A maternal RNA encoding smad1/5 is segregated to animal blastomeres during ascidian development.
Expressed Sequence Tag (EST) research on the ascidian Halocynthia roretzi revealed that Hrsmad1/5, a homolog of smad genes, is expressed in H. roretzi eggs. A comparison of amino acid sequences of smad family members showed that the isolated clone was a homolog of smad1 and smad5 of vertebrates. A molecular phylogenetic analysis showed that Hrsmad1/5 was separated from the common ancestor with the group containing smad1 and smad5. A northern blot analysis showed that transcript of Hrsmad1/5 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 Hrsmad1/5 was investigated by means of whole-mount in situ hybridization. Maternal transcripts of Hrsmad1/5 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 Hrsmad1/5 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 Hrsmad1/5 gene may play a role in the signal transduction process in epidermal precursor cells of ascidian embryos.
Kobayashi
A
A
Department of Zoology, Graduate School of Science, Kyoto University, Japan.
Sasakura
Y
Y
Ogasawara
M
M
Makabe
K W
KW
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001757:Blastomeres / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000096:biosynthesis* / Q000737:chemistry; D020224:Expressed Sequence Tags; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000096:biosynthesis* / Q000737:chemistry; D010802:Phylogeny; D016415:Sequence Alignment; D015398:Signal Transduction; D051785:Smad Proteins; D051902:Smad5 Protein; D015534:Trans-Activators / Q000096:biosynthesis* / Q000737:chemistry; D016212:Transforming Growth Factor beta / Q000378:metabolism; D014561:Urochordata / Q000196:embryology* / Q000235:genetics* / Q000378:metabolism
D004268:DNA-Binding Proteins; D010750:Phosphoproteins; D051785:Smad Proteins; D051902:Smad5 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta