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I'm Mike Levine at UC Berkeley and
I'm going to discuss the dorsal-ventral
patterning of the Drosophila embryo.
This process is controlled by
a sequence-specific transcription factor
called 'dorsal' (dl),
which is shown on the next slide.
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This is a side view of
a 2-hour Drosophila embryo,
stained with an antibody
against the dorsal protein.
Initially the protein is
distributed uniformly
throughout the cytoplasm of the egg and
early embryo,
but shortly after fertilization
the protein begins to enter nuclei.
Here you can see that in ventral
regions (bottom regions) the protein
has entered nuclei, but in the top
(or dorsal) regions of the embryo,
the dorsal protein
remains in the cytoplasm.
So regulated nuclear transport produces
a broad dorsal nuclear gradient with
peak levels present in ventral regions,
progressively lower levels in lateral and
more dorsal regions.
This dorsal nuclear gradient controls
dorsal-ventral patterning by regulating
something like 60 to 70
different target genes,
in a concentration-dependent fashion.
The next slide shows several
examples of dorsal target genes,
which reveal distinct threshold
readouts of the dorsal gradient.
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High levels of the dorsal gradient
activate the genes stained in yellow,
lower levels of the gradient activate
the transcripts encoded by the genes
visualized in blue, green and red.
Altogether the dorsal gradient establishes
several basic embryonic tissues,
high levels of the gradient establish
the mesoderm at the bottom of
the embryo as visualized by
the marker gene seen in yellow,
low levels of the gradient establish
the presumptive neurogenic ectoderm in
lateral regions as seen in the expression
of these genes in blue, green and red, and
finally the absence of the dorsal gradient
permits the expression of genes in
the very top of the embryo, not shown
here, which will form the dorsal ectoderm.
The dorsal gradient does not produce these
different thresholds of gene activity
alone, instead it works in concert
with two other sequence-specific
transcription factors,
shown on the next slide.
