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Fig. 4 Requirement of the Dr1B enhancer for gill cover growth. (A) Schematic of two mutant alleles in the D. rerio pou3f3b locus. el502 is a 2-bp deletion in the pou3f3b-coding sequence, and ci3010 is a 1,025-bp deletion of the Dr1B enhancer. (B) Representative adult mutants stained with Alcian blue and Alizarin red show reduced opercles (Op) and subopercles (Sop) compared with the wild-type control. Fused Sop bones are indicated with asterisks. All fish were 19-mm standard length. (Scale bar, 500 μm.) (C) Opercle area normalized to standard length, compared by one-way ANOVA with Tukey’s multiple-comparisons test. Horizontal lines denote the mean. (D) In situ hybridization for pou3f3b (magenta) in the Dr1B enhancer mutant relative to a sibling control reveals no pronounced loss of expression at 48 hpf. Background fluorescence (blue) shows the general structure of the arches. (Scale bar, 50 μm.)