Chamber-specific ablation and reporter lines were generated using the standard I-SceI meganuclease transgenesis technique (details in Methods). To perform ventricular cardiomyocyte ablation, Tg(vmhc:mCherry-NTR) zebrafish were treated with 5 mM MTZ as previously described9 (link). For lineage tracing experiments, Tg(vmhc:mCherry-NTR;amhc:CreERT2;β-act2:RSG) zebrafish were treated with 10 µM 4-hydroxytamoxifen as previously described5 (link). For Notch inhibition studies, zebrafish were treated with 100 µM DAPT. Live imaging, heart contraction, immunofluorescence, and whole mount in situ hybridization were performed as described in Methods.