Brachyury expression in tailless Molgulid ascidian embryos.
The T-box transcription factor gene Brachyury is important for the differentiation of notochord in all chordates, including the ascidians Halocynthia roretzi and Ciona intestinalis. We isolated Brachyury from molgulid ascidians, which have evolved tailless larvae multiple times independently, and found the genes appear functional by cDNA sequence analyses. We then compared the expression of Mocu-Bra in tailed Molgula oculata embryos to two tailless species, Molgula occulta (Mocc-Bra) and Molgula tectiformis (Mt-Bra). Here we show that both tailless species express Brachyury in the notochord lineage during embryogenesis. Initial expression of Mocu-Bra is normal in tailed M. oculata embryos; 10 precursor notochord cells divide twice to result in 40 notochord cells that converge and extend to make a notochord down the center of the tail. In contrast, in tailless Molgula occulta, Mocc-Bra expression disappears prematurely, and there is only one round of division, resulting in 20 cells in the final notochord lineage that never converge or extend. In M. occulta x M. oculata hybrid embryos, expression of Mocu-Bra is prolonged, and the embryos form a tail with 20 notochord cells that converge and extend normally. However, in Molgula tectiformis, a different tailless ascidian, Mt-Bra was expressed only in the 10 notochord precursor cells, which never divide, converge, or extend. In summary, neither Brachyury function nor the early establishment of the notochord lineage appears to be impaired in tailless embryos. In light of these results, we are continuing to investigate how and why notochord development is lost in tailless molgulid ascidian embryos.
Takada
Norio
N
Department of Zoology, Graduate School of Science, Kyoto University, Japan.
York
Jonathan
J
Davis
J Muse
JM
Schumpert
Brenda
B
Yasuo
Hitoyoshi
H
Satoh
Nori
N
Swalla
Billie J
BJ
D000595:Amino Acid Sequence; D000818:Animals; D003433:Crosses, Genetic; D018076:DNA, Complementary / Q000378:metabolism; D005326:Fetal Proteins; D015870:Gene Expression; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D009154:Mutation; D009672:Notochord / Q000196:embryology / Q000378:metabolism; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid; D020825:T-Box Domain Proteins / Q000096:biosynthesis*; D014561:Urochordata / Q000196:embryology* / Q000254:growth & development*; D000098908:Brachyury Protein
D018076:DNA, Complementary; D005326:Fetal Proteins; D020825:T-Box Domain Proteins; D000098908:Brachyury Protein