cDNA clones encoding the myogenic regulatory factors (MRFs) myogenin, MyoD and myf-5 were isolated by reverse-transcription polymerase chain reaction from larvae and embryos of the common carp (Cyprinus carpio L.). Myocyte-specific enhancer factor 2 (MEF2) cDNAs were identified from a cDNA library from adult carp. Northern blot analysis showed that MyoD, myf-5 and MEF2C transcripts were present in three-somite embryos, whereas myogenin and MEF2A transcripts were not detected until the 15-somite stage. Intense signals of myogenin and MyoD transcripts were observed even in 1-month-old juveniles. Levels of MyoD, myogenin and MEF2A transcripts declined between 1 and 7 months after hatching, and myf-5 gave only a weak signal in the oldest fish. In contrast, levels of MEF2C transcripts were considerably higher in 7-month-old juveniles than in 1-month-old larvae. mRNAs encoding carp myosin heavy chain and -actin were first detected at approximately the time of the first heartbeat, and levels were maximal in juveniles 1 month post-hatching. The relatively high levels of MRF mRNA in juvenile fish probably reflect the recruitment of new muscle fibres from the satellite cell population. It was concluded that the relative importance of the different members of the MyoD and MEF2 families of transcription factors for muscle differentiation changes during ontogeny in the carp.

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