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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.