Dorsoventral Patterning of the Zebrafish Embryo
6.13 Smad5 Is Involved in the Mediation of Early Bmp2b
Signaling
101
To assess the relationship between smad5 and bmp2b, early marker gene
expression patterns were compared between sbn smad5 and swr bmp2b
double mutant embryos. Despite the maternal effect of the sbn mutation
and the maternal expression of smad5, no alterations in dorsoventral
patterning of sbn mutant embryos were observed earlier than in bmp2b
mutant embryos, indicating that Smad5 might have no function prior to
the function of the zygotically generated signal Bmp2b.
The phenotype of sbn smad5-swr bmp2b double-heterozygous embryos indicates that Bmp2b and Smad5 interact. As described above, the
zygotic effect of the sbn smad5 mutation leads to a weak CI phenotype.
sbn-swr double-heterozygous embryos deriving from a cross of a sbn
heterozygous male and a swr heterozygous female, however, are much
more strongly dorsalized and display C3 dorsalization, although heterozygosity for swr alone has no effect at all. A similar enhancement is
also observed for the maternal effect of the sbn smad5 mutation (from
C4 to C5).
Epistasis analyses via double mutants to determine whether Smad5,
as expected of a signal transducer, does act downstream of Bmp2b are
not possible, since mutations in both genes generate the same type of
phenotype. Instead, the effect of overexpressed Bmp2/4 in sbn smad5
mutants was studied. At the onset of gastrulation, sbn smad5 mutant
embryos display a striking reduction in bmp2b mRNA levels, similar to
the phenotype of swr bmp2b mutants themselves. Injection of wild-type
smad5 mRNA into sbn mutant embryos leads to a rescue of bmp2b
mRNA levels to wild-type condition, whereas the injection of Xenopus
bmp2 or bmp4 mRNA showed no effect. This indicates that Smad5 does
indeed act downstream of Bmp signaling to mediate the aforementioned
positive autoregulation of bmp214 expression.
6.14 The Three Phases of Dorsoventral Patterning
In contrast to their unresponsiveness in early bmp2b expression,
bmp214-injected sbn mutant embryos showed a striking response in
their morphology during later stages of development. When moderate
6.13 Smad5 Is Involved in the Mediation of Early Bmp2b
Signaling
101
To assess the relationship between smad5 and bmp2b, early marker gene
expression patterns were compared between sbn smad5 and swr bmp2b
double mutant embryos. Despite the maternal effect of the sbn mutation
and the maternal expression of smad5, no alterations in dorsoventral
patterning of sbn mutant embryos were observed earlier than in bmp2b
mutant embryos, indicating that Smad5 might have no function prior to
the function of the zygotically generated signal Bmp2b.
The phenotype of sbn smad5-swr bmp2b double-heterozygous embryos indicates that Bmp2b and Smad5 interact. As described above, the
zygotic effect of the sbn smad5 mutation leads to a weak CI phenotype.
sbn-swr double-heterozygous embryos deriving from a cross of a sbn
heterozygous male and a swr heterozygous female, however, are much
more strongly dorsalized and display C3 dorsalization, although heterozygosity for swr alone has no effect at all. A similar enhancement is
also observed for the maternal effect of the sbn smad5 mutation (from
C4 to C5).
Epistasis analyses via double mutants to determine whether Smad5,
as expected of a signal transducer, does act downstream of Bmp2b are
not possible, since mutations in both genes generate the same type of
phenotype. Instead, the effect of overexpressed Bmp2/4 in sbn smad5
mutants was studied. At the onset of gastrulation, sbn smad5 mutant
embryos display a striking reduction in bmp2b mRNA levels, similar to
the phenotype of swr bmp2b mutants themselves. Injection of wild-type
smad5 mRNA into sbn mutant embryos leads to a rescue of bmp2b
mRNA levels to wild-type condition, whereas the injection of Xenopus
bmp2 or bmp4 mRNA showed no effect. This indicates that Smad5 does
indeed act downstream of Bmp signaling to mediate the aforementioned
positive autoregulation of bmp214 expression.
6.14 The Three Phases of Dorsoventral Patterning
In contrast to their unresponsiveness in early bmp2b expression,
bmp214-injected sbn mutant embryos showed a striking response in
their morphology during later stages of development. When moderate