Many of the dorsal abdominal clusters in robo1/learobo2-8 mutant animals have branches that exceed the normal level of extension at approximately 21 and 22 hours after egg laying, but this phenotype is not significantly greater than the defects observed in robo1 mutants.
Heterozygosity for learobo2-8 enhances the frequency of axons ectopically crossing the midline that is seen in robo1 chb4 double heterozygous embryos. robo1 learobo2-8 double heterozygous embryos have axons ectopically crossing the midline.
robo1 learobo2-8 double mutant embryos show misprojection of lch5 axons to the v'ch1 axon pathway in 11% of hemisegments (a similar frequency to robo1 single mutants). Stalling of the lch5 axons prior to turning point TP2, a characteristic of the learobo2-8 single mutant, is also seen in double mutant embryos, although at a higher frequency (6% of hemisegments). The dorsal cluster misprojection phenotype seen in learobo2-8 single mutant embryos is not seen in the robo1 learobo2-8 double mutant embryos; in the double mutant embryos the dorsal cluster axons follow a normal trajectory to the lateral region, although in 7% of hemisegments they grow abnormally from this point, either projecting along the v'ch1 route or growing intersegmentally. The lch5 cell bodies are located dorsal to their normal position in 11% of hemisegments and are aberrantly oriented in 3% of hemisegments of robo1 learobo2-8 double mutant embryos. Missing or aberrantly positioned spiracular branches and failure of motor axons to extend as far as the sensory neuron cluster are occasionally seen.
Expression of roboScer\UAS.T:Ivir\HA1,T:wg under the control of Scer\GAL4bs.Term partially rescues the failure of ganglionic branches to enter the central nervous system that is seen in learobo2-4/learobo2-8 embryos and the weak midline crossing phenotype of the ganglionic branches seen in these embryos is also rescued. Expression of sliScer\UAS.cKa under the control of Scer\GAL4twi.PG in a learobo2-8 background does not result in any new tracheal branches growing from trunk metameres T2 to T9.