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Spitz/EGFr signalling via the Ras/MAPK pathway mediates the induction of bract cells in Drosophila legs

Centro de Biología Molecular Severo Ochoa-C.S.I.C., Facultad de Ciencias-CV, Universidad Autónoma-Cantoblanco, 28049 Madrid, Spain

View larger version (144K): [in a new window]   Fig. 1. Bract-cell morphology, distribution and lineage. (A,B) Adult leg showing mechanosensory bristles with the associated bract (black arrows), bract-less chemosensory bristles (arrowhead) and trichomes (red arrows). (C) The wing bracts are restricted to the proximal costa (bar). (D) High magnification of the area labelled in C showing mechanosensory (arrow) and bract-less chemosensory bristles (arrowhead). (E,F) Cell-lineage analysis shows that bracts and bristles are not related by lineage: y+ clones in a y– background. Arrows indicate sets of bracts and bristles with different labelling. In all figures, photos that show legs correspond to tibia or basitarsus, apart from Fig. 4A-C and Fig. 5B,F in which, for better resolution, the femur is shown.

View larger version (137K): [in a new window]   Fig. 2. The RAS/MAPK pathway mediates the induction of bracts. (A) rolled1/rolled10A leg showing a complete lack of bracts. (B) gap1B2 leg. Mechanosensory bristles have two or more bracts; chemosensory bristles are not affected. (C-F) Clones of cells mutant for spi IIA14, EGFr4A, Drase2F and DrafEA75. Clones are outlined and cells are marked with forked (C,E,F) or pawn (D). All bristles within clones are bract-less (arrows).

View larger version (117K): [in a new window]   Fig. 3. The competence to specify bracts is spatially restricted. hsp70-GAL4/UAS-Raf * pupae heat shocked for 30 minutes at 37°C. (A) In the leg, all trichomes are transformed into bracts (compare with Fig. 1A). (B) In the wing, only cells in the proximal costa (arrow) are transformed (compare with Figs 1C,D). (C) In the notum, many bracts (arrows) appear among dorsocentral bristles. (D) In the haltere, only a region of the pedicellum shows extra bracts. In A,B,D, distal is rightwards. In C, anterior notum is left.

View larger version (107K): [in a new window]   Fig. 4. The competence to specify bracts is temporally restricted. hsp70-GAL4/UAS-Raf * pupae heat shocked for 30 minutes, 37°C at 0-4 (A), 4-8 (B) or 8-12 (C) hours APF. Note that in B, the patches of bracts do not appear close to the bristles. (D) Notch activation represses the specification of bracts. hsp70-GAL4/UAS-Nintra pupae heat shocked for 30 minutes, 37°C at 8-12 hours APF. All bracts are missing.

View larger version (87K): [in a new window]   Fig. 5. (A) Effect of poxn overexpression induced by heat shock for 30 minutes, 37°C at 8 hours APF. In the leg, MBs are transformed into ChBs and bracts are missing. (B) Overexpression of poxn and Raf* yields an additive phenotype, with trichomes transformed into bracts and MBs into ChBs. (C-E) poxn expression detected by poxn RNA in situ hybridisation in early pupal leg (C) and wing (E) of wild-type, and leg of In(1)sc10–1 (D). (F) hsp70-GAL4/UAS-sSpi leg. All the trichomes are transformed into bracts. (G,H) rho1-lacZ expression in wild-type and In(1)sc10–1 legs, respectively. (I) A rho1de11 clone marked with f is outlined. Note the absence of bracts within the clone (arrows) and the presence of bracts in the wild-type bristles that surround the clone (arrowheads). (J) UAS-rho1 and UAS-S overexpression by hsp70-GAL4. Note the presence of three bracts in some bristles (arrows).

View larger version (96K): [in a new window]   Fig. 6. Argos function is required to select which cell will become a bract. (A) aos loss-of-function produces extra bracts (arrow); aos15 leg. (B) Overexpression of aos removes all bracts (arrow); sca-GAL4/UAS-aos. (C) sple basitarsus showing mutant polarity. Arrows indicate sets of bracts and bristles pointing in different directions. (D) sple; gap1 basitarsus; extra bracts show the same polarity and appear in the same position as endogenous bracts (arrows).

View larger version (24K): [in a new window]   Fig. 7. A model of the specification of bract cells. (A) In chemosensory organ precursor cells, poxn expression represses the activation of Spi ligand, and this inhibits the specification of bract fate. (B,C) In mechanosensory organ precursor cells, Spi is activated and signals to a neighbouring cell to activate, through the EGFr, the RAS/MAPK pathway. In regions where cells are competent (expression of an unidentified gene ‘X’) the activation of the pathway drives the specification of the bract fate. This competence is temporally restricted. Thus, in early pupal development (B), epidermal cells may not be competent as cells do not differentiate as bracts after the activation of the pathway. The activation of the Dl/N signalling pathway, which mediates the lateral inhibition mechanism to specify a single SOP cell, also represses the specification of bracts by antagonising the RAS/MAPK pathway. (C) At 8 hours, APF bract cells are specified, and the activation of the aos gene in the bract cell inhibits the specification of more neighbouring cells as bracts.