(A-H) TEM of type Ib boutons of NMJs in A2 or A3 segments in wild-type controls (A, C, and E) and mutants (B, D, F, G, and H) at rest (A, B, E, and F) or stimulated (C, D, G, and H). Mutant boutons exhibit a significant depletion in the number of SVs (compare A and B; compare E and F). Most of the residual SVs are associated with active zones in mutants (B and F). Mutant terminals display omega structures (G) and collared pits (H) which are rarely observed in wild-type control boutons. (I-K) Quantification of the number of SVs (I), the size of SVs (J), and SV intermediates (K). Type Ib boutons of NMJs in third instar wild-type controls (n=20), DB25/DB25 (n=23), DB25/DB56 (n=20), wild-type controls (Stimulated) (n=21), and DB25/DB56 (Stimulated) (n=26) were analyzed. At least three animals for each genotype were used. P value: *, P<0.05; **, P<0.01; ***, P<0.001. Error bars indicate SEM. Scale bar: 200 nm in A-F; 20 nm in G and H.