88
M. Hild et al.
found, display C4 dorsalization, characterized by a body axis that is
wound up in a snailshell-like fashion. piggy tail mutants display C3
dorsalization, characterized by a wound-up tail, and last-a-fin and minifin mutants show a complete or partial loss of the ventral tail fin (C2,
CI) (Mullins et al. 1996).
On the other hand, dina mutants, which display the strongest ventralized phenotype found in the mutant screen, are characterized by smaller
heads and an enlarged tail with enlarged blood islands and multiple
ventral tail fins, classified as V3 ventralization, while mutants of the
second complementation group, mercedes, are much more weakly ventralized, displaying almost wild-type morphology with the exception of
the ventral tail fin which is duplicated (Hammerschmidt et al. 1996a).
These phenotypes of 36-hour-old mutant embryos result from alterations in dorsoventral organization during much earlier stages of development. Morphologically, the phenotypes of the most strongly dorsalized and ventralized embryos, swirl and dina, respectively, are first
visible at midgastrula stages. At the 2-somite stage (Fig. 2; see also
Hammerschmidt et al. 1996b), dina mutants display a reduction in the
size of anterior somites and posterior notochord, both derivatives of the
dorsolateral mesoderm, while both structures are greatly enlarged in
swirl mutant embryos. In addition to these dorsolateral alterations, both
mutants display shifts in the anteroposterior organization. In dina mutants, the posterior region is enlarged at the expense of the anterior
region, in swirl mutants, the anterior region is enlarged at the expense of
the posterior region (Fig. 2). This tight linkage of dorsal and anterior
fates on one hand, and ventral and posterior fates on the other hand,
results from the morphogenetic movements during gastrulation, epiboly, dorsal convergent extension, and involution, which let dorsal cells
end up in more anterior positions of the developing body axis than cells
that come from ventral regions of the pregastrula embryo.
6.5 Analysis of Dorsoventral Phenotypes
with Molecular Markers
A detailed analysis of dina and swirl mutant embryos was carried out
using molecular markers that help to identify cell fates long before the
specification of cells becomes morphologically visible. At late blastula
M. Hild et al.
found, display C4 dorsalization, characterized by a body axis that is
wound up in a snailshell-like fashion. piggy tail mutants display C3
dorsalization, characterized by a wound-up tail, and last-a-fin and minifin mutants show a complete or partial loss of the ventral tail fin (C2,
CI) (Mullins et al. 1996).
On the other hand, dina mutants, which display the strongest ventralized phenotype found in the mutant screen, are characterized by smaller
heads and an enlarged tail with enlarged blood islands and multiple
ventral tail fins, classified as V3 ventralization, while mutants of the
second complementation group, mercedes, are much more weakly ventralized, displaying almost wild-type morphology with the exception of
the ventral tail fin which is duplicated (Hammerschmidt et al. 1996a).
These phenotypes of 36-hour-old mutant embryos result from alterations in dorsoventral organization during much earlier stages of development. Morphologically, the phenotypes of the most strongly dorsalized and ventralized embryos, swirl and dina, respectively, are first
visible at midgastrula stages. At the 2-somite stage (Fig. 2; see also
Hammerschmidt et al. 1996b), dina mutants display a reduction in the
size of anterior somites and posterior notochord, both derivatives of the
dorsolateral mesoderm, while both structures are greatly enlarged in
swirl mutant embryos. In addition to these dorsolateral alterations, both
mutants display shifts in the anteroposterior organization. In dina mutants, the posterior region is enlarged at the expense of the anterior
region, in swirl mutants, the anterior region is enlarged at the expense of
the posterior region (Fig. 2). This tight linkage of dorsal and anterior
fates on one hand, and ventral and posterior fates on the other hand,
results from the morphogenetic movements during gastrulation, epiboly, dorsal convergent extension, and involution, which let dorsal cells
end up in more anterior positions of the developing body axis than cells
that come from ventral regions of the pregastrula embryo.
6.5 Analysis of Dorsoventral Phenotypes
with Molecular Markers
A detailed analysis of dina and swirl mutant embryos was carried out
using molecular markers that help to identify cell fates long before the
specification of cells becomes morphologically visible. At late blastula