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.
