Fig. 1. Bar expression in the basal undifferentiated cells and R1/6 photoreceptor cells is independently regulated. Antibodies used for staining are as indicated in each panel with matched colors in this and all subsequent figures. Posterior is towards the left and dorsal is upwards in all discs unless mentioned otherwise. (A) Two tier nuclei layers in the longitudinally sectioned eye disc. Photoreceptor nuclei stained by Elav (red) localize apically, whereas undifferentiated cell nuclei stained by BarH1 (green) localize in the basal region of the disc. R1/6 photoreceptors (yellow arrow) are labeled by both Elav and BarH1 antibodies. Bar is also expressed in peripodial cells (small white arrow) and basal undifferentiated cells (double arrowheads) behind the furrow (marked by large white arrow). (B) Complementary expression patterns of Bar and Ato in the basal undifferentiated cells along the furrow (marked by arrow). Two confocal images are combined to illustrate the Bar-Ato pattern. (C) Co-expression of Bar and Lz in a majority of basal undifferentiated cells (marked by white arrow) except in some Bar-expressing cells immediately posterior to the furrow (marked by yellow arrow). An eye disc is longitudinally sectioned. (D-I) Lz is not required for Bar expression in the basal undifferentiated cells. A lz^r15 LOF clone is identified by the absence of Lz staining (D,E,G,H; red) and marked with broken lines in F,I. Bar expression is downregulated in R1/6 cells (D-F, arrows), but not in the basal undifferentiated cells (G-I) within the lz^r15 LOF clone. (J) Bar is expressed in R1/6 cells in the apical photoreceptor level. An arrow indicates Bar expression in R1/6 cells of wild-type eye disc. (K,L) Gl is not required for Bar expression in the basal undifferentiated cells. A gl¹ mutant eye disc stained with antibodies for Elav (red) and BarH1 (green) is shown at the apical photoreceptor nuclei level (K) and the basal undifferentiated cell level (L). Bar expression in most R1/6 photoreceptors is strongly downregulated or absent, except a few cells marked by an arrow (K), while its expression in the basal undifferentiated cells is not much changed (L). Broken lines in K,L mark the position of morphogenetic furrow; Bar is expressed only behind the furrow.