Ultrastructure of synaptic boutons in dlg mutants. A–C and E show TEM micrographs of type Ib boutons at muscle 6 or 7 (abdominal segment 3) of third-instar (A) wild-type, (B) dlg^XI-2, (C) dlgA^40.2 and (E) dlgS97⁵ larvae. b, Bouton. The arrow in A points to an active zone. D, F, High-magnification views of a representative active zone in wild-type (D) and dlgS97 mutants (F). The arrowheads indicate the outer limits of the active zone. G, Morphometric analysis of synaptic boutons showing synaptic length (left panel), normalized SSR length (middle panel), and SSR thickness (right panel). Note that active zones are increased in length in dlg, dlgS97, and dlgA mutants, but normalized SSR length is decreased only in dlg^XI-2. Scale bar: A–C, E, 1.2 μm; D, F, 0.3 μm. For a comparison of dlgS97⁵ with dlgS97¹³⁸ mutant allele in other parameters, see supplemental Table 2 (available at www.jneurosci.org as supplemental material).