Fig. 8. The double mutant spg-ace shows a more severe brain phenotype (J, living embryo) than each mutant alone (D, G, living embryos). (B,C,E,F,H,I,K,L,M-P) pax2.1 expression. At midsomitogenesis, the MHB expression of pax2.1 is severely reduced and completely missing at pharyngula stages in the double mutant embryos (K,L). (M-P) Phenocopy of the double mutant phenotype produced by blocking Fgf receptors using the inhibitor SU5402. (N,P) spg embryos treated with the inhibitor (+ SU5402, P) reveal strong similarity to spg/ace double mutant embryos, which is reflected by pax2.1 staining (compare P with K; expression of pax2.1 within the otic placode is also strongly reduced by inhibition (S. Léger and M. B., unpublished). (Q) During the first steps of regionalization of the MHB and the hindbrain, positioning of the MHB is independent of Pou2 (a). During the establishment phase of the MHB organizer, Pou2 is upstream of several cognate MHB markers (b). In the hindbrain primordium, spg/pou2 and ace/fgf8 serve a combinatorial role in initiation of gbx2 and spry4.
