14. Developmental Mechanisms for Chordate Evolution
211
domain itself, however, is not unique to Brachyury (Pflugfelder et al. 1992). A
great deal of information has recently been accumulated regarding T -box genes
other than the Brachyury subfamily members, 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).
2 Expression Patterns of Invertebrate
Deuterostome Brachyury Genes
Because, as mentioned above, vertebrate Brachyury genes are responsible for the
formation of notochord, our laboratory as well as other laboratories attempted the
isolation and characterization of invertebrate deuterostome Brachyury genes.
2.1 Cephalochordates
Cephalochordates (lancelets or amphioxus) are small, fishlike creatures, consisting
of about two dozen species. Recent molecular phylogenie analysis (Wada and Satoh
1994) and expression patterns of developmental genes (Holland and GarciaFernandez 1996) strongly support the notion that the invertebrate group closest to
vertebrates is the group of cephalochordates. The amphioxus Brachyury genes have
been isolated from two species, Branchiostoma floridae (Holland et al. 1996) and
B. belcheri (Terazawa and Satoh 1997). Both species showed the same pattern of
Brachyury expression.
Each species contains two copies of Brachyury, AmBral and AmBra2 in B.
floridae and Am(Bb)Bral and Am(Bb)Bra2 in B. belcheri, suggesting an independent duplication of Brachyury in the phylogenetic lineage of amphioxus. However,
the two duplicated genes showed an almost identical expression pattern (Fig. lA,
B). The Am(Bb)Bral and Am(Bb)Bra2 are initially expressed in the involuting
mesoderm of the gastrula, then in the differentiating so mites of neurulae (Fig. 1A),
followed by the differentiating notochord and finally in the tail bud of ten-somite
stage embryos (Fig. 1B). This spatial and temporal distribution of amphioxus
Brachyury transcripts resembles that of vertebrate Brachyury genes, except that
Am(Bb)Bra expression in the somite continues for a longer duration.
2.2 Urochordates
Ascidians are one of the three urochordate groups. Because fertilized asci dian eggs
develop rather quickly into tadpole-type larvae with several distinct types of tissues
and organs, and because the lineage of embryonic cells is well-documented, the
ascidian embryo provides an appropriate experimental system to explore the expression and function of developmental genes (Satoh 1994). The notochord of the
211
domain itself, however, is not unique to Brachyury (Pflugfelder et al. 1992). A
great deal of information has recently been accumulated regarding T -box genes
other than the Brachyury subfamily members, 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).
2 Expression Patterns of Invertebrate
Deuterostome Brachyury Genes
Because, as mentioned above, vertebrate Brachyury genes are responsible for the
formation of notochord, our laboratory as well as other laboratories attempted the
isolation and characterization of invertebrate deuterostome Brachyury genes.
2.1 Cephalochordates
Cephalochordates (lancelets or amphioxus) are small, fishlike creatures, consisting
of about two dozen species. Recent molecular phylogenie analysis (Wada and Satoh
1994) and expression patterns of developmental genes (Holland and GarciaFernandez 1996) strongly support the notion that the invertebrate group closest to
vertebrates is the group of cephalochordates. The amphioxus Brachyury genes have
been isolated from two species, Branchiostoma floridae (Holland et al. 1996) and
B. belcheri (Terazawa and Satoh 1997). Both species showed the same pattern of
Brachyury expression.
Each species contains two copies of Brachyury, AmBral and AmBra2 in B.
floridae and Am(Bb)Bral and Am(Bb)Bra2 in B. belcheri, suggesting an independent duplication of Brachyury in the phylogenetic lineage of amphioxus. However,
the two duplicated genes showed an almost identical expression pattern (Fig. lA,
B). The Am(Bb)Bral and Am(Bb)Bra2 are initially expressed in the involuting
mesoderm of the gastrula, then in the differentiating so mites of neurulae (Fig. 1A),
followed by the differentiating notochord and finally in the tail bud of ten-somite
stage embryos (Fig. 1B). This spatial and temporal distribution of amphioxus
Brachyury transcripts resembles that of vertebrate Brachyury genes, except that
Am(Bb)Bra expression in the somite continues for a longer duration.
2.2 Urochordates
Ascidians are one of the three urochordate groups. Because fertilized asci dian eggs
develop rather quickly into tadpole-type larvae with several distinct types of tissues
and organs, and because the lineage of embryonic cells is well-documented, the
ascidian embryo provides an appropriate experimental system to explore the expression and function of developmental genes (Satoh 1994). The notochord of the