Comparative development of cardiopharyngeal mesoderm in vertebrates (mouse) and tunicates (Ciona). a Schematic of the mouse embryo at embryonic day (E) 10 and the mouse head at E14, and lineage tree depicting the origins of cardiac compartments and branchiomeric muscles in mice. First heart field (FHF) and its derivatives are indicated in red: left ventricle (LV), and parts of left atrium (LA) and right atrium (RA); second heart field (SHF) and derivatives are in orange: right ventricle (RV), parts of left and right atria, and outflow tract (OFT); branchiomeric skeletal muscles are in yellow; extraocular muscles are in purple. All cells derive from hypothetical common pan-cardiopharyngeal progenitors (dark green) that produce the FHF and the second Tbx1/10+ cardiopharyngeal progenitors (CPM, light green). Broken lines indicate that the common FHF/SHF progenitor remains to be identified in mice. b Schematic of the different muscle tissues of the Ciona juvenile, and lineage tree depicting clonal relationships and gene expression in the cardiopharyngeal precursors. The first heart precursors (FHP) (red) and second heart precursors (SHP) (orange) contribute to the heart (red and orange mix). The exact contributions of the FHP and SHP to the compartments and cell types in the juvenile heart remain to be elucidated. Atrial siphon muscle precursors (ASM, yellow) form atrial siphon and longitudinal muscles (LoM, yellow) of the body wall. Oral siphon muscles (OSM, blue) arise from a different, non-cardiac lineage (A7.6, see text for details). Daughter cells of the Mesp+ B7.5 blastomeres (white) produce anterior tail muscles (ATM, gray) and trunk ventral cells (TVC, dark green). The latter are pan-cardiopharyngeal progenitors that express Nk4 and divide asymmetrically to produce the FHP (red) and Tbx1/10 + STVCs (light green disk). The latter divide again asymmetrically to produce SHP (orange) and the Islet+ precursors of ASM and LoM.