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Fig. 1. Relationship between Notch activation and DV compartmentalization. (A)
Signaling interactions at the DV boundary. Delta (Dl) is expressed by both
dorsal and ventral cells in response to Notch activation (blue arrow), but
signals most effectively (thick black arrow) to dorsal cells owing to the
presence of Fng, and only signals poorly (thin arrow) to ventral cells.
Activated Notch in dorsal cells acts together with the dorsal-specific gene
ap (not shown) to promote expression of Ser. Ser is blocked (black T)
from signaling (gray arrow) to other dorsal cells by Fng, and thus is limited
to signaling back across the compartment boundary (thick black arrow) to
ventral cells, where it activates Notch. As shown here, Notch activation also
results in the elevation of F-actin (green lines) along the cell interface
where peak signaling occurs; the resolution of confocal microscopy is such
that this usually appears as a single line (see also
Fig. 4). (B-D) Third instar
wing imaginal discs, stained for ap-lacZ expression (red) to mark
dorsal cells, and Wg (blue), to mark Notch activation. Expression of
ap-lacZ is a reliable marker of dorsal provenance, as it is not
affected by Notch signaling or changes in cell location
(Blair et al., 1994 ;
Micchelli and Blair, 1999;
Rauskolb et al., 1999). In
this and subsequent figures, discs are oriented with ventral down and anterior
to the left. (B'-D''') Individual stains of the discs
shown in B-D. (B) AyGal4 UAS-fng, with a clone of Fng-expressing
cells (arrow) marked by co-expression of GFP (green). Ectopic expression of
Fng can effectively reposition the compartment boundary away from the normal
DV interface by simultaneously creating an ectopic stripe of Notch activation
within ventral cells, and eliminating normal Notch activation at the DV
interface (Rauskolb et al.,
1999). The normal DV interface (arrowhead) is relatively straight
and smooth, and is disturbed by the Fng-expressing clone. (C,D) AyGal4
UAS-Dl, with clones of Dl-expressing cells (arrows) marked by
co-expression of GFP (green). Ectopic expression of Dl can effectively
reposition the compartment boundary away from the normal DV interface by
simultaneously creating an ectopic stripe of Notch activation within dorsal
cells, and eliminating normal Notch activation at the DV interface. In C, the
clone edge is smoother apically, as evidenced by Wg expression, than basally,
as evidenced by GFP expression. In D, the ectopic Notch activation stripes are
smooth, but do not completely register with the wild-type DV boundary.
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-tubulin (green), ap-lacZ (red), and DNA (blue).
Arrows indicate examples of dividing cells at the DV boundary, bars mark their
axis of division. This can occur at different orientations relative to the DV
boundary. (B) Histogram showing the percent of spindles at the DV boundary
(blue bars, out of 86 total) or away from the DV boundary (red bars, out of
456 total) found in different orientations relative to the boundary. The angle
was defined by the axis of the spindle and the tangent of the nearest DV
interface. Angles were binned into 15° increments, thus a random
distribution would predict 16.7% of spindle orientations (one sixth of the
total) in each bin (dashed line). There is a slight bias for cells at the
boundary to divide parallel to it, but not enough to account for
compartmentalization.
