Fig. 2. Expression of chn in embryos and imaginal disks. Expression was detected by in-situ hybridization with an antisense RNA chn probe labeled with DIG. (A,B) Views of a stage-5 embryo focusing at the lateral surface or internally at the invaginating mesoderm (arrowhead), respectively. Note the weak segmental pattern in A at the level of the ectoderm. (C,D) Lateral views of a stage-11 embryo showing (arrowheads) expression in the mesoderm and in patches of the lateral ectoderm located between the tracheal pits, respectively. (E) High magnification view of a similar embryo showing the position of the tracheal pits (arrowheads) and the hybridization in between them. (F) At stage 15, expression is almost exclusively found in the developing PNS (arrowheads) and in the CNS (arrow). (G) A high magnification view of the region under the red line in F shows that expression occurs in cells of the clusters of sensory neurons (compare with Fig. 3A). The image is centered on the dorsal and lateral clusters. (H) In third instar wing imaginal disks, chn expression occurs in proneural clusters (arrowheads) and in the posterior notum and wing hinge (arrows). (I) Expression of chn in proneural clusters depends on ac/sc, as it is absent in In(1)sc^10.1 disks. (J) Overexpression of UAS-sc in the posterior compartment of the wing disk (en-Gal4 driver) causes ectopic expression of chn (arrowheads). (K) chn is expressed in a number of patches of the leg disks, which correspond to proneural clusters since they are absent in a In(1)sc^10.1 mutant background (not shown), excepting for the femoral clusters, which express ato (Jarman et al., 1995). (L) Expression of chn in a third instar eye/antenna disk. Expression occurs ahead and/or at the morphogenetic furrow (arrow), at the presumptive head capsule (arrowheads) and at the second antennal segment (red arrowhead). This pattern is strongly reminiscent of that of the proneural gene atonal (Jarman et al., 1995).