spinal cord; t, telencephalon. Arrowheads indicate r3 and r5. Bracket indicates hindbrain.

nr2f5 expression. A: Lateral view, anterior to left. B,C: Dorsal view, co-stained with krox20 (red). D–I: Dorsal view (D–F) and lateral view (G–I) during segmentation stages. J,K: Dorsal view. L,M: Lateral view. l, lens; of, optic fissure; pa, pharyngeal arch; r, retina; s, somite; sc, spinal cord. Arrowheads indicate r3 and r5. Bracket indicates hindbrain.

nr2f expression at 12 hours post fertilization (hpf) with retinoic acid (RA) and DEAB treatment 8–10 hpf. A–C: Expression of RA target gene cyp26a1 to confirm treatment efficiency, co-stained with myoD (red) to mark the myotome. D–F: MHB and hindbrain markers pax2a, krox20 (red), and hoxd4a show changes in tissue specification. G–I: nr2f1a expression. Arrow indicates changes in lateral plate mesoderm (LPM) expression. J–L: Transverse sections showing changes in nr2f1a transcript levels in the endoderm (arrows) and LPM (arrowheads) at anteroposterior levels designated by dashed lines in G–I. M–O: nr2f2 expression. P–R: nr2f5 expression. N,Q: Arrowheads indicate a posterior extension of hindbrain expression. Brackets indicate loss of hindbrain expression.

nr2f expression at 12, 16, and 18 hours post fertilization (hpf) with retinoic acid (RA) and DEAB treatment 10–12 hpf. A–I: nr2f1a, nr2f2, and nr2f5 expression at 12 hpf. J–O: nr2f1a and nr2f2 expression at 16 hpf after RA or DEAB treatment from 10–12 hpf. P–U: nr2f1a and nr2f2 expression at 18 hpf after RA or DEAB treatment from 10–12 hpf showing a return of anterior hindbrain expression (asterisk). Arrowheads indicate a posterior extension of hindbrain expression. Brackets indicate loss of hindbrain expression.

nr2f expression at 12 and 18 hours post fertilization (hpf) after SU5402 treatment. A–F: nr2f1a and nr2f2 expression at 12 hpf. SU5402 treatment from 8–12 hpf (B,E) or 10–12 hpf (C,F). G–L: nr2f1a and nr2f2 expression at 18 hpf. SU5402 treatment from 8–18 hpf (H,K) or 10–12 hpf and washed out (I,L). Arrowheads indicate rhombomere-specific expression in the hindbrain. Asterisk indicates gap in expression, possibly between midbrain and hindbrain.

Acknowledgments
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