SERCA2 is essential for cardiac development. (A–F) SERCA2 expression in early development. SERCA2 RNA is expressed in (A–C) the bilateral cardiac precursors; (D) the heart tube at 26 hpf; (E) the somites and mesenchyme of the fin buds (arrows); and (F) the facial, jaw, and pectoral fin muscles at 72 hpf. (G–K2) Comparison of SERCA2 expression in wild-type (G–K) and tre mutants (G2–K2). (G and G2) At 30 hpf, SERCA2 expression in tre heart tubes is comparable to wild type. However, at (H and H2) 48 hpf, (J and J2) 53 and 72 hpf (not shown) wild-type embryos show diffuse expression of SERCA2 in the atrium, whereas tre mutants show robust expression of SERCA2 in the atrium. At (I and I2) 48 hpf and (K and K2) 53 and 72 hpf (not shown), probes for pancardiac genes cmlc2 (and tbx20, not shown) exhibit the usual diffuse signal in the atrium for both wild-type and tre embryos, indicating robust expression of SERCA2 in tre mutants is not an artifact of morphology. (L–T) SERCA2 knockdown phenotypes. Embryos injected with 17 ng of (M, P, and S) SERCA2 morpholino (MO2) or (Q and T)5 µM CPA have heart rates H30–50% slower than normal. Hearts show weak contractility in both chambers and fail to loop but no fibrillation or arrhythmia (other than heart rate). Most fail to circulate blood. SERCA2-inhibited embryos are touch-insensitive although not completely paralyzed. a, atrium; v, ventricle.