Fig. 3

The loss of PG1 hox genes causes defects in both neural crest, and hindbrain associated neuronal populations.

In hoxb1a^−/− mutants three neural crest streams exit the hindbrain (B), similar to wildtype (A). The r2 and r4 streams are fused in hoxb1b^−/− mutants (9/9 embryos) (C) and all three neural crest streams are less distinct in hoxb1a^−/−;hoxb1b^−/− mutants, with the r4 stream being reduced (8/9 embryos) (D). The Rol2 neurons (r2) (arrow), Mauthner neurons (r4) (filled arrowhead), and Mid3cm neurons (r6) (empty arrowhead) are reticulospinal neurons (labeled by RMO44) that project contralaterally across the midline (E). In hoxb1a^−/− mutants the Mauthner neuron (r4) is lost (filled arrowhead), however there is a neuron resembling Rol2 in its position (10/10 embryos)(F). In hoxb1b^−/− (12/12 embryos) (G) and hoxb1a^−/−;hoxb1b^−/− (3/3 embryos)(H) mutants, the Mauthner (filled arrowhead) and the Mid3cm (empty arrowhead) neurons are absent. Visualized with the transgenic islet1:GFP line, trigeminal BMNs differentiate to form two clusters in r2 (arrow) and r3 (filled arrowhead), the facial BMNs are born in r4 and subsequently their cell bodies migrate posteriorly to r6/7 and then laterally to their final positions (empty arrowhead) (I). The facial BMNs fail to migrate posteriorly in hoxb1a^−/− mutants (11/11 embryos)(J, empty arrowhead). Hoxb1b^−/− mutants specify fewer facial BMNs, however they migrate appropriately (12/12 embryos) (K, empty arrowhead). In hoxb1a^−/−;hoxb1b^−/− mutants there are fewer facial BMNs which fail to migrate posteriorly (5/5 embryos)(L, empty arrowhead). All embryos have been genotyped for hoxb1a^sa1191 and hoxb1b^ua1006. All images are dorsal views, anterior, to the left, (A–D) 22hpf, (E–L) 48hpf.