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N. Satoh et al.
1 Introduction
The evolutionary pathway from advanced invertebrates through primitive chordates to vertebrates has been a subject of extensive investigation and vigorous discussion for more than a century (HaeckeI1868; Garstang 1928; Berrilll955; Jefferies
1986). The phylum Chordata consists of three subphyla, Urochordata,
Cephalochordata and Vertebrata. Chordates exhibit a notochord, a dorsal hollow
nerve cord, and pharyngeal gill slits, which are hallmarks of their body plan. In
addition, echinoderms, hemichordates and chordates may share a common ancestor and form the monophyletic group of deuterostomes (Brusca and Brusca 1990;
Willmer 1990; Wada and Satoh 1994; Nielsen 1995). Therefore, chordates originated from an ancestor more than 550 million years ago by organizing their characteristic features as mentioned above. Because the notochord is the most prominent feature of chordates, the elucidation of molecular developmental mechanisms
underlying the formation of notochord will lead to a better understanding of mechanisms underlying the origin and evolution of chordates (Satoh and Jeffery 1995).
Among a dozen genes that are implicated in the formation of the chordamesoderm of vertebrate embryos, Brachyury is of particular interest, because this gene is
involved in notochord differentiation (Herrmann and Kispert 1994). Since its identification in 1927, the mouse Brachyury (1) locus has been implicated in mesoderm
formation and notochord differentiation (Chesley 1935). Homozygotic mutant mouse
embryos die on day 11 of gestation with deficiencies in the posterior mesoderm
formation in the primitive streak, an absent notochord and severe reduction of the
allantois. In 1990, this gene was cloned (Herrmann et al. 1990). Transient expression of Brachyury is detected in nascent and migrating mesoderm generated from
the primitive streak, then continuously in the notochord. At later stages, during
axis elongation, Brachyury is expressed in the tail bud and in the notochord. The
cloning of mouse Brachyury was followed by isolation and characterization of its
homologues in Xenopus (Xbra; Smith et al.I991), zebrafish (ZJ-Tor no tail; SchulteMerker et al. 1992) and chick (Ch-T; Kispert et al. 1995b). Vertebrate Brachyury
genes are expressed in the embryonic area from which mesendoderm is generated;
that is, in the primitive streak in the mouse embryo, the marginal zone of the frog
embryo, or the germ ring in the fish embryo, and later, in the notochord and in the
tail bud during axis elongation. These domains of Brachyury expression reflect the
dual function of the gene during vertebrate development, namely in the notochord
differentiation and in the mesoderm formation. In addition, zebrafish no tail has
been shown to be allelic with Brachyury (Schulte-Merker et al. 1994). Therefore,
in vertebrates, not only the pattern of Brachyury expression but also its function are
likely to be conserved (Herrmann and Kispert 1994; Smith 1997; Papaioannou and
Silver 1998).
Recent studies revealed that vertebrate Brachyury genes encode proteins that
share a domain of about 180 amino acids in the N-terminal half. This domain or Tdomain serves DNA binding activity, and thus the Brachyury protein acts as a
transcriptional factor (Kispert and Herrmann 1993; Kispert et al. 1995a). The T-
N. Satoh et al.
1 Introduction
The evolutionary pathway from advanced invertebrates through primitive chordates to vertebrates has been a subject of extensive investigation and vigorous discussion for more than a century (HaeckeI1868; Garstang 1928; Berrilll955; Jefferies
1986). The phylum Chordata consists of three subphyla, Urochordata,
Cephalochordata and Vertebrata. Chordates exhibit a notochord, a dorsal hollow
nerve cord, and pharyngeal gill slits, which are hallmarks of their body plan. In
addition, echinoderms, hemichordates and chordates may share a common ancestor and form the monophyletic group of deuterostomes (Brusca and Brusca 1990;
Willmer 1990; Wada and Satoh 1994; Nielsen 1995). Therefore, chordates originated from an ancestor more than 550 million years ago by organizing their characteristic features as mentioned above. Because the notochord is the most prominent feature of chordates, the elucidation of molecular developmental mechanisms
underlying the formation of notochord will lead to a better understanding of mechanisms underlying the origin and evolution of chordates (Satoh and Jeffery 1995).
Among a dozen genes that are implicated in the formation of the chordamesoderm of vertebrate embryos, Brachyury is of particular interest, because this gene is
involved in notochord differentiation (Herrmann and Kispert 1994). Since its identification in 1927, the mouse Brachyury (1) locus has been implicated in mesoderm
formation and notochord differentiation (Chesley 1935). Homozygotic mutant mouse
embryos die on day 11 of gestation with deficiencies in the posterior mesoderm
formation in the primitive streak, an absent notochord and severe reduction of the
allantois. In 1990, this gene was cloned (Herrmann et al. 1990). Transient expression of Brachyury is detected in nascent and migrating mesoderm generated from
the primitive streak, then continuously in the notochord. At later stages, during
axis elongation, Brachyury is expressed in the tail bud and in the notochord. The
cloning of mouse Brachyury was followed by isolation and characterization of its
homologues in Xenopus (Xbra; Smith et al.I991), zebrafish (ZJ-Tor no tail; SchulteMerker et al. 1992) and chick (Ch-T; Kispert et al. 1995b). Vertebrate Brachyury
genes are expressed in the embryonic area from which mesendoderm is generated;
that is, in the primitive streak in the mouse embryo, the marginal zone of the frog
embryo, or the germ ring in the fish embryo, and later, in the notochord and in the
tail bud during axis elongation. These domains of Brachyury expression reflect the
dual function of the gene during vertebrate development, namely in the notochord
differentiation and in the mesoderm formation. In addition, zebrafish no tail has
been shown to be allelic with Brachyury (Schulte-Merker et al. 1994). Therefore,
in vertebrates, not only the pattern of Brachyury expression but also its function are
likely to be conserved (Herrmann and Kispert 1994; Smith 1997; Papaioannou and
Silver 1998).
Recent studies revealed that vertebrate Brachyury genes encode proteins that
share a domain of about 180 amino acids in the N-terminal half. This domain or Tdomain serves DNA binding activity, and thus the Brachyury protein acts as a
transcriptional factor (Kispert and Herrmann 1993; Kispert et al. 1995a). The T-