BcrAbl induced crossovers depend on the P3-motif of Fra.

<p>Stage 16 embryos stained with mAb 1D4 against Fasciclin II (FasII) are depicted with anterior up. All UAS (U) transgenes are pan-neurally expressed using the 1407-Gal4 (1407-G4) driver line recombined onto the <i>fra³</i> chromosome; thus, phenotypes are compared to heterozygous <i>fra³</i> rather than a wild-type embryo. [<b>A</b>] A <i>fra³</i> heterozygote exhibits a nerve cord that is essentially wild-type. Expression of [<b>B</b>] BcrAbl but not [<b>C</b>] Ablwt results in FasII axons crossing the midline incorrectly (arrowhead in B). [<b>D</b>] In homozygous <i>fra³/fra⁴</i> mutants, the FasII fascicles may partially fuse, and small breaks in the connective may appear. Removal of both copies of <i>fra</i> abolishes [<b>E</b>] BcrAbl and [<b>F</b>] Ablwt induced crossover defects. In both cases, FasII expressing axons are observed projecting towards the periphery (arrows), including axons from the medial most pCC/MP2 pathway (arrow in E). [<b>G</b>] Quantification of ectopic crossing errors (per embryo) observed using BcrAbl is graphed. White bar indicates crossovers in a <i>fra³</i> 1407-Gal4 heterozygote, black bar is a <i>fra³</i> 1407-Gal4/<i>fra⁴</i> homozygote and gray bars are <i>fra³</i> 1407-Gal4/<i>fra⁴</i> homozygote's expressing the indicated Fra transgene. Expression of BcrAbl in a <i>fra³</i> 1407-Gal4/<i>fra⁴</i> homozygote reduces ectopic crossovers, but these are restored by co-expression of Fra transgenes (* P<0.05, or ** P<0.01). Compared to Fra^wt, expression of Fra^ΔP3 restores only about half of the expected crossovers (φ P<0.05). [<b>H</b>] Re-expression of U-Fra^wt transgene restores the crossover defects (arrowhead in G) in <i>fra³/fra⁴</i> embryos expressing BcrAbl, while [<b>I</b>] expression of Fra^ΔP3 restores about half of the ectopic crossovers.