A. In wild type, photoreceptor neurons (ELAV, magenta) differentiate clustered around central R8 cells (SENSELESS, green). Yellow bar indicates columns 1–4, corresponding to the Second Mitotic Wave. B. GMR-RasV12 promotes ectopic neurogenesis posterior to column 5 (eg arrow). Between columns 2–4 most ommatidia (72%) had the normal number of ELAV-positive cells, and only 0.50 extra neurons per ommatidium were seen on average. C. Reduced N signaling results in ectopic R8 specification within the morphogenetic furrow. An average of 0.36 extra neurons were seen in column 4 where R8 cells were unaffected(eg arrow; see also Figure 4B). 68% of the ommatidia in column 4 had the normal number of ELAV-positive cells. There are many more ectopic neurons anteriorly, but as columns 2–3 contained many R8 cells it is unclear how many other neurons differentiated as a direct consequence of loss of N function and how many were recruited by R8's. D. GMR-RasV12 promotes more and earlier ectopic neurogenesis when N function is reduced (compare panel B). Arrow indicates extra ELAV-positive cells around ommatidia with single R8 cells. There were 4.1 extra ELAV positive neurons per ommatidium, on average, and in some preparations all the cells appeared to be ELAV positive.