○Guorong Li1,2 Jun Takeuchi3 Peijun Zhang4 Hidetoshi Saiga5 Kinya Yasui1
Inst. Mol. Embryol. Gen., Kumamoto Univ.1，Dept. Biol., Shandong Teachers' Univ., China2，The Hospital for Sick Children, Canada3，EMBL, Inst. Oceanol. Chinese Acad. Sci., China4，Dept. Biol. Sci., Grad. Sch. Sci., Tokyo Metropolitan Univ.5
To identify T-box genes expressed maternally or in early development of lancelets, screening was performed by using cDNA library of early gastrulae of Branchiostoma belcheri. Totally 133 T-box clones have been isolated. Of these, 130 clones are Tbrain/eomesodermin and one is brachyury2. The other two clones are from a single gene. The last clones show affinity to vertebrate Tbx2 and 3, but differ from AmphiTbx2/3 of B. floridae. We also performed PCR by using cDNA and genomic DNA libraries of B. Belcheri with 21 primer sets designed according to AmphiTbx4/5 (Ruvinsky et al., AF262564). However, we failed to amplify any fragments. Since there is no Tbx4/5 in ascidians, we suspect if lancelets have no counterpart of Tbx4/5. Accordingly, we tentatively call the clone BbTbx2/3/4/5. Phylogenetic tree suggests that BbTbx2/3/4/5 is closer to the Tbx2 subfamily than the Tbx6 subfamily, although almost identical T- box sequence has been registered as AmphiTbx6/16 (Ruvinsky et. al., 2000). Of the three genes, only Tbr/eomes was expressed maternally in B. Belcheri. The T-domain of this gene preserves a functional sequence found in VegT. BbTbx2/3/4/5 is initially expressed at the blastopore margin except the dorsal lip in early gastrulae. As gastrulation progressed further, expression domain is restricted to the caudal end. In neurulae, expression is detected around the neurenteric canal and the overlying ectoderm. In larvae, expression in the posterior end of endoderm and mesoderm disappears, but that in ectoderm remains, at which ectodermal cells become enormously elongated forming tail fin. The tail fin is the only fin formed in lancelets.
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