β-Specem21/Y embryos show midline axon guidance defects, with medial longitudinal fascicles ectopically crossing the midline.
Homozygotes fail to hatch (~90%) or die as early first instar larvae (~10%). Hatched larvae are lethargic and display limited movement. General anatomy of neuromusculature, epidermis, denticles and mouthparts is normal. Morphology of neuromuscular junction, and number of boutons on muscle 6/7 is normal. Mutants have severely reduced synaptic transmission. Evoked EJCs are reduced to approximately one quarter wild type values in voltage clamped muscles. Glutamate response is normal suggesting that the defect is presynaptic. Synaptic transmission is only insignificantly reduced during high-frequency stimulation. However the frequency of sEJCs and mEJCs are significantly reduced, suggesting a Ca2+-independent defect in synaptic vesicle fusion. The amplitude of mEJCs is unaffected, consistent with unaffected postsynaptic receptors. Distribution of active zones and synaptic vesicle clustering is normal. The distribution/polarization of synaptic proteins Syn, Csp, syt, Syx1A dlg1 is distinctly abnormal.
Most hemizygous male embryos die before hatching, although they appear to be fully developed and motile (22 +/- 6% hatch). The mutant larvae survive for 1-3 days, whether or not they hatch. Homozygous larvae show reduced midgut acidification compared to wild-type larvae.