The number of ventral muscles that cross the midline in lea1 Sdcunspecified double mutant embryos is significantly increased compared to either single mutant. The longitudinal axons collapse along the ventral midline and form a single fascicle. roboQ1-15 Sdcunspecified embryos do not show enhancement of the muscle and central nervous system midline crossing phenotypes seen in roboQ1-15 embryos. sli2 Sdcunspecified double heterozygous embryos have additional longitudinal transverse muscles. The number of Fas2-expressing inner fascicles that cross the midline in the double heterozygotes (3.3%) is increased compared to sli2 single heterozygotes (0.6%). Sdcunspecified homozygous embryos that are also heterozygous for one of roboQ1-15, lea1 or sli2 show an enhanced axonal guidance defect with multiple midline crossings of the fascicles.