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Fig. 4 Loss of Cos2 function results in de-repression of the Hh pathway in the myotome of the zebrafish embryo. (A) Diagrammatic representation of the synteny between regions of human (Hs; LG15) and zebrafish (Dr; LG7) chromosomes bearing the cos2 genes. fes, proto-oncogene tyrosine protein kinase; rhcg, Rh type-C glycoprotein; polg, DNA polymerase subunit 1; mrpl46, mitochondrial ribosomal protein L46; mrps11, mitochondrial ribosomal protein S11. (B) Nucleotide sequence of the first protein coding exon of cos2 and the flanking introns. The start codon is indicated in green. The sequences targeted by the start and splice MOs are highlighted. (C) Flat mount of a two-somite stage wild-type embryo, showing the pattern of ptc1 transcription. The precursors of the SSFs and MPs (arrows) and the neural plate (asterisk) are indicated. (D) Similar flat mount of a cos2 morphant with expanded domain of ptc1 expression in the somitic mesoderm (n=9/16). (E) Lateral view of the myotome of a 24 hpf wild-type embryo showing the SSFs (stained for expression of the homeodomain protein Prox1, green) and the MPs [stained for expression of the homeodomain proteins of the Engrailed (Eng) family, red]. Within the slow muscle lineage, Prox1 is expressed in SSFs as well as MPs, while Eng proteins are expressed exclusively in the MPs. Prox1- and Eng-positive MP nuclei (yellow) are indicated (arrows). (F) A similar stage cos2 morphant with supernumerary MP cells in the myotome (n=6/7). (G) Medial view of the myotome of a 24 hpf wild-type embryo. MFFs that surround the MPs and express low levels of Eng are indicated (arrows). (H) MFFs are increased in numbers in cos2 morphants (n=5/7). (I) Myotome of an embryo co-injected with cos2 MOs and cos2 sense mRNA, showing effective suppression of supernumerary MP induction that is observed in cos2 morphants (n=12/12).