CDaseslab-1 embryos appear anatomically and morphologically normal. Their neuromuscular junctions appear morphologically normal. Although mutants move spontaneously and in response before hatching, movements are less vigorous than in controls. A minority briefly generate peristaltic body contractions sufficient to emerge partly or completely from the egg case. In contrast, CDaseslab-1/+ siblings display wild-type movement and behavior.
Electrophysiological assays performed at late embryonic stages or early larval stages reveal that CDaseslab-1 mutants show impaired neurotransmission due to a presynaptic defect. Nerve-evoked EJC amplitude and fidelity are reduced compared to controls and show transmission failures and near-failures. The incidence of failures progressively increases at higher stimulation frequencies. However, postsynaptic glutamate-gated currents are normal. Additionally, hypertonic saline-evoked synaptic vesicle fusion is significantly impaired in CDaseslab-1, CDaseslab-1/CDaseslab-2 and CDaseslab-1/Df(3R)slab-3 mutants.
FM1-43 dye uptake is reduced in CDaseslab-1 presynaptic terminals, indicating a smaller cycling vesicle pool. Ultrastructural analyses of CDaseslab-1/Df(3R)slab-3 mutants reveal that the mutant presynaptic terminals have normal clustered and docked readily-releasable pools of vesicles. However, CDaseslab-1 boutons have fewer synaptic vesicles within the nonclustered population removed from the active zone and the boutons contain increased numbers of synaptic vesicles linked together in multi-vesicle clusters or chains. Furthermore, the boutons exhibit increased numbers of vesicles tethered by filamentous structures to the plasma membrane.