Fig. 4. robo2 and robo3 are required for normal SerT expression. (A-C) Stage 16 ventral nerve cords from robo2^x123 (A,B) and robo3¹ (C) loss-of-function mutant embryos stained for serotonin uptake in red (A,C) and white (B). The slightly disorganized axon tracts of the mutant CNS are labeled in green with mAb BP102. robo2 and robo3 loss-of-function nerve cords show a random loss of SerT activity in approximately half of hemisegments. (B) SerT activity is lost in both a single hemisegment (arrow) and an entire segment (arrowhead). (D) In situ hybridization for SerT mRNA in a stage 16 robo2 mutant indicates a loss of SerT transcription in the same pattern as the loss of SerT activity. Mutant nerve cords were identified by axonal morphology or, in the case of D, irregular width. (E,F) Serotonin uptake is shown in red and eagle-lacZ is shown in green in wild type (E) and a robo2^x123 mutant (F). (E) In a wild-type cord, both serotonergic neurons in each hemisegment express eg and therefore stain for lacZ. (F) Although robo2 mutants lack SerT activity in many neurons, lacZ expression remains normal. Serotonergic cell bodies positive for lacZ but not SerT activity are highlighted in the insets below (asterisk). Scale bar: 20 µm for D-F; 10 µm for A-C.