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Fig. 5. A requirement for EGFR signalling in smooth-cuticle cells. (A)
wg-gal4 and prd-gal4 drivers were used to analyse the fusion
phenotype; their expression patterns were visualized using RNA in situ
hybridisation. (B) DN-EGFR was expressed in smooth-cuticle cells in the
regions of the embryos indicated by the red lines using arm-gal4,
prd-gal4 and wg-gal4 drivers. Note that the prd-gal4
and wg-gal4 drivers are expressed at higher levels per cell than the
arm-gal4 driver. Expression of DN-EGFR in alternating smooth-cuticle
domains resulted in strong denticle belt fusions in essentially all
paired domains, and never in non-paired parasegments. The
wg-gal4 driver expressed only in two cell rows in each stripe, but
incomplete fusions were evident (far right). (C) The specificity of activating
and inhibiting EGFR signalling on the fusion phenotype was tested by modifying
the fusion phenotype of rhomboid-3 rhomboid-1 double mutant
(ru1rho17M43) embryos. Note that the analysis
was performed only in paired domains (alternating smooth-cuticle
stripes), and that these weak EGFR, ras and raf transgenes
caused no phenotypes when expressed by themselves in wild-type embryos using
prd-gal4. Transgenes were expressed at 25°C (graph on left),
except rasN17 and DN-raf, which were expressed at 29°C
(graph on right). Note that at 29°C fewer denticle belt fusions occurred
in rhomboid-3 rhomboid-1 double mutant embryos. (D) Although the
width of the medial fusion varied in spitz group embryos, expressing
DN-EGFR laterally in smooth-cuticle cells along the entire circumference of
the parasegment (see A above) resulted in fusions that were never wider than
the middle two thirds of denticle belts.
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