Maternal information stored in particular regions of the egg cytoplasm has an important function in the determination of developmental fate during early animal development1,2. Ascidians show mosaic development3,4; such autonomous development has been taken as evidence that prelocalized ooplasmic factors specify tissue precursor cells during embryogenesis. Interest has been concentrated on the mechanisms underlying the formation of muscle cells in the tail, as yellow-coloured myoplasm in eggs is preferentially segregated into muscle-lineage blastomeres5. Here we show that maternal messenger RNA of the macho-1 gene is a determinant of muscle fate in the ascidian Halocynthia roretzi. The macho-1 mRNA encodes a zinc-finger protein, and the mRNA is localized to the myoplasm of eggs. Depletion of the mRNA specifically resulted in the loss of primary muscle cells in the tail, as shown by the expression of muscle-specific molecular markers. The myoplasm of macho-1-deficient eggs lost its ability to promote muscle formation. Injection of synthesized macho-1 mRNA caused ectopic muscle formation in non-muscle-lineage cells. Our results indicate that macho-1 may be both required and sufficient for specification of muscle fate, and that the mRNA is a genuine, localized muscle determinant.
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We thank T. Nishikata for providing myosin antibody; N. Satoh for providing muscle actin cDNA; K. Kobayashi for help in the cytoplasmic transfer experiments; and M. L. King for critical reading of our manuscript. We also thank members of the Asamushi Marine Biological Station and the Otsuchi Marine Research Center for their help in collecting live ascidian adults, and members of the Misaki Marine Biological Laboratory for their help in maintaining these ascidians. This work was supported by the Research for the Future Program of the Japanese Society for the Promotion of Science, and a grant from the Human Frontier Science Program organization.
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Nishida, H., Sawada, K. macho-1 encodes a localized mRNA in ascidian eggs that specifies muscle fate during embryogenesis. Nature 409, 724–729 (2001). https://doi.org/10.1038/35055568
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