Cloning and characterisation of Myf5 and MyoD orthologues in Xenopus tropicalis

The myogenic regulatory genes MyoD and Myf5 are members of the bHLH transcription factor superfamily. These genes are expressed as an early response to mesoderm induction in the frog Xenopus laevis. This paper describes our work to determine the conservation of sequence, expression and function of the early myogenic genes in the closely related diploid species Xenopus tropicalis. To this end we have cloned and sequenced Xenopus tropicalis homologues of Myf5 and MyoD and found a high degree of conservation in the nucleotide and amino acid sequences of both genes. The expression of Myf5 and MyoD in Xenopus tropicalis was assayed by in situ hybridisation and the expression patterns were found to be similar to those described in Xenopus laevis. The bHLH myogenic regulatory factors are known to be able to auto-and cross-activate expression of the myogenic genes. Interestingly, however, we found that while injection of mRNA coding for either Xenopus tropicalis myf5 (Xtmyf5) or Xenopus tropicalis myoD (XtmyoD) was capable of activating expression of the endogenous Xenopus laevis myoD (XmyoD) and cardiac actin genes, neither was capable of inducing expression from the endogenous Xenopus laevis (Xmyf5) gene. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.