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The role of apterous in establishing the wing margin.
a. Mature right wing disc. Directions (compass at left) are: A, anterior; P, posterior; V, ventral; D, dorsal. Prospective D vs. V regions of the disc are indicated (dark vs. light shading), as are the wing pouch (thin dashed line) and future margin (thick dashed line). The D part expresses apterous (ap); the V part does not. The margin expresses wingless (wg), as do the pouch edge and a notal stripe (not shown). The Wg band straddles the D/V boundary (vs. the Dpp stripe which abuts the A/P line; Fig. 6.6a).
b. Effects of xenotopic ap clones inferred from adult defects and GOF data. When ap is misexpressed ventrally (above) it turns wg ON at the edge of this GOF area . Based on their ectopic margins (c) , apLOF clones in the D region (below) must also turn wg ON peripherally.
c. The normal margin (center) has a triple row (TR) of stout or slender MS (mechanosensory, straight) and CS (chemosensory, curved) bristles. The medial TR (mTR) has stout bristles, and the dorsal TR (dTR) has CS bristles, while the ventral TR (vTR) has a 4:1 ratio of slender MS to evenly spaced CS bristles. New TRs develop around apGOF tissue in the V region (upper oval)  and apLOF clones in the D region (lower oval)  whenever these spots are anterior to vein 2. The apGOF V spots have a 'dTR-mTR-E-vTR' polarity (E = clone edge) from inside to outside, while the apLOF D clones have opposite polarity .
d. Right adult wing (D surface). Only stout bristles are drawn. Along the front is TR. More posteriorly are a double row (DR) and a posterior row (PR). DR (like TR) has innervated bristles, but PR has nonsensory hairs [1741, 1837]. The DR/PR transition (at vein 3) recedes to the A/P line (~ vein 4 ) if En expression in the A region (which arises in the 3rd instar ) is blocked [1837, 4229], and it advances anteriorly if Hh is overexpressed . Why the anterior extent of the en-ON area is greater distally than proximally (not shown) is a mystery .
e, f. Symmetric (D/D or V/V) phenotypes caused by GOF (e) or LOF (f) alterations at the Dorsal wing (Dlw) locus . Strangely (via transvection?), the same mutation (Dlw1) switches cell fates in opposite ways (V-to-D vs. D-to-V) depending on whether it is in heterozygous (Dlw1/+ fly = GOF) or homozygous (Dlw1/Dlw1 clones = LOF) condition. In each case the effect is fully penetrant but partly expressed (i.e., not all bristles are transformed). V/V wings can also be created by using ap-Gal4 with UAS-Ser and UAS-fringe to restore Notch signaling, a wg-ON stripe, and a wing margin (albeit a V/V one) in an apLOF background [2850, 3158].
Panel a is after , b and c are based on [882, 1038, 1741] though clones are typically more elongated along the proximal-distal axis than shown here [521, 1373, 1545], d is adapted from [350, 524, 1224, 1741, 1837], and e and f sketch phenotypes from . See also App. 7.