Adult appendages have A/P and D/V axes, which are aligned with the corresponding axes of the fly. Each appendage spans one complete body segment. Appendages also have a proximal-distal (P/D) axis (proximal being closest and distal being farthest from the body). To illustrate the principles by which appendages are patterned, we focus on generation of the A/P axis in developing wing discs. The A/P axes of other imaginal discs are determined by similar mechanisms. Although the specific genes involved in establishing the D/V and P/D axes of appendages are different from those used to create the A/P pattern, the principles underlying these developmental events are much the same.
As mentioned above, an appendage, such as a wing, spans a single segment, which is subdivided into anterior and posterior domains (Fig. 4.2). As described in more detail below (Fig. 4.3) subdivision of ap-
Veins form at boundaries along the A/P axis of the wing disc primordium
C. Localized misexpression of Dpp activates nested expression of sal and omb
FIGURE 4.2. Wing vein development.
B. Dpp diffusessal^ gene expression in L2 vein
D. Localized loss of sal function shifts the position of the L2 vein to the new sal border
FIGURE 4.2. Wing vein development.
pendages into two territories can be traced back to formation of imag-inal discs during embryonic development. Imaginal disc cells straddle the A/P boundary of the embryonic segments from which they derive. The position of the A/P boundary within imaginal discs is maintained with great precision in the face of the thousandfold increase in cell number that takes place during larval development.
As discussed in Chapter 3, the segment-polarity genes engrailed (en) and hedgehog (hh) are expressed in the embryo in stripes confined
■ Anterior Versus Posterior Cells ■
Cells lying in the anterior and posterior portions of a segment can be distinguished from one another by several criteria. The most obvious difference between anterior and posterior cells is that, like oil and water, they do not mix with one another. Because anterior and posterior cells cannot intermingle, they are said to lie within discrete compartments. There is a sharp line dividing imaginal discs into anterior versus posterior compartments which is referred to as the A/P boundary. The position of the A/P boundary in a given imaginal disc is invariant. For example, in the wing, there are four major longitudinal veins running the length of the wing that are numbered L2-L5, L2 being the anterior-most vein and L5 being the posterior-most vein. There also is a vein circumnavigating the anterior margin of the wing known as the L1 vein. The A/P boundary in the wing disc runs in a stereotyped position just anterior to L4 (Fig. 4.2B).
to the posterior portion of each segment. These two genes play essential roles in specifying the identity of posterior cells. In mutants lacking function of either of these two genes, the posterior portion of each segment develops as a mirror duplication of the anterior half of the segment (see Fig. 3.3F). en encodes a transcription factor (En) that activates expression of various target genes including the hh gene. hh encodes a secreted signal (Hh) that plays an essential role during embryonic development in maintaining the subdivision between the anterior and posterior halves of the segment. The en and hh genes continue to play critical organizing roles during the development of imaginal discs.
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