Fig. 5. Transient expression of tsh in the PE results in ectopic eye development. Anterior is towards the left in all images. Except for the cross-section in B, dorsal is upwards. (A,B) L3 eye disc of a hs-Gal4; UAS-tsh larvae, in which tsh was transiently induced during L2 through a heat-pulse, shows clusters of 22C10 (red) photoreceptors on the PE (marked by Hth nuclei, blue). (A) Focal plane through the PE showing the axons (white arrowhead) and cell bodies (white arrow); the ME photoreceptor cell bodies are also seen in this plane (orange arrow). (B) Confocal z-section through the same disc where the normal (orange) and the tsh-induced PE (white) photoreceptors (arrows) plus associated axons (arrowheads) can be distinctly seen. (C,E) Expression patterns of MD705-GAL4 (705>GFP) and MS-1096-GAL4 (1096>GFP) in late L3 eye discs. (C) MD705-GAL4 drives expression along the dorsoanterior margin and PE. (E) MS1096-GAL4 in both dorsal and ventral PE and margin of the eye disc, although more strongly ventrally. Fate maps (Haynie and Bryant, 1986) indicate that the dorsoanterior disc margin and PE give rise to dorsal anterior head capsule (including the frontal, vertical and post-vertical bristles) while anteroventral margin and PE develops into anteroventral head capsule (including the vibrissae, post-gena and lower post-occipital bristles). (D) MD705- and (F) MS1096-driven tsh expression results in PE photoreceptor differentiation (arrow). In 1096>Tsh discs, the region where these photoreceptors develop loses tsh and hth expression (close-up in Fi and Fii). (G) MS1096-driven tsh expression results in ventral ectopic eyes (arrow) that replace ventral head structures (loss of postgenal structures) and in small dorsal patches of photoreceptors normally accompanied by a bristle (arrowhead). (H) MS1096-driven expression of ey does not induce ectopic eyes in equivalent positions (arrow) in the head, although it does so in the wing hinge (J), where tsh is present (Bessa et al., 2002). (I) Expression of MS1096 detected by X-Gal histochemistry in MS1096-GAL4/UAS-lacZ adult heads. Signal is detected surrounding the eye in the dorsoanterior (arrowhead) and ventroanterior head capsule (arrow), in agreement with the fate map of the corresponding PE domains in eye discs. Bolwig's organ nerve (bn) runs along the peripodial layer of the eye disc and is detected by 22C10.