(A) Schematic of trunk muscle processing for analysis, pink represents fish fillet. Colours identify sib (myogkg125/+, blue) or myog-/- (myogkg125, red) samples throughout the figure. (B) qPCR analysis shows downregulation of myog, mymk, mymx and mrf4 mRNAs in adult myog-/-. Symbol shapes denote paired sib and myog-/- samples, n = 4–6 fish/genotype, paired t-test. (C) Schematic of myofibre isolation for morphometric analysis (top) and representative images (bottom) showing smaller myog-/- myofibre (red brackets) compared to age-matched sib. Scale bar = 100 μm. (D) Measure of absolute myofibre length, n = 3 fish/genotype, n = 110–120 myofibres/fish, unpaired t-test. (E) Representative images and measure of unaltered sarcomere length on freshly isolated myofibres, n = 3 fish/genotype, n = 10 myofibres/fish, unpaired t-test. Scale bar = 10 µm. (F) Representative images of isolated fixed adult myofibres show size reduction in myog-/- (red brackets). Scale bar = 100 µm. (G–J) Quantification of number of nuclei/100 µm (G), absolute number of nuclei per myofibre (H), myofibre diameter (I), and nuclear domain size (myofibre volume per nucleus) (J) showing significant changes in myofibres from juvenile (Juv, 1 month-old) and adult (Ad, 8 months-old) stages within (coloured p) or among (black p) genotypes. n = 3 fish/genotype, n = 30–50 adult myofibres/fish, n = 15–20 juvenile myofibres/fish, one-way ANOVA. (K) Relationship of number of nuclei and loge(Surface Area) indicates different growth mode between sib and myog-/- (i.e. significant slope difference, black p), despite significant correlation of loge(SA) with nuclear number within genotype (coloured p) (see Materials and methods). (L) Increase in Surface Area (SA) from juvenile to adult stage (=Ad_SA – Juv_SA) indicates reduced growth rate in myog-/-. Data from Figure 1—figure supplement 1E, unpaired t-test. All graphs report mean ± SEM. Statistical significance within (coloured p) or between (black p) genotypes is indicated.