In situ mRNA hybridisation and immunohistochemistry were performed as described (Hammond et al., 2007 (link)). Fluorescein- or digoxigenin-tagged probes used were mef2c and mef2d (Ticho et al., 1996 (link)), myod and myogenin (Weinberg et al., 1996 (link)), smyhc1 (Bryson-Richardson et al., 2005 (link)), myhz1, mylz2, smbpc, tnnc, tpma (Xu et al., 2000 (link)), acta1, actc, hsp90a (I.M.A.G.E clones 6997034, 7284336 and 7259827, respectively). Anti-Mef2 was raised in rabbit against a C-terminal peptide of human MEF2 (c-21, Santa Cruz, used 1:200). Anti-Mef2c is a rabbit polyclonal made against aa140-238 of hMEF2C (McDermott et al., 1993 (link)) (81.8% identity to aa139-237 of zebrafish Mef2c, used 1:1000), A4.1025 (recognise all MyHC 1:5 (Dan-Goor et al., 1990 (link))), F59 and S58 (anti-slow MyHC 1:5; DSHB (Devoto et al., 1996 (link))), EB165 (anti-fast MyHC, DSHB 1:1), anti-α-Actinin (Sigma 1:500), anti-Tropomyosin (CH1, Sigma 1:1000), anti-Titin T12 (a gift from D. Furst, University of Bonn, Germany, 1:1), anti-cardiac actin (Ac1-20.4.2, Progen), anti-MyBP-C (rabbit polyclonal, gift from M. Gautel, King's College London, UK), anti-Pax3/7 (DP312; Hammond et al., 2007 (link)). Slow MyHC was detected with S58 in dual staining with EB165 and with F59 elsewhere. Secondary reagents were Alexa-conjugates (Invitrogen) or peroxidase-conjugates (Vector). Embryos were dissected, flatmounted in glycerol or Citifluor (Agar) and images recorded on a Zeiss Axiophot with Axiocam using Openlab software, or on a Zeiss LSM510. Except where stated otherwise, confocal images are short stacks of one side of the embryo at around somites 10-13.