92
M. Hild et al.
tested several candidate genes previously isolated from other organisms
where they show activities like those proposed for the gene products
affected in the zebrafish mutants. As ventralizing signals which might
be affected in dorsalized mutants, three candidates had been isolated,
the Bone Morphogenetic Proteins Bmp2, Bmp4, and Bmp7, as dorsalizing signals which might be affected in the ventralized mutants, the Bmp
antagonists Noggin, Chordin, and Follistatin. The zebrafish homologues
of these genes were cloned (Bauer et al. 1998; Martinez-Barbera et al.
1997; Miller-Bertoglio et al. 1997; Nikaido et al. 1997; Schulte-Merker
et al. 1997) and their expression patterns investigated. Two of the bmp
genes, bmp2b and bmp7, are expressed from blastula stages onwards in
a broad ventral domain, reflecting the initial broad ventral specification
within the zebrafish blastula (see Fig. 3A,B for bmp2b). The expression
of bmp4 starts later, shortly before the onset of gastrulation. During
early gastrula stages, bmp2b, bmp4, and bmp7 show a very similar,
graded expression pattern with mRNA levels progressively dropping
from ventral to dorsal (see Fig. 3D for bmp4). During further development, however, the expression patterns of the three genes diverge. At
late gastrula stages, bmp2b shows the broadest ventral expression domain, sparing just the embryonic axis, while bmp7 is expressed in the
ventral half of the embryo, and bmp4 only in a rather small ventral
posterior domain. In addition to this ventral expression, all three bmp
genes are differentially expressed in the dorsal midline of the embryo,
the prechordal plate, or the notochord anlage. During somitogenesis
stages, bmp2b is initiated in the eyes and the otic vesicles, bmp4 in the
eyes, the head region, the dorsal ectoderm, the lateral mesoderm, and in
ventral regions of the somites. bmp7 is expressed in a rather small
region of the brain, probably the pineal gland, in addition to strong
expression remaining in ventral regions of the embryo. Of the dorsalizing candidates, only the zebrafish chordin homologue, named chordino,
is expressed at the right time and place to be a good candidate for the
Spemann organizer signal affected in dino mutants. Like bmp2 and
bmp7, chordino expression starts at blastula stages, when it is expressed
complementarily to bmp2b and bmp7 in the region of the presumptive
Spemann organizer (Fig. 3C). The expression of Jollistatin starts later,
shortly after the onset of gastrulation when it is expressed in the presumptive anterior paraxial mesoderm. The expression pattern suggests
that Follistatin might act as an antagonist of Bmp2b during later stages
M. Hild et al.
tested several candidate genes previously isolated from other organisms
where they show activities like those proposed for the gene products
affected in the zebrafish mutants. As ventralizing signals which might
be affected in dorsalized mutants, three candidates had been isolated,
the Bone Morphogenetic Proteins Bmp2, Bmp4, and Bmp7, as dorsalizing signals which might be affected in the ventralized mutants, the Bmp
antagonists Noggin, Chordin, and Follistatin. The zebrafish homologues
of these genes were cloned (Bauer et al. 1998; Martinez-Barbera et al.
1997; Miller-Bertoglio et al. 1997; Nikaido et al. 1997; Schulte-Merker
et al. 1997) and their expression patterns investigated. Two of the bmp
genes, bmp2b and bmp7, are expressed from blastula stages onwards in
a broad ventral domain, reflecting the initial broad ventral specification
within the zebrafish blastula (see Fig. 3A,B for bmp2b). The expression
of bmp4 starts later, shortly before the onset of gastrulation. During
early gastrula stages, bmp2b, bmp4, and bmp7 show a very similar,
graded expression pattern with mRNA levels progressively dropping
from ventral to dorsal (see Fig. 3D for bmp4). During further development, however, the expression patterns of the three genes diverge. At
late gastrula stages, bmp2b shows the broadest ventral expression domain, sparing just the embryonic axis, while bmp7 is expressed in the
ventral half of the embryo, and bmp4 only in a rather small ventral
posterior domain. In addition to this ventral expression, all three bmp
genes are differentially expressed in the dorsal midline of the embryo,
the prechordal plate, or the notochord anlage. During somitogenesis
stages, bmp2b is initiated in the eyes and the otic vesicles, bmp4 in the
eyes, the head region, the dorsal ectoderm, the lateral mesoderm, and in
ventral regions of the somites. bmp7 is expressed in a rather small
region of the brain, probably the pineal gland, in addition to strong
expression remaining in ventral regions of the embryo. Of the dorsalizing candidates, only the zebrafish chordin homologue, named chordino,
is expressed at the right time and place to be a good candidate for the
Spemann organizer signal affected in dino mutants. Like bmp2 and
bmp7, chordino expression starts at blastula stages, when it is expressed
complementarily to bmp2b and bmp7 in the region of the presumptive
Spemann organizer (Fig. 3C). The expression of Jollistatin starts later,
shortly after the onset of gastrulation when it is expressed in the presumptive anterior paraxial mesoderm. The expression pattern suggests
that Follistatin might act as an antagonist of Bmp2b during later stages