Fig. EV1

Myofiber volume scales linearly with myofiber nuclear number regardless of the growth conditions. (A) Schematic highlights the fast myofiber compartment. SP, spinal cord. NO, notochord. BV, blood vessels. I, intestine. (B) Histological examination of the labeled myofibers in the Tg(mylpfa:palm-mTurquoise2) transgenic line (left). White dashed box outlines the magnified region shown to the right. F310 Ab stains fast myofibers. (C) Schematic highlights the slow myofiber compartment. (D) Histological examination of the labeled myofibers in the palmuscle-Dual: Tg(mylpfa:palm-mTurquoise2; mylpfa:H2A-mCherry) double transgenic line (left). White dashed box marks the magnified region shown to the right. Orange dashed lines outline the entire trunk compartment. F59 Ab stains slow myofibers. White arrows point to slow myofibers. White asterisks mark the fast myofiber nuclei. (E, F) Linear regression coefficients analyses of myofiber number versus nuclear number across all 32 myotomes in individual animals at 10 dpf (E) and 14 dpf (F). (G) Timeline of the growth manipulation and tracking scheme. Representative images of fish grown under either slow growth (SG; 1 animal in 20 ml) or fast growth (FG; 1 animal in 200 ml) conditions at 14 dpf. White arrows point to the emergence of swim bladder, median fin fold, and caudal fin structures under the FG condition. White dashed lines mark the fin fold region. (H–M) Quantification of anatomical traits under SG and FG: Standard length, SL (H); Snout-operculum length (I); Snout-vent length (J); Height at nape (K); Height at anterior of anal fin, HAA (L) and Eye diameter (M). (N) A cross-sectional image of a myotome, individual myofibers are depicted in pseudocolor by Cellpose. (O–R) Quantitation of the myofiber cross-sectional area in either the dorsal or ventral compartment of Myotome #12 at 10 dpf (O, P) and 14 dpf (Q, R). (S) Quantification of individual myofiber volume at 10 dpf under either slow growth (SG) or fast growth (FG) conditions. (T) Linear regression coefficients were calculated for myofiber volume versus nuclear number (10 dpf). (U) Quantification of the individual myofiber volume at 14 dpf under either the SG or the FG conditions. (V) Linear regression coefficients were calculated for myofiber volume versus nuclear number (14 dpf). Data from biological replicates are shown as mean ± standard deviation (H–M, S, U) or violin plots (solid lines, median; dashed lines, quartiles; O–R). Significance was examined by examined by two-tailed Student’s t-test (H–L) or two-tailed Mann–Whitney test (M, O–R). P values are shown above the horizontal lines for intergroup comparisons. n = number of animals (H–M, O–R) and m = number of myofibers (O–S, U). Four animals (E, F) and ten animals (S, U) were used in each condition. Scale bars, 50 µm (B, D); 1 mm (G).