Hedgehog dependent and independent smyhc1 and smyhc2 expression. All panels show lateral views. (A) smyhc1 (1 to 249 bp) in situ hybridisation at 48 hpf showing expression in secondary superficial slow fibres at the dorsal and ventral edges of the somites of a smo mutant. (B) smyhc2 (1 to 155 bp) in situ hybridisation at 48 hpf showing wild-type levels of expression in the iob of smo mutant (see Fig. S1 in the supplementary material for comparison with wild type). (C) Untreated and cyclopamine treated 55 hpf embryos showing Hh-independent smyhc1:gfp in the secondary superficial slow fibres and Hh-independent smyhc2 (1 to 324 bp) in situ hybridisation signal in the sca and iob. (D) 96 hpf wild-type sib and smo mutant. The smo mutant has slow MyHC S58 antigen in iob, sca and secondary superficial slow fibres; smyhc2 (1 to 324 bp) in situ hybridisation signal in iob and sca; but lacks scp and icp smyhc2 (1 to 324 bp) in situ hybridisation signal. (E) Untreated embryo and embryo exposed to cyclopamine from the 20-somite stage. This late cyclopamine exposure does not eliminate the smyhc1:gfp surface slow fibres but does eliminate smyhc2 (1 to 324 bp) in situ hybridisation signal in the scp and icp, as well as causing a dorsoventral narrowing of the posterior tail. Fifteen out of 15 embryos with this treatment showed this phenotype. (F) High-magnification views of posterior tail in untreated embryo and embryo exposed to cyclopamine from the 20-somite stage. In untreated embryos, smyhc2 (1 to 324 bp) in situ hybridisation signal colocalises with the general muscle MF20 antigen in the scp and icp dorsal and ventral to the smyhc:gfp surface slow fibre layer. The treated embryos lack scp and icp smyhc2 (1 to 324 bp) in situ hybridisation signal and general muscle MF20 antigen dorsal or ventral to the smyhc:gfp surface slow fibre layer. Scale bars: 25 μm. Abbreviations: icp, infracarinalis posterior; iob, inferior obliquus; sca, supracarinalis anterior; scp, supracarinalis posterior.
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