Fig. 1

Homozygous you mutants lack HH sensitive cell fates. Panels A–D you^ty97 mutants show a reduction in ptc expression within the somites and neural tube. Wild type (WT) embryos at the 10-somite (A) or 22-somite stages (C) show robust expression of ptc compared to similar stage homozygous you^ty97 mutant embryos panels B and D. A–B dorsal views, panels C, D lateral views, anterior to the left. Panels E–L comparative phenotype of different you alleles and intra allelic compounds at 26 hpf. Lateral views anterior to the left. (E) Wildtype myotomes demonstrate a distinct chevron shape and the presence of a horizontal myosepta (arrows). (F) you^ty97 homozygotes exhibit u-shaped somites and lack a horizontal myosepta. (G) ptc is strongly expressed in wildtype myotomes at this stage (26 hpf). (H) ptc expression is reduced in homozygous you^ty97 mutants. (I) Homozygous you^tm146c mutants exhibit a much less severe alteration in myotomal shape. (J) Intra allelic homozygote compounds between you^ty97 and you^tm146c exhibit a myotomal phenotype intermediate between the homozygote phenotype of the two separate alleles. (K) Myotomal ptc expression is indistinguishable from wild type in homozygous you^tm146c mutant embryos. (L) ptc expression in you^tm146c/ty97 allelic compound homozygote embryos is reduced compared to you^tm146chomozygotes. (M, N) you^ty97/syu^del double mutant embryos exhibit an additive phenotype. (M) 26 somite syu^del homozygous mutant embryos exhibit a more severe reduction in somitic ptc staining than homozygous you^ty97 mutants. (N) Ptc staining is reduced to barely detectable levels within similarly staged double mutant you^ty97 and syu^del embryos. (O, P) myoD expression at the 15-somite stage, specifically within the adaxial cells, is down regulated compared to wildtype (O) in you^ty97 homozygote embryos (P), arrows indicate gaps in adaxial expression. Panels Q–T engrailed mRNA expression in the pioneer cells at the 26-somite stage is severely down regulated as compared to wildtype (Q, S) within homozygous you^ty97 mutants (R, T). Arrowheads in panels Q and R mark the midbrain–hindbrain expression of eng which is unaffected within you homozygote mutant embryos. (U, V) Floor plate expression of axial (hnf3β) is reduced or absent compared to wildtype (U) within you^ty97 homozygote mutant embryos (V). (W, X) At 26 somites, slow myosin heavy chain gene expression is severely reduced compared to wild type embryos (W) within you^ty97 homozygous mutants embryos (X).