The Xenopus MyoD gene: an unlocalised maternal mRNA predates lineage-restricted expression in the early embryo.

Expression of the mouse MyoD gene appears to represent a critical point in the commitment of cultured cells to muscle. In Xenopus, myogenic commitment begins during mesoderm induction which is initiated early in development by endogenous growth factors. To study MyoD gene expression during induction, a Xenopus MyoD gene and homologous cDNAs were selected from Xenopus libraries and analysed. Two different cDNAs have been sequenced. They code for proteins closely related to each other and to mouse MyoD and are likely to be expressed from duplicated Xenopus MyoD genes. Surprisingly, MyoD mRNA is first detected during oogenesis and the maternal species is not localized exclusively to the region of the blastula fated to muscle. Zygotic MyoD mRNA accumulates slowly above maternal levels beginning at the MBT and new transcripts are localized to the somitic mesoderm. Expression outside of somites has been detected in developing heads of tailbud embryos and can be induced in blastula animal pole explants treated with mesoderm-inducing factors. The early expression of MyoD in Xenopus development suggests that it may play a part in the induction of muscle mesoderm and generally strengthens the evidence that MyoD is determinative in muscle commitment. In addition, the initiation of MyoD transcription at the MBT and its stimulation by mesoderm-inducing factors implies that MyoD gene expression is an immediate early response to mesoderm induction.