Dorsoventral Patterning of the Zebrafish Embryo
89
stages, the expression pattern of bmp2b and chardina were unaffected in
both mutants, indicating that the initial coarse dorsoventral pattern,
which is supposed to be under maternal control (see above), is set up
normally, including a normal induction of the Spemann organizer. However, severe alterations are observed beginning shortly before the onset
of gastrulation, when the initial pattern is supposed to be refined under
the control of zygotic signals from the Spemann organizer and antagonizing ventralizing signals. In normal embryos, eve}, a marker for
presumptive ventral mesoderm (Joly et al. 1993), is initially expressed
in a broad fashion. Beginning shortly before the onset of gastrulation, its
expression becomes progressively restricted to ventral-most regions,
indicating the progressive dorsalization of mesoderm in dorsolateral
regions initially specified as ventral. In dina mutants, this ventral retraction of the eve} expression domain does not occur, whereas in swirl
mutants, eve} expression is progressively lost in the entire embryo,
including ventral-most regions. On the other hand, the expression domain offkd3, a marker for presumptive neuroectodermal cells in dorsal
animal regions (Hammerschmidt et al. 1996a), is much narrower in dina
mutants, but expanded into ventral-most regions in swirl mutants. In an
animal view, the expression domains of the ventral mesodermal marker
eve} and the neuroectodermal markerfkd3 are perfectly complementary
in wild-type, dina, and swirl mutants, indicating that dorsalization of the
mesoderm and neural induction are indeed tightly linked, and most
likely regulated by the same signals that are affected in dina and swirl
mutants. dina mutants appear to display a defect in a dorsalizing signal
generated from the Spemann organizer, while swirl mutants apparently
have a defect in an antagonizing signal required for the maintenance of
ventral specification. The phenotype of snailhause and samitabun mutants is very similar to that of swirl mutants, suggesting that the same
processes are affected.
6.6 Early Expression Patterns of bmp2b, bmp4, bmp7,
chordino, andfollistatin
We were interested to learn which genes might be affected in the various
dorsoventral patterning mutants. Before starting laborious approaches
such as positional cloning (see for example, Zhang et al. 1998), we
89
stages, the expression pattern of bmp2b and chardina were unaffected in
both mutants, indicating that the initial coarse dorsoventral pattern,
which is supposed to be under maternal control (see above), is set up
normally, including a normal induction of the Spemann organizer. However, severe alterations are observed beginning shortly before the onset
of gastrulation, when the initial pattern is supposed to be refined under
the control of zygotic signals from the Spemann organizer and antagonizing ventralizing signals. In normal embryos, eve}, a marker for
presumptive ventral mesoderm (Joly et al. 1993), is initially expressed
in a broad fashion. Beginning shortly before the onset of gastrulation, its
expression becomes progressively restricted to ventral-most regions,
indicating the progressive dorsalization of mesoderm in dorsolateral
regions initially specified as ventral. In dina mutants, this ventral retraction of the eve} expression domain does not occur, whereas in swirl
mutants, eve} expression is progressively lost in the entire embryo,
including ventral-most regions. On the other hand, the expression domain offkd3, a marker for presumptive neuroectodermal cells in dorsal
animal regions (Hammerschmidt et al. 1996a), is much narrower in dina
mutants, but expanded into ventral-most regions in swirl mutants. In an
animal view, the expression domains of the ventral mesodermal marker
eve} and the neuroectodermal markerfkd3 are perfectly complementary
in wild-type, dina, and swirl mutants, indicating that dorsalization of the
mesoderm and neural induction are indeed tightly linked, and most
likely regulated by the same signals that are affected in dina and swirl
mutants. dina mutants appear to display a defect in a dorsalizing signal
generated from the Spemann organizer, while swirl mutants apparently
have a defect in an antagonizing signal required for the maintenance of
ventral specification. The phenotype of snailhause and samitabun mutants is very similar to that of swirl mutants, suggesting that the same
processes are affected.
6.6 Early Expression Patterns of bmp2b, bmp4, bmp7,
chordino, andfollistatin
We were interested to learn which genes might be affected in the various
dorsoventral patterning mutants. Before starting laborious approaches
such as positional cloning (see for example, Zhang et al. 1998), we