The abdomen of Drosophila: does planar cell polarity orient the neurons of mechanosensory bristles?

Caroline CG Fabre, José Casal, Peter A Lawrence

In the adult abdomen of Drosophila, the shafts of mechanosensory bristles point consistently from anterior to posterior. This is an example of planar cell polarity (PCP); some genes responsible for PCP have been identified. Each adult bristle is made by a clone of four cells, including the neuron that innervates it, but little is known as to how far the formation or positions of these cells depends on PCP. The neurons include a single dendrite and an axon; it is not known whether the orientation of these processes is influenced by PCP. We describe the development of the abdominal mechanosensory bristles in detail. The division of the precursor cell gives two daughters, one (pIIa) divides to give rise to the bristle shaft and socket cell and the other (pIIb) generates the neuron, the sheath and the fifth cell. Although the bristles and their associated shaft and socket cells are consistently oriented, the positioning and behaviour of the neuron, the sheath and the fifth cell, as well as the orientation of the axons and the dendritic paths, depend on location. For example, in the anterior zone of the segment, the axons grow posteriorly, while in the posterior zone, they grow anteriorly. Manipulating the PCP genes can reverse bristle orientation, change the path taken by the dendrite and the position of the cell body of the neuron. However, the paths taken by the axon are not affected. PCP genes, such as starry night and dachsous orient the bristles and position the neuronal cell body and affect the shape of the dendrites. However, these PCP genes do not appear to change the paths followed by the sensory axons, which must, therefore, be polarised by other factors.