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M. Hild et al.
other, swr a72 , Bmp2b protein is C-terminally extended by six additional
amino acids.
Finally, snailhouse was shown to map within 0.16 cM of zebrafish
bmp7 (no recombination in over 300 mutant F2 embryos tested=600
meioses using a SSCP). Sequencing of the only available allele, snhty68,
revealed a glycine-valine exchange in the signal peptide of Bmp7 protein which appears to abolish the secretion of the mutant protein. Upon
overexpression in zebrafish embryos and Xenopus animal cap explants,
this mutant form of Bmp7 revealed a severely reduced ventralizing
activity. The same was true for Xenopus Bmp7 after introduction of the
same mutations via site-directed mutagenesis. bmp4, Jollistatin and
noggin did not map to any of the other dorsoventral mutations.
Altogether, these results indicate that bmp2b and bmp7 are essential
for ventral development, and that chordino is essential for dorsal development during early dorsoventral patterning of the zebrafish embryo.
However, in contrast to what is suggested by their activity in Xenopus
animal cap assays, both bmp2b and bmp7 are not required for the initial
induction of mesoderm. Interestingly, targeting of the mouse bmp2,
bmp7, and chordino homologues did not affect early dorsoventral patterning but later processes of mouse development. bmp2-deficient mice,
for example, display defects in amnion and heart development (Zhang
and Bradley 1996), while bmp7-deficient mice show defects in eye and
kidney formation (Dudley et al. 1995; Luo et al. 1995). On the other
hand, these later processes do not appear to be affected in the zebrafish
mutants. The dorsoventral defects of bmp2b, bmp7, and chordino mutants can be rescued via injection of the respective mRNAs at the
I-somite stages. Rescued individuals are viable and can be raised to
fertile adults, although injected RNA is usually degraded by the end of
gastrulation (Hammerschmidt et al. 1998). The analysis of rescued
homozygous mutant adults revealed no apparent alterations in the case
of snailhouselbmp7 and chordino, while swirl/bmp2b mutants displayed
rather severe balancing problems. Two organs have been described to be
involved in balancing and sensing the orientation in the fish, the lateral
line and the inner ears. bmp2b consistently shows a strong expression in
the otic vesicles during late stages of somitogenesis. A more thorough
analysis of the inner ear defects in rescued swr bmp2b mutant adult fish
is currently under way.
M. Hild et al.
other, swr a72 , Bmp2b protein is C-terminally extended by six additional
amino acids.
Finally, snailhouse was shown to map within 0.16 cM of zebrafish
bmp7 (no recombination in over 300 mutant F2 embryos tested=600
meioses using a SSCP). Sequencing of the only available allele, snhty68,
revealed a glycine-valine exchange in the signal peptide of Bmp7 protein which appears to abolish the secretion of the mutant protein. Upon
overexpression in zebrafish embryos and Xenopus animal cap explants,
this mutant form of Bmp7 revealed a severely reduced ventralizing
activity. The same was true for Xenopus Bmp7 after introduction of the
same mutations via site-directed mutagenesis. bmp4, Jollistatin and
noggin did not map to any of the other dorsoventral mutations.
Altogether, these results indicate that bmp2b and bmp7 are essential
for ventral development, and that chordino is essential for dorsal development during early dorsoventral patterning of the zebrafish embryo.
However, in contrast to what is suggested by their activity in Xenopus
animal cap assays, both bmp2b and bmp7 are not required for the initial
induction of mesoderm. Interestingly, targeting of the mouse bmp2,
bmp7, and chordino homologues did not affect early dorsoventral patterning but later processes of mouse development. bmp2-deficient mice,
for example, display defects in amnion and heart development (Zhang
and Bradley 1996), while bmp7-deficient mice show defects in eye and
kidney formation (Dudley et al. 1995; Luo et al. 1995). On the other
hand, these later processes do not appear to be affected in the zebrafish
mutants. The dorsoventral defects of bmp2b, bmp7, and chordino mutants can be rescued via injection of the respective mRNAs at the
I-somite stages. Rescued individuals are viable and can be raised to
fertile adults, although injected RNA is usually degraded by the end of
gastrulation (Hammerschmidt et al. 1998). The analysis of rescued
homozygous mutant adults revealed no apparent alterations in the case
of snailhouselbmp7 and chordino, while swirl/bmp2b mutants displayed
rather severe balancing problems. Two organs have been described to be
involved in balancing and sensing the orientation in the fish, the lateral
line and the inner ears. bmp2b consistently shows a strong expression in
the otic vesicles during late stages of somitogenesis. A more thorough
analysis of the inner ear defects in rescued swr bmp2b mutant adult fish
is currently under way.