Timing of initiation of muscle-specific gene expression in the ascidian embryo precedes that of developmental fate restriction in lineage cells.

The lineage of muscle cells in the ascidian embryo is well documented. Of the B-line blastomeres, B7.4 cells become restricted to give rise to muscle at the 44-cell stage, whereas B7.8 and B7.5 cells become restricted to muscle at the 64-cell stage. In this study we addressed three issues concerning the timing of initiation of muscle-specific structural gene transcription: (i) whether the initiation of the transcription of muscle-specific genes is correlated to that of developmental fate restriction; (ii) if there is a relationship in the timing, whether it is applicable to B7.4-, B7.5- and B7.8-sublineages; and (iii) whether two muscle-specific genes, one for actin and the other for the myosin heavy chain, show the same pattern of temporal expression. Whole-mount in situ hybridization revealed that hybridization signal of actin mRNA was first evident in B6.2 (the progenitor of B7.4) at the 32-cell stage, in B7.8 at the 64-cell stage, and in B7.5 around the 76-cell stage, respectively, suggesting that the transcription of the gene is initiated prior to the developmental fate restriction in the B7.4-sublineage. Presence of actin transcripts in the 32-cell embryos was confirmed by means of reverse transcription-polymerase chain reaction (RT-PCR). The myosin heavy chain gene was expressed in the same manner as the actin gene. Therefore, we concluded that the timing of initiation of transcription of the muscle-specific genes is not always associated with that of the fate restriction and that the temporal profile of the gene expression differs among the sublineages, even though two different genes have the same expression profile.