Fig. 3. The wea mutant atrium lacks atrial myosin heavy chain. (A-D) Lateral views at 48 hpf, anterior to the left. (A,B) Whole-mount immunofluorescence with MF20 (TRITC) and S46 (FITC). In wild-type embryos (A), double exposure indicates MF20 staining in the ventricle (red), and overlap of MF20 and S46 staining in the atrium (yellow). Ventricle (V) and atrium (A) are indicated. Inset shows MF20 staining only; immunoreactivity in both chambers is clear. In wea mutant embryos (B), double exposure indicates that MF20 stains the ventricle (red), but neither MF20 nor S46 stain the atrium. Inset, showing MF20 alone, reinforces that MF20 stains only the ventricle in wea mutants. (C,D) Whole-mount immunofluorescence with the anti-tropomyosin antibody CH1 (FITC). CH1 stains both chambers in wild-type (C) and wea mutant embryos (D). Note that the wea mutant ventricle appears smaller than the wild-type ventricle.