Zebrafish hoxb1b is required for hindbrain segmentation. A-O. Wild type (A, E, I, M), hoxb1a^-/-(B, F, J, N), hoxb1b^-/-(C, G, K, O) or doubly hoxb1a^-/-;hoxb1b^-/- embryos (D, H, L) were assayed by in situ hybridization for expression of hoxb1a in r4 (blue stain in A-D), krox20 in r3/r5 (red stain in A-D, I-O and blue stain in panels E-H), pax2 at the midbrain-hindbrain boundary (blue stain in E-H), fgf3 in r4 (blue stain in I-L), hoxd4a in r7 (blue stain in E-H) and hoxb3a in r5/r6 (blue stain in M-O). P. Quantification of segmentation defects in hoxb1b^-/- embryos. Rhombomere lengths were measured as indicated in the inset. MHB-r6 measures the full distance from the anterior limit of the MHB to the posterior limit of r6. p-values were computed using Students’ t-test and error bars represent standard error. N = 10 embryos. All embryos are flat mounted in dorsal view with anterior to the top. A-H and M-O are at 22hpf, while I-L are at 14hpf. r = rhombomere; MHB = midbrain/hindbrain boundary.

hoxb1a and hoxb1b are required for neuronal differentiation in the hindbrain. 48hpf wild type (A, F), hoxb1a^-/-(B, G), hoxb1b^-/-(C, D, H, I) or doubly hoxb1a^-/-;hoxb1b^-/- embryos (E, J) were assayed by immunostaining for the differentiation of branchiomotor neurons (islet1/2 staining in A-E) and Mauthner neurons (3A10 staining in F-J). Colored arrowheads indicate r4 (yellow), r5 (blue) and r6 (red). All embryos are flat mounted in dorsal view with anterior to the top.

Retinoic acid and hoxb1b act independently to activate hoxb1a transcription. 19hpf wild type (A, C, E) or hoxb1b^-/-(B, D, F) embryos were treated with DMSO (control; A, B), 10uM DEAB (C, D) or 100 nM RA (E, F) and assayed by in situ hybridization for expression of hoxb1a in r4 (blue stain in A-F) and krox20 in r3/r5 (red stain in A-F). All embryos are flat mounted in dorsal view with anterior to the top.