stjDD106 and stjDD106/Df(2R)Exel7128 mutants die as late embryos. They do not exhibit any gross morphological abnormalities, but fail to hatch from their egg cases. In contrast with the wild-type 'beads on a string' morphology, neuromuscular junctions in 21h old stjDD106 (and stjDD106/Df(2R)Exel7128) embryos completely lack boutons, with only thin neurites extending onto the muscle surface throughout the body-wall musculature.
In stjDD106 mutant embryos, motor neurons successfully navigate to and fix on their target muscles at the appropriate time. They exhibit normally shaped and sized growth cones. As early as 16 hours after egg laying, however, during the prevaricosity stage, neuromuscular junctions become abnormal, with thinner neurites and atypical branching. Although muscles 6, 7, 12 and 13 are each contacted by the neurons, the nerve often does not end by bifurcating into a T shape at the boundary between muscles 12 and 13, but instead bifurcates more ventrally, on muscle 6, in a Y shape that extends to the other muscles. Other aspects of embryonic development continue normally (for example, gut folding, ventral nerve cord retraction and trachea air-filling), but neuromuscular junction development never progresses past the prevaricosity stage and no boutons ever arise.
In electron micrographs of stjDD106 mutant neuromuscular junctions exhibit long, thin processes containing active zones are found in place of the large round profiles of boutons. Neuronal processes average only 0.6υm across, compared to 3 υm in wild-type. Profiles wider than 1.3υm are never encountered. Despite these morphological changes, the active zones in stjDD106 mutant synapses look normal, having closely apposed pre- and post-synaptic densities of similar sizes, clusters of synaptic vesicles and T bars.
Homozygous stjDD106 and transheterozygous stjDD106/Df(2R)Exel7128 mutants exhibit spontaneous miniature excitatory junction currents of wild-type amplitude, indicating that functional release sites are properly aligned with functional receptors, despite the absence of boutons. The frequency of these mEJPs is diminished by 64%, suggesting that fewer release sites are present and consistent with the 70% reduction in active zone area in the mutants, and the 61% decrease in the area of nerve-muscle contacts.
Stimulation of motor axons in stjDD106 mutants does not evoke a postsynaptic response. The probability of release is reduced at least 1000-fold in the mutant compared to wild-type.