212
N. Satah eta!.
ascidian tadpole consists of 40 cells, aligned in single file along the center of the
larval tail. Lineage of notochord cells is completely described (Conklin 1905; Nishida
1987). Thirty-two of the 40 notochord cells occupy the anterior four-fifths of the
notochord and derive from 8 of the A-line primordial notochord blastomeres of the
1l0-cell embryo, while the other 8 posterior-most cells come from 2 of the B-line
primordial blastomeres of the 1l0-cell embryo (Fig. 1C'). These 10 blastomeres
subsequently divide twice to form 40 notochord cells.
By taking advantage of these features, we determined that the expression of
Brachyury (As-I) of llalocynthia roretzi is transient, that As-T is expressed exclusively in blastomeres of the notochord lineage, and that the timing of As-T expression coincides with that of the developmental fate restriction of the blastomeres
(Fig. 1C; Yasuo and Satoh 1993). Therefore, in contrast to vertebrate and
cephalochordate Brachyury genes, the expression of the asci dian Brachyury is restricted to the differentiating notochord cells. This was confirmed in the Brachyury
gene (Ci-Bra) of another ascidian Ciona intestinalis (Corbo et al. 1997a).
2.3 Hemichordates
Hemichordata is a small phylum, containing the enteropneusts (acorn worms) and
pterobranchs. The relationship of the hemichordates and the echinoderms was recognized from their similar larval forms (Brusca and Brusca 1990; Nielsen 1995).
On the other hand, from the time of Bateson (1885), the hemichordates have been
linked to the chordates because they share chordate-specific structures and organization that can be related to the chordate body plan. However, there had been no
reports on hemichordate developmental genes before 1997.
In 1998, Tagawa et al. determined the ~xpression pattern of the Brachyury
(PfBra) gene of the acorn worm Ptychodera flava. A distinct expression was first
detected at the early gastrula stage, and the expression was restricted to the blastopore or the base of the invaginating archenteron (Fig. 1D). During the next phase
of hemichordate gastrulation, the blastopore closed, and then opened again as an
anus by the early tornaria larval stage. The expression remained at the base of the
archenteron during gastrulation, and this pattern of the PfBra expression continued during formation of the larva (Fig. 1D, E), until it disappeared in the 10-dayold larvae. In addition, another distinct expression became evident as early as the
middle gastrula stage in the region that eventually forms the mouth or stomodeum
(Fig. 1E). The PfBra expression in the stomodeum remained during formation of
the mouth, and this expression became undetectable prior to the disappearance of
the expression at the base of the archenteron. Therefore, PfBra is expressed in the
blastopore and the stomodeum of gastrulae and early tornaria larvae.
The stomochord is an organ in the adult proboscis, and its homology to chordate notochord has been suggested for a long time (Bateson 1885). This organ is
formed during metamorphosis, which usually occurs after 3 to 5 months of larval
swimming. Peterson et al. (1999a) examined whether or not PfBra is expressed in
the stomochord. They found that PfBra is not expressed in the stomochord. Instead,
N. Satah eta!.
ascidian tadpole consists of 40 cells, aligned in single file along the center of the
larval tail. Lineage of notochord cells is completely described (Conklin 1905; Nishida
1987). Thirty-two of the 40 notochord cells occupy the anterior four-fifths of the
notochord and derive from 8 of the A-line primordial notochord blastomeres of the
1l0-cell embryo, while the other 8 posterior-most cells come from 2 of the B-line
primordial blastomeres of the 1l0-cell embryo (Fig. 1C'). These 10 blastomeres
subsequently divide twice to form 40 notochord cells.
By taking advantage of these features, we determined that the expression of
Brachyury (As-I) of llalocynthia roretzi is transient, that As-T is expressed exclusively in blastomeres of the notochord lineage, and that the timing of As-T expression coincides with that of the developmental fate restriction of the blastomeres
(Fig. 1C; Yasuo and Satoh 1993). Therefore, in contrast to vertebrate and
cephalochordate Brachyury genes, the expression of the asci dian Brachyury is restricted to the differentiating notochord cells. This was confirmed in the Brachyury
gene (Ci-Bra) of another ascidian Ciona intestinalis (Corbo et al. 1997a).
2.3 Hemichordates
Hemichordata is a small phylum, containing the enteropneusts (acorn worms) and
pterobranchs. The relationship of the hemichordates and the echinoderms was recognized from their similar larval forms (Brusca and Brusca 1990; Nielsen 1995).
On the other hand, from the time of Bateson (1885), the hemichordates have been
linked to the chordates because they share chordate-specific structures and organization that can be related to the chordate body plan. However, there had been no
reports on hemichordate developmental genes before 1997.
In 1998, Tagawa et al. determined the ~xpression pattern of the Brachyury
(PfBra) gene of the acorn worm Ptychodera flava. A distinct expression was first
detected at the early gastrula stage, and the expression was restricted to the blastopore or the base of the invaginating archenteron (Fig. 1D). During the next phase
of hemichordate gastrulation, the blastopore closed, and then opened again as an
anus by the early tornaria larval stage. The expression remained at the base of the
archenteron during gastrulation, and this pattern of the PfBra expression continued during formation of the larva (Fig. 1D, E), until it disappeared in the 10-dayold larvae. In addition, another distinct expression became evident as early as the
middle gastrula stage in the region that eventually forms the mouth or stomodeum
(Fig. 1E). The PfBra expression in the stomodeum remained during formation of
the mouth, and this expression became undetectable prior to the disappearance of
the expression at the base of the archenteron. Therefore, PfBra is expressed in the
blastopore and the stomodeum of gastrulae and early tornaria larvae.
The stomochord is an organ in the adult proboscis, and its homology to chordate notochord has been suggested for a long time (Bateson 1885). This organ is
formed during metamorphosis, which usually occurs after 3 to 5 months of larval
swimming. Peterson et al. (1999a) examined whether or not PfBra is expressed in
the stomochord. They found that PfBra is not expressed in the stomochord. Instead,