Fig. 2.

The JAK/STAT signaling pathway regulates tsl expression in the BCs and PFCs. (A–C) Expression of tsl⁰⁶¹⁷-lacZ (A) upon overexpression of upd with the slboGal4 driver (B) and upon overexpression of dome^DN with slboGal4 (C), which represses tsl⁰⁶¹⁷-lacZ expression in the BCs and PFCs but not in the CCs. Blue arrowhead in C points the anterior polar cells, which fail to migrate. (D and D′) Immunostaining showing that the most anterior positive cells correspond to the anterior polar cells, where the slboGAL4 driver is not expressed. Green, β-gal; red, Fasciclin III, which accumulates at the contact surface between the two polar cells. (E and F) Overexpression of upd with the slboGAL4 driver activates the BC/PFC enhancer. (G and H) However, overexpression of upd does not activate the CC enhancer. (I and J) Expression driven by a tsl(B) construct with mutations in the four putative STAT binding sites found at the BC/PFC enhancer. At stage 9, no expression can be detected, and at stage 10, only weak expression is seen in the BCs (black arrowhead) but not at the PFCs (I). On the contrary, expression in the CCs is not affected, as shown at stage 11, when occasionally some PFCs express low levels of lacZ (arrow) (J). Expression at the CCs acts as an internal control for expression level of BCs and PFCs. (K) Schematic representation of the B fragment showing the four putative STAT binding sites of the BC/PFC enhancer; note that the CC enhancer remains unaffected. A–J correspond to stages 10B/11.