Fig. 4. Injection of pou2 mRNA in spg^m793 mutants can rescue segmental expression of krx20 and val. (A-F) Dorsal views of pou2 expression (purple/blue) during late gastrula to early somitogenesis stages in comparison with that of val in r5/r6 (red; D,E) and krx20 in r3/r5 (red; F). (A-C) High-level pou2 expression in the hindbrain primordium is marked by a red arrow. (G-J) Restriction of pou2 expression to the epiblast. Sagittal (H) and transverse (G,I,J) sections through the neural plate and notochord of wild-type embryos stained for pou2 (blue) and ntl (red). (I) Arrows and arrowheads mark strong lateral and medial pou2 expression in the hindbrain primordium, respectively. Embryonic stages are indicated. (K-R) Dorsal views of the hindbrain primordia of uninjected and pou2 mRNA-injected embryos at the 5- to 6-somite stage. The genotypes of the embryos are wild-type or spg^m793, as indicated at the top. Genotypes were determined by an allele-specific restriction polymorphism detected in genomic DNA amplified from the embryos shown. Expression of krx20 (K-N) and val (O-R) was detected by WISH. pax2.1 expression was also assayed (K-N) to compare rescue of hindbrain gene expression in spg^m793 mutants with that of the MHB. While pax2.1 expression in the MHB is not shown, pax2.1 in the otic placodes is visible on the images (K-N; lateral edges between hindbrain and otic placodes highlighted by dotted white lines). Embryos in O and Q were determined to be +/+ for spg. Anterior is up in all whole-mount embryos shown. 2-6, rhombomeres 2-6; ep, epiblast; h, hindbrain; hy, hypoblast; m, midbrain; no, notochord; tb, tailbud stage.