Differential expression of two MyoD genes during early development of the trout : comparaison with myogenin

In the trout Oncorhynchus mykiss genome, two distinct MyoD genes (TMyoD and TMyoD2) and a single myogenin gene (Tmyogenin) have been identified. The two MyoD genes are believed to arise from a recent tetraploidization of the salmonid genome. During the anterior-to-posterior wave of somite formation, the TMyoD transcript is present initially in adaxial cells of both the presomitic mesoderm and the forming somites. A lateral extension of TMyoD labelling is observed in maturing somites when progressively they acquire the characteristic chevron shape. In contrast, the initial expression of TMyoD2 takes place in somites which have been formed already and is limited to cells of the posterior domain of the somites. Later, when all the myotomes have acquired their chevron shape, TMyoD2 transcript disappears progressively from the inner part of the myotomes until it is present only in the superficial part where slow oxidative fibres differentiate. The expression of Tmyogenin is detected first in adaxial cells of forming somites. Shortly after the formation of the somite, Tmyogenin expression extends from the adaxial cells to the posterior lateral regions of the somites and then progresses towards the anterior region. TMyoD, TMyoD2 and Tmyogenin are dynamically and differentially expressed in the developing somite suggesting that they are piaying distinet roles in the early myogenesis of trout.