Dorsoventral Patterning of the Zebrafish Embryo
99
associate with Smad4 trimers and translocate to the nucleus where they
participate in transcriptional complexes (see Attisano and Wrana 1998,
Kretschmar and Massague 1998, for reviews). We have cloned the
zebrafish homologues of Smadl and SmadS.
Developmental Northern analysis reveals that high levels of smad5
transcripts are provided maternally. At early blastula stages, smad5
mRNA levels have dropped to about SO%. Levels go up again after the
onset of zygotic transcription at midblastula stages, but decline soon
afterwards. smadl, on the other hand, starts to be expressed at gastrula
stages, long after the peak of smad5 expression. Whole mount in situ
hybridization revealed that both maternal and zygotic smad5 transcripts
are uniformly and ubiquitously distributed, whereas the expression of
smadl is restricted to the ventral side of the gastrula embryo. This
expression is absent in bmp2b mutant embryos, but enhanced after
overexpression of bmp2b in wild-type embryos, indicating that smadl
transcription is positively regulated by Bmp2b signaling.
6.12 somitabun Is smadS
Zebrafish smadl and smad5 genes were mapped, revealing that smad5
maps close to another dorsalizing mutation, somitabun sbntc24. Subsequent direct linkage analysis between the smad5 gene and the somitabun mutation revealed no recombination in over 200 examined meioses, indicating that gene and mutation are located within O.S cM of each
other. Consistent with the maternal and zygotic expression of smad5,
the somitabun mutation has both a dominant maternal and a dominant
zygotic effect. When a heterozygous somitabun female is crossed to a
wild-type male, all offspring show strong dorsalization of C4 strength,
characterized by a wound-up trunk and tail. The observation that all
offspring show the same phenotype, independent of their own genotype
(either wild type or heterozygous for sbn) indicates the overwhelming
maternal effect of the mutation. However, sbn also has a weak zygotic
effect. When heterozygous males are crossed to wild-type females, SO%
of the offspring show a weak Cl dorsalization, characterized by the
absence of the ventral tail fin.
Cloning of the smad5 cDNA from somitabun mutant embryos revealed a single amino acid exchange, a threonine to isoleucine exchange
99
associate with Smad4 trimers and translocate to the nucleus where they
participate in transcriptional complexes (see Attisano and Wrana 1998,
Kretschmar and Massague 1998, for reviews). We have cloned the
zebrafish homologues of Smadl and SmadS.
Developmental Northern analysis reveals that high levels of smad5
transcripts are provided maternally. At early blastula stages, smad5
mRNA levels have dropped to about SO%. Levels go up again after the
onset of zygotic transcription at midblastula stages, but decline soon
afterwards. smadl, on the other hand, starts to be expressed at gastrula
stages, long after the peak of smad5 expression. Whole mount in situ
hybridization revealed that both maternal and zygotic smad5 transcripts
are uniformly and ubiquitously distributed, whereas the expression of
smadl is restricted to the ventral side of the gastrula embryo. This
expression is absent in bmp2b mutant embryos, but enhanced after
overexpression of bmp2b in wild-type embryos, indicating that smadl
transcription is positively regulated by Bmp2b signaling.
6.12 somitabun Is smadS
Zebrafish smadl and smad5 genes were mapped, revealing that smad5
maps close to another dorsalizing mutation, somitabun sbntc24. Subsequent direct linkage analysis between the smad5 gene and the somitabun mutation revealed no recombination in over 200 examined meioses, indicating that gene and mutation are located within O.S cM of each
other. Consistent with the maternal and zygotic expression of smad5,
the somitabun mutation has both a dominant maternal and a dominant
zygotic effect. When a heterozygous somitabun female is crossed to a
wild-type male, all offspring show strong dorsalization of C4 strength,
characterized by a wound-up trunk and tail. The observation that all
offspring show the same phenotype, independent of their own genotype
(either wild type or heterozygous for sbn) indicates the overwhelming
maternal effect of the mutation. However, sbn also has a weak zygotic
effect. When heterozygous males are crossed to wild-type females, SO%
of the offspring show a weak Cl dorsalization, characterized by the
absence of the ventral tail fin.
Cloning of the smad5 cDNA from somitabun mutant embryos revealed a single amino acid exchange, a threonine to isoleucine exchange