Mutant synapses at the larval neuromuscular junction show profound undergrowth, with a reduced number of small, tightly spaced type I boutons and an increased number of satellite boutons.
Shows incomplete late larval lethality at 29oC. Arboreal branches of the neuromuscular junctions (NMJs) of muscle 4 are dramatically shortened in homozygous larvae and synaptic boutons are both smaller and more tightly spaced than in wild type, greatly reducing the synaptic terminal area. Mutant terminals contain abundant clusters of "mini-boutons"; very small boutons that are only occasionally seen in wild-type terminals. The mini-boutons are surrounded by subsynaptic reticulum (SSR) as is characteristic of wild-type type I boutons. Mature type I boutons are also significantly smaller than wild-type. The terminals of the NMJs on both muscles 4 and 12 also show a significant decrease in all orders of terminal branches, resulting in a very significant decrease in the total numbers of arboreal branches. Type I bouton numbers are very significantly reduced at the NMJs on muscles 4, 6 and 12. The area of muscles 4 and 12 is not significantly different from wild type. The neuromuscular junctions of mysb9/mys11 larvae contain small, tightly clustered boutons surrounded by numerous mini-boutons. Homozygous and mysb9/mys11 larvae show altered synapse specificity at the muscle 12 NMJ. More than 30% of NMJs show a "backbranched" phenotype; at least one arboreal branch leaves muscle 12 and branches back onto muscle 13 from a site away from the point of nerve entry onto muscle 12. 50% of mysb9/mys11 muscle 12 NMJs show a "pulled away" phenotype; boutons are suspended in the space between muscles 12 and 13 and defasciculation, or branching, occurs before the nerve has actually reached the point of insertion on the muscle. SSR morphology appears qualitatively unaffected in type I NMJs of mysb9/mys11 larvae and the SSR mean cross-section area is not significantly different from wild type. Boutons appear ultrastructurally normal and the presynaptic membranes are normally attached to the postsynaptic membrane. Synaptic vesicle density is indistinguishable from wild type. At the muscle 4 NMJ, most homozygous larvae have nerve-evoked excitatory junctional current (EJC) amplitudes in the normal range, while some have unusually small EJC amplitudes. Overall, the mean EJC amplitude is only slightly reduced compared to wild type. EJC amplitudes are similarly unaffected in mysb9/mys11 larvae. At the muscle 6 NMJ, homozygous larvae have a significantly decreased EJC amplitude, while mysb9/mys11 larvae appear normal. The frequency of miniature EJCs is normal at the NMJ, and the mean (quantal) amplitude is very slightly increased in homozygous larvae.
Wing blisters are seen in approximately 1-2% of hemizygotes, and approximately 60-80% of hemizygotes have held-out wings.