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Fig. 6. Model of Fz and Stbm interactions in the pupal wing epithelium and
border cell cluster. In the wing, Fz and Stbm mutually reinforce the
localisation of each other in opposing junctions of neighbouring cells
(rounded black arrows) and inhibit the localisation of each other in adjacent
regions of the same cell (grey bars). Distally localised Fz within the same
cell promotes the production of a single distal actin-rich trichome, via Dsh
and RhoA function (Axelrod,
2001; Strutt,
2001; Strutt et al.,
1997). In addition, proximally localised Stbm is thought to
promote trichome formation at the opposite end by an uncharacterised mechanism
(Adler et al., 2004). In the
border cell cluster, Fz and Stbm are localised to the junctional regions in
which the epithelial polar follicle cells and the partly epithelial border
cells make contact. Because Fz-expressing polar follicle cells promote the
migration of Stbm-expressing border cells in a contact-dependent manner, we
infer that Fz in the junctions of polar cells promotes the localisation of
Stbm to the junctions of border cells. In turn, this would lead to Fz
localisation to the non-junctional (mesenchymal) migratory regions of the
border cells. Fz in border cells locally modulates the formation of
appropriate actin structures, probably via Dsh and RhoA, as in the wing. In
addition, Stbm in the junctions of border cells promotes the formation of
actin structures at a distance in the migratory region. In this way, Stbm
localised to junctions and Fz in the migratory region both independently
promote migration. Consistent with our mosaic analysis, this scheme predicts
that (i) contact with an Fz-expressing polar cell promotes border cell
migration (Fig. 4B,C), (ii)
Fz-expressing polar cells are only able to promote the migration of border
cells that express Stbm (Fig.
1D, Fig. 4D), and
(iii) Stbm is required in the polar cells for efficient border cell migration
(Fig. 4B), but the border cells
do not need to touch the Stbm-expressing polar cell
(Fig. 4D).
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