A maternal RNA encoding <i>smad1/5</i> is segregated to animal blastomeres during ascidian development

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<jats:p>Expressed Sequence Tag (EST) research on the ascidian <jats:italic>Halocynthia roretzi</jats:italic> revealed that <jats:italic>Hrsmad1/5</jats:italic>, a homolog of <jats:italic>smad</jats:italic> genes, is expressed in <jats:italic>H. roretzi</jats:italic> eggs. A comparison of amino acid sequences of <jats:italic>smad</jats:italic> family members showed that the isolated clone was a homolog of <jats:italic>smad1</jats:italic> and <jats:italic>smad5</jats:italic> of vertebrates. A molecular phylogenetic analysis showed that <jats:italic>Hrsmad1/5</jats:italic> was separated from the common ancestor with the group containing smad1 and smad5. A northern blot analysis showed that transcript of <jats:italic>Hrsmad1/5</jats:italic> was abundant in the fertilized egg. The amount of the transcript remained constant until the gastrulae and then rapidly decreased at the neurulae. The spatial expression of <jats:italic>Hrsmad1/5</jats:italic> was investigated by means of whole‐mount <jats:italic>in situ</jats:italic> hybridization. Maternal transcripts of <jats:italic>Hrsmad1/5</jats:italic> were detected in the entire fertilized egg. The signals were localized preferentially to the animal blastomeres of the 8‐, 16‐, 32‐ and 64‐cell stages. The zygotic expression of <jats:italic>Hrsmad1/5</jats:italic> started in prospective epidermal blastomeres in the animal hemisphere at the 64‐cell stage but not in cells of the central nervous system, and it decreased rapidly around the neural‐plate stage. At the tail‐bud stage, signals were detected broadly all through the trunk and in a small part of the epidermis in the tail region. This is the first report of a maternal RNA that preferentially accumulates in the animal hemisphere in early ascidian embryos. Animal blastomeres of ascidian embryos differentiate mainly into epidermis in a cell‐autonomous manner and partly differentiate into neural tissues by induction. The <jats:italic>Hrsmad1/5</jats:italic> gene may play a role in the signal transduction process in epidermal precursor cells of ascidian embryos.</jats:p>

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