Fig. 1. The development of the mechanosensory bristle organ of the adult peripheral nervous system of Drosophila. (A) An adult bristle (machrochaete) stained for neur^A101-lacZ to reveal the nucleus of the socket cell (blue) and anti 22C10 antibody staining to mark the neurone (brown). The bristle sense organ consists of two more cells: the prominent bristle cell and a sheath or thecogen cell, which is not visible in the picture. (B) A wing imaginal disc of the late third larval instar stage, stained with anti Hnt antibody (red) to reveal the SOPs of the machrochaete. The disc contains a scaGal4 insertion that activates UAS GFP in the cells of the proneural clusters (green). The double staining reveals that the clusters are arranged in a stereotypic pattern that allows the identification of each cluster individually. ANP and PNP, anterior and posterior notopleural; APA and PPA, anterior and posterior postalar; DC, dorsocentral; SC, scutellar clusters. (C) Development of the bristle sense organ. The SOP is selected from a proneural cluster during the process of lateral inhibition, which is mediated by the Notch signalling pathway (not shown). The SOP, recognizable by the high level of expression of the proneural protein Ac, signals through the Notch ligand Delta to its neighbours (pink lines). Activation of the pathway results in the Su(H)-dependent switch to the epidermal fate in the neighbours of the SOP. The high levels of Ac and Sc proteins in the SOP are achieved through the activation of the SOP-E of the sc gene (Culi and Modolell, 1998). Once the SOP is selected, it switches off the expression of the proneural genes and initiates expression of neur^A101-lacZ, sens and hnt. It then divides to generate the second order precursor cells pIIa and pIIb. pIIa divides to give rise to the socket and bristle cells. pIIb divides to generate a third-order precursor pIIIb and a glial cell. The glial cell migrates away and does not contribute to the formation of the sense organ. pIIIb further divides to give rise to the neurone and the sheath cell that protects the neurone. In this lineage, the Notch-signalling pathway is employed several times to help the cells to choose the correct fate. In the first step, pIIb sends a Notch-mediated inhibitory signal (pink line) that prevents pIIa from joining the pIIb fate and developing the pIIa fate. Later Notch is required to send an inhibitory signal from the bristle to the socket and from the neurone to the sheath cell to prevent the receiving cells from choosing the same fate as the sending cell. The differentiated neurone can be detected through the expression of the neurone specific 22C10 and Elav marker. (D) The consequence of loss of Notch function during bristle development. Owing to the lack of Notch signalling, all cells of a Notch mutant proneural cluster choose the SOP fate. As a result of the missing communication between the progenies of the SOP, an excess of neurones develops at the expense of the other fates of the sensillum. These supernumerary neurones can be visualized by anti 22C10 or anti Elav antibody staining.

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