Fig. 1. Mirror image duplication can be induced by an ectopic source of the morphogen molecule. (A) A model for morphogen signaling. A morphogen emanating from the expressing cell (S) sets the positional value of a cell by forming a concentration gradient across the developmental field in which the cell resides; the value of the gradient at each point in the field is a function of the distance of the receiving cell from the morphogen-secreting cells (left). Introduction of an ectopic source (S') of morphogen can induce mirror image duplication (underline; right). (B) Wing imaginal disc (red) of third instar Drosophila larva. The imaginal disc is a two-sided sac comprising a columnar cell layer that contains presumptive wing blade (wb) and thorax (t) regions, and an overlying squamous peripodial membrane (pm); it is set aside from the embryonic epidermis and develops at the larval stage. The imaginal disc is subdivided into anterior (A) and posterior (P) compartments along the anteroposterior axis. hedgehog (hh) is expressed in the posterior compartment; hh mRNA is visualized with in situ hybridization (left). Schematic on right modified with permission from Bryant and Levinson (Bryant and Levinson, 1985). (C) Ectopic expression of hh, by making a clone of cells expressing hh, induces a mirror image duplication of the anterior wing structure. Hh produced in the P compartment is secreted into the A compartment (top). A clone of cells ectopically expressing hh in the A compartment induces a complete mirror image duplication of the A compartment (bottom). Wing veins I V are indicated. Reproduced with permission from Tabata (Tabata, 2001). (D) Ectopic production of Shh, induced by implanting shh-expressing cells into the anterior limb bud, induces a mirror image duplication of the wing structure. shh is expressed in the region corresponding to the ZPA in the wing bud (top). Implanted cells that ectopically produce Shh in the anterior of the limb bud induce a mirror image duplication of the wing structure (bottom). Digits (II, III and IV) are labeled on the schematic, and radius (R), ulna (U) and humerus (H) are labeled in the photographs on the right. Reproduced with permission from Riddle et al. (Riddle et al., 1993). Photographs courtesy of C. Tabin.