The maternal muscle determinant in the ascidian egg

The ascidian, Halocynthia roretzi. (a) Adult. (b) Neurula with closing neural tube on the dorsal side.1 (c) Tadpole larva just before hatching at 35 h of development.

Fate specification in the vegetal hemisphere at the 32‐cell and 64‐cell stages.34 Anterior is to the left. An FGF signal (arrows) secreted from the vegetal endoderm blastomeres promotes asymmetric cell divisions in the marginal zone. The signal induces notochord and mesenchyme fates. The default fates are nerve cord and muscle, respectively. Sister blastomeres are connected with bars.

The primary (green) and secondary (orange and red) muscle lineages.33 Primary muscle cells are derived from the B‐line (posterior‐vegetal) blastomeres. Secondary muscle cells originate from the A‐line (anterior‐vegetal) and b‐line (posterior‐animal) blastomeres. macho‐1 is involved in muscle fate determination in the primary lineage. Anterior is to the left. Note that the anterior‐posterior order of muscle progenitor cells is reversed during the gastrula and neurula stages.

macho‐1 as a muscle determinant. (a),(b) Larval morphology and myosin expression in tail muscle cells in the control experiment. (c)–(e) Larvae in which antisense oligonucleotide against macho‐1 mRNA has been injected. (f)–(h) Muscle actin mRNA expression at the 110‐cell stage. (i) Myosin expression in larvae into which macho‐1 mRNA has been injected. (j) Nuclear localization of flag‐tagged macho‐1 protein that was translated from injected mRNA, in every nucleus at the 110‐cell stage. (Reprinted with permission from Ref 7. Copyright 2001 Nature Publishing Group)

Ooplasmic movements and localization of various factors. Cytoplasmic and cortical reorganization proceed in two major phases during the first cell cycle. (a) Yellow myoplasm in Styela partita. Copy of the original drawings by Conklin (1905).5 Yellow cytoplasm is localized to the unfertilized egg cortex. It is concentrated at the vegetal pole just after fertilization, then gradually moves toward the future posterior pole before the first cleavage takes place. It is segregated into the posterior‐vegetal muscle lineage blastomeres, and eventually into tail muscle cells of the tadpole larva. (b) Distribution of muscle‐forming activity inferred from the results of cytoplasmic transfer experiments. (c) Localization of maternal mRNA of macho‐1. Ani, animal pole; Veg, vegetal pole; Ant, anterior; Post, posterior. (Reprinted with permission from Ref 7. Copyright 2001 Nature Publishing Group)