Cdkn1c drives fast myogenesis through Myod. In situ mRNA hybridization (A,D) or protein immunodetection (B,C) at 24 hpf (A–C; lateral flatmount) or 72 hpf (D, dorsal flatmount) of embryos injected with cdkn1c MO. (A). Cdkn1c MO exacerbated somitic fast muscle loss induced by myod MO, and ablated residual fast muscle in mutant myf5hu2022;myod knockdown. Bar: 100 μm (B). Cdkn1c MO alone diminishes fast muscle differentiation, without reducing slow muscle. Dashed lines indicate positions of upper and lower transverse sections. Bars: flatmounts 100 μm, somite lateral zoom and sections 50 μm (C). Cdkn1c MO diminished Myod immunoreactivity in nuclei of nascent fast muscle and Myogenin immunoreactivity in more mature somites, particularly the expression at the posterior somite border (arrows). Bar: 50 μm. (D). Cdkn1c MO (9 ng) reduced ventral head myogenesis in a similar manner to myod MO (2 ng). Low doses of cdkn1c MO (3 ng) or myod MO (0.25 ng) that alone had little effect, synergised to reduce myogenesis in intermandibular, interhyoideus and hyohoideus muscles. Bar: 100 μm. Chart quantifies the extent of ventral hyoid muscle defects after injection of the indicated ng of each MO/embryo, with differences tested by χ2 test. All embryos were classified as having either ‘normal’, ‘weak’ or ‘no’ mylz2 mRNA in the three stated muscles. ‘Normal’ embryos (dark bars) had strong mylz2 expression, as shown in control panel. ‘Weak’ embryos (light bars) had mylz2 expression visibly below the control range, as shown in the 2 ng myod MO panel. ‘No’ embryos entirely lacked mylz2 expression, as in the 9 ng cdkn1c MO panel, are shown as white space above the bars. Data shown has n = 20 for each condition from within a single experiment.