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Fig. 1 wt1a-positive cells lose eGFP expression upon entering the heart tube. (A) Schematic representation of the in vivo imaging of the developing heart tube. (B) Time-lapse images of the developing heart tube between 52 and 68 h post-fertilization (hpf) in the double transgenic line epi:eGFP;myl7:mRFP. Gray images are single-channel detailed images of the areas outlined in the merged panels. There is an opposite gradual shift in the expression levels between eGFP and RFP over time. (C) Quantification of the ratio of eGFP and mRFP levels in cells of the heart tube according to the distance from the sinus venosus (SV). (D) Schematic representation of tissue dissection for SMARTer-seq of dorsal pericardium, proepicardium and heart tubes of zebrafish embryos. (E,E′) Volcano plots. Magenta dots indicate upregulated genes in the heart tube. Green dots indicate genes upregulated in proepicardium (E) or pericardium (E′). (F) Quantification of normalized counts for the epicardial marker genes wt1a and wt1b, and the myocardial gene myl7. Data are mean±s.d. (G-H‴) Whole-mount immunofluorescence against GFP and myosin heavy chain (MHC) of wt1a:CreErt2;ubi:switch zebrafish embryos, recombined between 24 and 96 hpf. The heart of a 5 days post-fertilization (dpf) embryo is shown. GFP-expressing cells are those derived from wt1a progenitors. (H-H‴) Detailed views of single slices of the region outlined in G. Asterisks indicate cells that co-express GFP and MHC. (I) Schematic representation of the downregulation of eGFP and upregulation of mRFP in cardiomyocyte progenitors upon their entry into the heart tube. Scale bars: 50 µm. dp, dorsal pericardium; ht, heart tube; sv, sinus venosus.