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Fig. 7. R1-R6 in bsg²⁶⁵ mutant terminals exhibit a mutant ultrastructural phenotype. (A) Cross section of a control cartridge innervated by non-mutant axons (control animals were generated by recombining a wild-type chromosome arm using the same basic procedure used to create mutant terminals as already described). This control exhibits a wild-type structure (Meinertzhagen and O'Neil, 1991) in which a ring of photoreceptor terminals (R) surrounds the axon profiles of lamina cells L1 and L2. The entire cartridge is surrounded by lamina epithelial glia (*). Terminals contain normal mitochondria (m) and synaptic profiles composed of synaptic vesicles, capitate projections (arrowheads) and T-bar synaptic ribbon release sites (arrow). (B) Cartridge innervated by bsg²⁶⁵ photoreceptors; the terminals are of variable sizes, most are larger than controls in A, which are shown at the same magnification, although one terminal is very small (*). At this distal section plane in the lamina, close to the eye, there are more mitochondrial profiles than normal. (C) In the same lamina as B, but cut at a proximal level, R1-R6 terminals lack mitochondrial profiles. Such profiles disappear in the distal third of the lamina's depth, with a cut-off that is sharply localized. At that level, cartridge cross-sections, as here, have some terminals with (m) and some without (*) mitochondrial profiles. (D) Individual R1-R6 terminal from C, exhibits misplaced rough ER (long arrows), never normally seen in either control or wild-type terminals and pleiomorphic profiles of synaptic vesicles (short arrow); capitate projections are mostly lacking, except those that are shallow (arrowhead), whereas T-bar ribbons are normal (double arrowhead). Bar, 1 µm.

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