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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.
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