Embryos produced by homozygous dl females form normal cellular blastoderm but at gastrulation develop into
yolk-filled tube of dorsal hypoderm. Hair pattern of cuticle
characteristic of dorsal hypoderm; ventral structures, such as
denticle belts, lacking. Normally, dorsal infoldings occupy
entire circumference of embryo. Evidence of anterior-posterior differentiation includes possible mouth armature
structures anteriorly, small spiracles posteriorly, and orientation of hairs. The periodicity of stripes of ftz expression
in pre gastrulation embryos, as revealed by antibody staining,
displays the pattern normally characteristic of the dorsum
circumferentially in embryos produced by dl females (Carroll,
Winslow, Twombly, and Scott, 1987, Development 99: 327-32.
Embryos produced by dl/dl and dl/Df(2L)TW137 indistinguishable, suggesting dl to be amorphic. Penetrance complete;
expression constant. Embryos of dl2 females lack all structures normally derived from the ventral half of the egg,
including mesoderm, endodermal gut, ventral nervous system,
and ventral hypoderm.
dl1 and to a lesser extent dl2, females produce embryos with
reduced capacity for neurogenesis in response to an absence of
dl function (Campos-Ortega, 1983, Wilhelm Roux's Arch. Dev.
Biol. 192: 317-26). dl germ line dependent; homozygous
germ-line clones produce dorsalized embryos (Schupbach and
Wieschaus, 1986, Dev. Biol. 113: 443-48).
Embryos of dl/+ females produced at 29 develop into comparatively normal-looking larvae; they mainly lack internal
organs, such as mesoderm and parts of the anterior and posterior gut; often ventral hypoderm including denticle belts
reduced; phenotype sensitive to genetic background. At 22,
dl/+ females produce normal embryos. Developmental fate of
ventrally located cells on cellular blastoderm apparently
shifted to that of more dorsally located cells. The phenotype
of embryos produced by dl/dl females partially rescued by the
injection of wild-type cytoplasm but not RNA (Santamaria and
Nusslein-Volhard, 1983, EMBO J. 2: 1695-99; Anderson and
Nusslein-Volhard, 1944, Nature 34: 225-27).
Developmental profiles show transcript to be present only in
ovaries and pre-cellular-blastoderm stages of embryogenesis.
In situ hybridization indicates that ovarian transcript accumulates in nurse cells from stage 5 to 11; number of transcripts per genome equivalent in these polytene cells remains
low and constant until stage 10, at which time there is a
dramatic increase in the relative numbers of transcripts.
After a lag of one or two nuclear divisions, transcript begins
to accumulate in the oocyte; by stage 12 there is little
detectable transcript in the nurse cells. It appears as though
the nurse-cell transcript is transferred to the oocyte and
thus to the embryo; transcript seems to be uniformly distributed in stage 14 oocytes (Steward, Ambrosio, and Schedl).
dl protein is uniformly distributed throughout cytoplasm of
early embryo; in the syncytial blastoderm a gradient of
expression is achieved by the graded transport of dl protein
into nuclei, with the highest nuclear concentrations found
ventrally; protein remains cytoplasmic dorsally. Maternal dorsalizing mutants prevent nuclear localization and ventralized
embryos show dorsal as well as ventral nuclear localization
(Steward, Zusman, Huang, and Schedl, 1988, Cell 55: 487-95;
Rushlow, Han, Manley, and Levine, 1989, Cell 59: 1165-77;
Steward, 1989, Cell 59: 11179-88; Roth, Stein, and Nusslein-Volhard, 1989, Cell 59: 1189-1202).