Fig. 8. Cell death pathways in the Drosophila retina. Levels of activated effector caspases depend on two opposing pathways. Inhibitor of apoptosis proteins (DIAPs) block caspase activation. Initiator caspases promote caspase activation. EGFR-dependent survival signalling acts through DIAPs by inhibiting the HID, a DIAP inhibitor. A pro-apoptotic signal mediated by N also acts through this pathway, antagonising EGFR-dependent survival signalling. N has this role only in pupal retina, not in eye imaginal discs, and is antagonised by primary pigment cells and/or cone cells. Thus, caspase activity is blocked by multiple extracellular signals. Dronc and p35-sensitive initiator caspases act redundantly for effector caspase activation in most eye cells. No extracellular signals have yet been found that promote caspase activation directly and it is possible that no such regulation exists, although our results raise the possibility that in eye disc cells Hid might activate caspases independently of DIAPs. In addition, our results indicate that Dronc has effects in dying cells independent of effector caspases. The data do not rule out more elaborate models in which EGFR is also a receptor for primary pigment cells and/or cone cell survival signals upstream of N, as well as acting downstream of N. However, such models require that EGFR function upstream of N depends on primary pigment cells and/or cone cells whereas EGFR function downstream of N does not.
