Fig. 3

Crossing is not required for reception of anterior or dorsal guidance cues. A-Cdcc knockdown embryos demonstrating phenotypes consistent with loss of reception of attractive ventral cues, including anterior growth with a failure to grow ventrally (a′-c′) Summary diagrams illustrating affected axons. AAsterisk indicates affected CoPA, while arrowhead points to affected CoPA axon. In (B), an ipsilateral projection that grows dorsally and anteriorly without crossing. Asterisk and arrowheads indicate affected cell body and axons, respectively. C A ventral extension (arrowhead) from an affected CoPA (asterisk) grows at an oblique angle to the ventral spinal cord (compare to wild-type pathfinding, Figure 2a-b). (D-E) robo3 (twt^tx209) embryos exhibit two distinct phenotypes. D′-E′ Summary diagrams illustrating affected axons. In (D), robo3 (twt^tx209) CoPA axons fail to enter the midline and ascend instead on the ipsilateral side of the spinal cord. Asterisk indicates affected CoPA neuron, arrowheads indicate ipsilateral projection in this image, in which only the ipsilateral spinal cord was imaged. In (E) robo3 (twt^tx209), CoPA axons enter the midline but remain for some distance while ascending before exiting the midline on the contralateral side (“midline ascending”). Asterisk denotes CoPA cell body, arrowheads indicate axons ascending within the midline. In all images, dorsal is up, anterior to the left. (F-G) Comparative phenotypes from offspring of heterozygous incross (F) versus outcross to wild type (G). Only in offspring of heterozygous incross is the ipsilateral (IL) CoPA phenotype observed. The midline ascending (MA) phenotype is present in both conditions.