The effect of smyd1b single or smyd1a; smyd1b double mutation on myofibril organization in slow muscles. Zebrafish embryos of WT control, smyd1bsa15678, and smyd1amb5; smyd1bsa15678 mutant embryos were fixed at 72 hpf. Thick filaments in slow muscles were analyzed by immunostaining with anti-MyHC (F59) antibody (A–C), whereas the M-line organization in fast muscles was analyzed by Myom3-RFP localization in the slow myofibers of trunk muscles (D–F). Lateral view of myosin light chain organization in slow muscle fibers of WT control (A), smyd1bsa15678 (B), and smyd1amb5; smyd1bsa15678 (C) mutant embryos at 72 hpf. Lateral view of M-line organization of Myom3-RFP in slow muscle fibers of WT Myom3mn0067gt control (D), Myom3mn0067gt; smyd1bsa15678 (E), and Myom3mn0067gt; smyd1amb5; smyd1bsa15678 (F) mutant embryos at 72 hpf. Scale bar, 30 µm (A, D).