AN INTRINSIC GENETIC PROGRAM FOR AUTONOMOUS DIFFERENTIATION OF MUSCLE-CELLS IN THE ASCIDIAN EMBRYO

The B-line presumptive muscle cells of ascidian embryos have extensive potential for self-differentiation dependent on determinants prelocal ized in the myoplasm of fertilized eggs, Ascidian larval muscle cells therefore provide an experimental system with which to explore an intr insic genetic program for autonomous specification of embryonic cells. Experiments with egg fragments suggested that maternal mRNAs are one of the components of muscle determinants. Expression of larval muscle actin genes begins as early as the 32-cell stage, prior to the develop mental fate restriction of the cells, The timing of initiation of the actin gene expression proceeds the expression of an ascidian homologue of vertebrate MyoD by a few hours. Mutations in the proximal E-box of the 5' flanking region of the actin genes did not alter the promoter activity for muscle-specific expression of reporter gene, These result s, together with results of deletion constructs of fusion genes, sugge st that muscle determinants regulate directly, or indirectly via regul atory factors other than MyoD, the transcription of muscle-specific st ructural genes leading to the terminal differentiation.