sli[1], robo[5] transheterozygous stage 16 embryos exhibit thinner than normal fascicles and midline crossing defects.

A kuz^H143 background enhances the FasII midline crossing phenotype seen in sli¹, robo⁵ transheterozygous stage 16 embryos.

A kuz^e29-4 background enhances the FasII axon midline crossing phenotype seen in sli¹, robo⁵ transheterozygous stage 16 embryos.

A kuz¹¹² background enhances the FasII midline crossing phenotype seen in sli¹, robo⁵ transheterozygous stage 16 embryos.

One copy each of sli¹ and robo⁵ enhances the FasII axon midline crossing phenotype seen in kuz^H143 stage 16 embryos.

Double heterozygous sli¹/+, robo⁵/+ embryos exhibit minor defects in midline crossing.

In robo1⁵/+, sli¹/+ transheterozygous embryos, the medial longitudinal pathway occasionally crosses the midline. The addition of RhoGAP93B^{dsRNA.Scer\UAS} (driven by Scer\GAL4^elav.PLu) significantly enhances this phenotype.

When RhoGAP93B^Scer\UAS.cHa is driven by RhoGAP93B^Scer\UAS.cHa in a sli¹/+, robo1⁵/+ background, the axon scaffold of the ventral midline is severely disrupted. The ectopic crossing defect see at low frequency in sli¹/+, robo1⁵ embryos is enhanced.

In robo1⁵/+, sli¹/+ transheterozygous embryos, the medial longitudinal pathway occasionally crosses the midline. The addition of RacGAP84C^Scer\UAS.cRa (driven by Scer\GAL4^elav.PLu) has no effect on this phenotype.

sli¹, robo⁵ transheterozygotes exhibit approximately 2.7 defects in longitudinal axon guidance per animal. An average of 24% of segments show defects. Also 27% of embryos also exhibit defects in pCC/MP1.

Heterozygous dock⁰⁴⁷²³ enhances the longitudinal axon guidance and pCC/MP1 defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 7.4 defects are seen per animal. 67% of segments (calculated as number of defects/segments) show defects, and 72% of embryos show defects in pCC/MP1.

Heterozygous dock³ enhances the longitudinal axon ectopic midline crossing defect seen in transheterozygous sli¹, robo⁵ mutants. An average of 5.7 defects are seen per animal. 52% of segments (calculated as number of defects/segments) show defects.

Overexpression of Pak^{Scer\UAS.T:Hsap\MYC} under the control of Scer\GAL4^elav.PLu enhances the longitudinal axon guidance and pCC/MP1 defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 6.1 defects are seen per animal. 55% of segments (calculated as number of defects/segments) show defects.

Heterozygous Pak⁴ enhances the frequency of central nervous system axon defects seen in sli¹, robo⁵ mutants. An average of 6.1 defects are seen per animal. 54% of segments (calculated as number of defects/segments) show defects.

Heterozygous Pak¹¹ enhances the frequency of central nervous system axon defects seen in sli¹, robo⁵ mutants. An average of 5.8 defects are seen per animal. 53% of segments (calculated as number of defects/segments) show defects.

Overexpression of Pak^{Scer\UAS.T:Myr1} under the control of Scer\GAL4^elav.PLu enhances the longitudinal axon defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 14.3 defects are seen per animal.

sli¹, robo⁵ transheterozygotes exhibit approximately 2.7 defects in longitudinal axon guidance per animal. An average of 24% of segments show defects. Also 27% of embryos also exhibit defects in pCC/MP1. Heterozygous dock⁰⁴⁷²³ enhances the longitudinal axon guidance and pCC/MP1 defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 7.4 defects are seen per animal. 67% of segments (calculated as number of defects/segments) show defects, and 72% of embryos show defects in pCC/MP1. Heterozygous dock³ enhances the longitudinal axon ectopic midline crossing defect seen in transheterozygous sli¹, robo⁵ mutants. An average of 5.7 defects are seen per animal. 52% of segments (calculated as number of defects/segments) show defects. Overexpression of Pak^{Scer\UAS.T:Hsap\MYC} under the control of Scer\GAL4^elav.PLu enhances the longitudinal axon guidance and pCC/MP1 defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 6.1 defects are seen per animal. 55% of segments (calculated as number of defects/segments) show defects. Heterozygous Pak⁴ enhances the frequency of central nervous system axon defects seen in sli¹, robo⁵ mutants. An average of 6.1 defects are seen per animal. 54% of segments (calculated as number of defects/segments) show defects. Heterozygous Pak¹¹ enhances the frequency of central nervous system axon defects seen in sli¹, robo⁵ mutants. An average of 5.8 defects are seen per animal. 53% of segments (calculated as number of defects/segments) show defects. Overexpression of Pak^{Scer\UAS.T:Myr1} under the control of Scer\GAL4^elav.PLu enhances the longitudinal axon defects seen in transheterozygous sli¹, robo⁵ mutants. An average of 14.3 defects are seen per animal. 130% of segments (calculated as number of defects/segments) show defects.

A few axon bundles cross the midline incorrectly in embryos trans-heterozygous for robo¹ and sli¹. This combination increases the penetrance of crossovers to about 60%.

When one copy of Abl⁴ is also present in trans-heterozygous robo¹ and sli¹ embryos, several abnormal crossovers are observed in all embryos examined, and some axon bundles collapse towards the midline in a manner reminiscent of a sli¹ phenotype.

Heterozygous sli¹ enhances the number of abnormal axon crossovers observed in Abl⁴ mutants.

The addition of one mutant copy of Abl⁴ doubles the number of crossovers observed in heterozygous sli¹ mutants.

Compared with heterozygous sli¹ mutants alone, pan-neural overexpression of Abl^Scer\UAS.cHa using Scer\GAL4^elav.PLu enhances the frequency of abnormal axonal crossovers. Scer\GAL4^elav.PLu>Abl^Scer\UAS.cHa expression causes axons to cross the midline inappropriately in most segments of all embryos in sli¹ heterozygous mutants.

In sli¹/+,robo⁵/+ double heterozygote embryos the occasional axon crosses the midline. This phenotype is dramatically enhanced by Nedd4^{Scer\UAS.T:Hsap\MYC}.

Over half of sim², sli¹ embryos exhibit a 'collapsed axon' phenotype, about a third a have fused commissures. Midline cells are displaced ventrally in these embryos. Half of jing⁰¹⁰⁹⁴, sli¹ double homozygous embryos exhibit a 'collapsed axon' phenotype, about a third a have fused commissures. Midline cells are displaced ventrally in these embryos.

28% of segments contain Fas2-positive neurons inappropriately crossing the midline in sli¹/robo¹ double heterozygous embryos, in contrast to either single heterozygote which do not show this defect. 26% of segments contain Fas2-positive neurons inappropriately crossing the midline in sli¹/robo⁴ double heterozygous embryos, in contrast to either single heterozygote which show defects in 0 or 1% of segments. sli¹ dominantly enhances the robo⁵ phenotype, resulting in a lateral compression of the axon scaffold in the central nervous system. sli¹ robo⁵ double homozygotes have a phenotype similar to that of sli¹ single homozygotes.