214
N. Satoh et al.
PjBra is expressed in the mesoderm of the adult proboscis, collar and the very
posterior region of the trunk. They suggested a role of Brachyury in the mesoderm
formation of hemichordate adults, which, they thought, is relevant to the mesoderm
(notochord) formation in chordate embryos.
2.4 Echinoderms
The phylum Echinodermata consists of five major groups, crinoids, asteroids, ophiuroids, echinoids and holothuroids. Among them, most of the developmental genes
have been characterized in echinoid sea urchins (Davidson et al. 1998). The sea
urchin Brachyury (HpTa) was isolated from Hemicentrotus pulcherrimus (Harada
et al. 1995). In sea urchin embryos, the HpTa is expressed in the secondary mesenchyme founder cells, vegetal plate of the mesenchyme blastula (Fig. IF), extending
tip of the invaginating archenteron and, finally, the secondary mesenchyme cells at
the late-gastrula stage (Fig. 1G). The secondary mesenchyme cells give rise to four
types of cells, circum-esophageal muscles, pigment cells, basal cells and coelomic
pOllches. Differentiation of the secondary mesenchyme cells into these four types
occurs after the prism stage. At this stage, the HpTa expression was undetectable.
Therefore, we were unable to identify which components of the secondary mesenchyme are the HpTa-positive founder cells. A similar expression pattern of Brachyury
is shown in another sea urchin Strongylocentrotus purpuratus (SpTa; Peterson et
al. 1999b).
2.5 The Invertebrate Deuterostome Brachyury Genes are
Orthologues of Vertebrate Brachyury
A<; mentioned in the Introduction, a great deal of information has recently been
accumulated regarding T-box genes other than the Brachyury subfamily, and it is
now thought that T -box genes constitute a novel family of transcription factors that
playa crucial role in the development of various animal groups (Papaioannou and
Silver 1998). Therefore, it remains possible that the invertebrate deuterostome
Brachyury genes mentioned above are members of T-box genes other than the
Brachyury subfamily. In order to determine this question, we performed molecular
phylogenetic analysis. We aligned 146 amino acid sites of the T-domains based
upon maximum similarity, by which molecular phylogenetic analysis was done
llsing the neighbor-joining method (Saitou and Nei 1987). As shown in Fig. 2, the
amphioxus Am(Bb)BraJ and Am(Bb)Bra2, ascidian As-T and Ci-Bra, acorn worm
PjBra, and sea urchin HpTa formed a clade with the vertebrate Brachyury subfamily members, and this grouping was supported by the 100% bootstrap value. Therefore, it may be concluded that all of the invertebrate deuterostomeBrachyury genes
are orthologues of vertebrate Brachyury genes.
N. Satoh et al.
PjBra is expressed in the mesoderm of the adult proboscis, collar and the very
posterior region of the trunk. They suggested a role of Brachyury in the mesoderm
formation of hemichordate adults, which, they thought, is relevant to the mesoderm
(notochord) formation in chordate embryos.
2.4 Echinoderms
The phylum Echinodermata consists of five major groups, crinoids, asteroids, ophiuroids, echinoids and holothuroids. Among them, most of the developmental genes
have been characterized in echinoid sea urchins (Davidson et al. 1998). The sea
urchin Brachyury (HpTa) was isolated from Hemicentrotus pulcherrimus (Harada
et al. 1995). In sea urchin embryos, the HpTa is expressed in the secondary mesenchyme founder cells, vegetal plate of the mesenchyme blastula (Fig. IF), extending
tip of the invaginating archenteron and, finally, the secondary mesenchyme cells at
the late-gastrula stage (Fig. 1G). The secondary mesenchyme cells give rise to four
types of cells, circum-esophageal muscles, pigment cells, basal cells and coelomic
pOllches. Differentiation of the secondary mesenchyme cells into these four types
occurs after the prism stage. At this stage, the HpTa expression was undetectable.
Therefore, we were unable to identify which components of the secondary mesenchyme are the HpTa-positive founder cells. A similar expression pattern of Brachyury
is shown in another sea urchin Strongylocentrotus purpuratus (SpTa; Peterson et
al. 1999b).
2.5 The Invertebrate Deuterostome Brachyury Genes are
Orthologues of Vertebrate Brachyury
A<; mentioned in the Introduction, a great deal of information has recently been
accumulated regarding T-box genes other than the Brachyury subfamily, and it is
now thought that T -box genes constitute a novel family of transcription factors that
playa crucial role in the development of various animal groups (Papaioannou and
Silver 1998). Therefore, it remains possible that the invertebrate deuterostome
Brachyury genes mentioned above are members of T-box genes other than the
Brachyury subfamily. In order to determine this question, we performed molecular
phylogenetic analysis. We aligned 146 amino acid sites of the T-domains based
upon maximum similarity, by which molecular phylogenetic analysis was done
llsing the neighbor-joining method (Saitou and Nei 1987). As shown in Fig. 2, the
amphioxus Am(Bb)BraJ and Am(Bb)Bra2, ascidian As-T and Ci-Bra, acorn worm
PjBra, and sea urchin HpTa formed a clade with the vertebrate Brachyury subfamily members, and this grouping was supported by the 100% bootstrap value. Therefore, it may be concluded that all of the invertebrate deuterostomeBrachyury genes
are orthologues of vertebrate Brachyury genes.