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Fig. 2. Parasegment-specific activation and repression of slp1 by Runt.
(A) Wild-type slp1 expression in a gastrula stage embryo as
visualized by in situ hybridization. (B) The phasing of slp1
expression relative to the expression of different pair-rule transcription
factors. A strip of cells along the anteroposterior axis is depicted across
the bottom with slp1-expressing cells indicated by shading. The
higher expression level of the even-numbered stripes is indicated by darker
shading. The four-cell wide Runt stripes are depicted above this strip as a
trapezoid, reflecting the higher expression levels in the center of the
stripes. By contrast, Eve and Ftz stripes are depicted as triangles with peak
expression in the most anterior cells, whereas the uniform expression of Opa
is depicted as a broad rectangle that spans the presegmental region. The
regulatory circuitry responsible for generating the slp1 pattern is
also depicted. Activation by Runt + Opa is indicated with an arrowhead,
whereas repression by either Eve, or the combination of Runt + Ftz, is
indicated with a horizontal bar. (C) Transient elimination of runt
activity in an embryo hemizygous for the temperature-sensitive
runt[YP17] mutation leads to loss of odd-numbered slp1
stripes and expansion of some of the even-numbered stripes. These changes are
interpreted to be due to loss of Runt-dependent activation and repression as
indicated in D. (E) Double in situ hybridization showing the complementary
expression of slp1 (blue) and ftz (brown) mRNAs in embryos
with a high-level of NGT-driven Runt. This embryo was obtained by
crossing homozygous NGT40 females with homozygous
UAS-runt[232] males. As indicated in F, slp1 expression in
these embryos fills the presumptive odd-numbered parasegments and is repressed
throughout the even-numbered parasegments.
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