The maternal muscle determinant in the ascidian egg

Muscle formation in ascidian embryos has been investigated for more than a century as a representative example of cell fate specification by localized maternal factors within the egg cytoplasm. Observations of colored cytoplasm in combination with micromanipulation techniques have suggested the presence of a muscle-forming factor. The molecular basis has been elucidated with the discovery of macho-1. macho-1 mRNA is already present in the unfertilized egg, and translocates to the posterior region of the egg during ooplasmic movements. It encodes a zinc-finger transcription factor that positively regulates the expression of target genes. macho-1-binding cis-elements have been identified in muscle-specific zygotic genes. Maternally localized macho-1 appears to have originated in the ascidian lineage, but it activates a muscle-forming developmental program that is shared by the vertebrates. macho-1 is also involved in establishment of the anterior-posterior axis as a competence factor in mesenchyme induction in the posterior region. It is suggested that translation of the macho-1 protein is initiated at the eight-cell stage, and that the protein is inherited by all descendant blastomeres of the posterior-vegetal region. The macho-1 activities in nonmuscle descendants are suppressed or modified by cell interactions during the cleavage stages. In addition to the primary muscle specified by maternal macho-1, ascidian embryos develop secondary muscle, whose fate is determined by cell interactions. Dozens of maternal mRNAs show similar localization to macho-1, and these are known as postplasmic/PEM RNAs, being also involved in various posterior-specific developmental events. Evolutionary aspects relevant to macho-1 and tail muscle formation are also discussed in this article.