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Fig. 8. Model of tsh and hth regulation in the wing disc. In all
panels: P, pouch; DH, distal hinge; PH, proximal hinge; N, notum; green, Vg;
blue, Hth; pink, Tsh+Hth. (A) Early in the second instar, prior to the
formation of the DV compartment boundary, the ventro-anterior wedge of Wg
represses tsh in the prospective appendage (1). Following this
establishment period, PcG members maintain tsh repression
independently of Wg and Dpp signaling (2), although tsh remains
sensitive to Wg-mediated repression. (B) Late in the second instar, after the
formation of the DV boundary, Dpp (yellow arrows) represses hth in
cells that have upregulated vg. (C) Third instar wing disc, showing
the complex relationship between Vg, Hth, and the Dpp gradient. Unlike
tsh, hth repression requires continuous signal input. In the medial
region of the pouch, high levels of Dpp completely repress Brk, allowing the
expression of a putative hth repressor (R), irrespective of Vg. In
the lateral regions of the pouch, moderate Dpp levels incompletely repress
Brk, and Vg is required to ensure sufficient levels of R to maintain
hth repression. In the lateral DH, low levels of Dpp signaling fail
to repress Brk, which represses R, permitting the co-expression of Vg and Hth.
Note that the above model does not apply to the PH, where hth is
insensitive to repression by Dpp signaling, possibly due to the presence of
Tsh.
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