Dendritic targeting relative to the ventral midline. (a) Control and (b,c,e,f) experiments showing confocal images (maximal Z-projections) of RP2 neurons at 25–31 hours AEL, visualised with UAS-mCD8::GFP (green) in the context of a set of axon tracts visualised by anti-FasciclinII staining (magenta). Dendrites between the lateral and central intermediate Fasciclin II fascicle are defined as 'lateral'; dendrites located between the central intermediate fascicle and the midline as 'medial'. Misexpression lines are indicated in the bottom left hand corner of each panel. (b) Misexpression of commissureless (comm) leads to aberrant midline crossing of dendritic branches (arrowhead), though no apparent increase of dendrites targeted towards the midline between the intermediate and medial FascilinII tracts. The high variability in phenotype is partly due to the varying lengths the dendritic tree mis-routed across the ventral midline. (c) Misexpression of frazzled (fra) causes increased targeting of dendrites into the medial neuropile (arrowhead). Black asterisk indicates the cell body of the contralateral RP2 neuron. (d,d') Ventral (d) and lateral (d') views of stage 13 embryos driving expression of GSd433 with engrailed-GAL4 and stained by in situ hybridisation using an anti-sense probe against robo2. The staining shows the segmentally repeated stripes characteristic for engrailed. The reaction had to be terminated before the endogenous robo2 expression pattern appeared (see Additional file 3) due the high levels of expression. (e,f) Misexpression of robo2 by GSd433 (e) or robo (f) leads to a reduction to near absence (robo) of branches innervating the medial neuropile (arrowheads), and some dendritic branches positioned aberrantly lateral of the lateral Fasciclin II axon tract (arrows). (g) Quantification showing ratios of medial/lateral dendrites; *P = 0.04, **P < 0.001, t-test, N = 5; error bars indicate the standard error. Anterior is left. Scale bars: (a-c,e,f) = 10 μm; (d,d') = 140 μm.