Figure 6.

Robo1^ΔIg1 cannot rescue midline crossing defects in robo1 mutants. (A–D) Stage 16 embryos stained with anti-HRP (magenta) and anti-FasII (green) antibodies. Lower images show FasII channel alone from the same embryos. FasII-positive axons cross the midline inappropriately in every segment in robo1 null mutants (B, arrow with asterisk). This phenotype is completely rescued by a robo1 genomic rescue transgene expressing full-length Robo1 protein (C) but is not rescued by an equivalent rescue transgene expressing Robo1^ΔIg1 (D). (E–H) Stage 13 embryos stained with anti-FasII to examine the trajectory of the pCC axon, which pioneers the medial FasII pathway. In heterozygous robo1/+ embryos (E) or robo1 null mutants rescued by Robo1 (G), the pCC axon extends anteriorly and does not cross the midline (arrows in E and G). In robo1 null mutants, the pCC axon inappropriately crosses the midline and fasciculates with its contralateral homolog (F, arrow with asterisk). Ectopic crossing of pCC is not rescued by expression of Robo1^ΔIg1 (H, arrow with asterisk). For quantification of ectopic crossing phenotypes in the genotypes shown in (A–D), see Table 2.