The frequency of midline crossing is much higher in mewM6/Y;sliunspecified/+ and ifk27e/Y;sliunspecified mutants and also involves more lateral tracts. The semidominant interaction of sliunspecified and scbunspecified is also observed in sliunspecified/Df(2R)XTE-18 embryos. Tigx has a semidominant interaction with sliunspecified, Fas2 labelling of fascicles between segments is reduced and midline guidance defects are observed in one in three segments. LanA9-32 mutants, when double heterozygous with sliunspecified, exhibit midline crossover of Fas2-labelled axons in over 30% of segments. The degree of defasciculation and midline guidance errors in all axon tracts of the triple heterozygote of scbunspecified/sliunspecified;LanA9-32/+, appears to be additive of the individual phenotypes. In addition, a narrowing of the central nervous system and the medial displacement of all axon tracts is also seen in this mutant combination.
Fas2-positive axons cross the central nervous system midline in about 12% of sliunspecified kuzunspecified double heterozygotes, in contrast to either single heterozygote. kuzunspecified homozygotes that are also heterozygous for sliunspecified have a central nervous system (CNS) phenotype similar to kuzunspecified single mutants, but the position of the longitudinal connectives is shifted towards the CNS midline. Removal of one copy of kuz+ in a sliunspecified homozygous background does not enhance the sli mutant phenotype.
Expression of sliScer\UAS.cUa under the control of Scer\GAL4sli.PS rescues the midline crossing phenotype seen in sliunspecified, but does not restore proper lateral position of the FasII-positive fascicles.
Expression of sliUncleave.Scer\UAS under the control of Scer\GAL4sli.PS rescues the midline crossing phenotype seen in sliunspecified, but does not restore proper lateral position of the FasII-positive fascicles.