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Fig. 7. retn mutations cause neuronal pathfinding errors. (A) Mushroom bodies in retn-Gal4⁸⁹/+ show a clear separation of ß-lobes (arrows). (B) In retn-Gal4⁸⁹/retn^z2-428 larvae, ß-lobe fusion is evident (arrow). Compare with single larval MB in Fig. 6D, which is unlinked to its paired MB. (C) retn-Gal4⁸⁹/retn^z2-428 pupae also show ß-lobe fusion (arrow) and poor fasciculation of neuronal projections (arrowhead). (D) ß lobes of Canton-S adult female brain do not cross the midline. In 90% (n=6) Canton-S adult male brains there was no lobe fusion but one animal did have some crossing ß-lobe fibers; a low frequency of ß-lobe fiber crossing has been noted in wild-type animals (Moreau-Fauvarque et al., 1998; Michel et al., 2004). The gamma lobe fibers label weakly with Fas2 (Crittenden et al., 1998). Arrow and arrowhead indicate the midline between the ß-lobes and the -lobe, respectively. (E) ß-Lobes of retn^dri8/retn^z2-428 adult female have Fas2-positive axons that cross the midline giving a fused appearance. In this mutant female, the right -lobe is also smaller than in wild-type females, as though there are fewer Fas2-postive axons. ß-Lobe fusion of Fas2-positive axons was also found in 75% (n=4) of retn^dri8/retn^z2-428 and 33% (n=3) of retn^RO44/retn^RO44 adult male brains. Anterior is upwards. Arrow and arrowhead indicate the midline between the ß-lobes and the -lobe, respectively. (F,H,J) retn-Gal4⁸⁹/+; UAS-mGFP. (G,I,K) FRTG13, retn-Gal4⁸⁹/FRTG13, retn-Gal4⁸⁹; UAS-mGFP clones following heat shock of hs-FLP; FRTG13, retn-Gal4⁸⁹/FRTG13, Gal80; UAS-mGFP/+. (F) Mid-pupal SOG neurons in retn-Gal4⁸⁹/+ show dense arborization (arrowhead) and projections extending towards the protocerebrum (arrows). (G) Mid-pupal SOG neurons in retn-Gal4⁸⁹ homozygous clones have little dendritic branching (arrowhead) and poor extension of distal processes (arrows). G is at a higher magnification than F. (H) Confocal section of midline-crossing transect (F) shows tight fasciculation of neurites (arrow). (I) Confocal sections of SOG transect (G) shows poor fasciculation of the same neuronal projections (arrow). (J) Mid-pupal photoreceptors R1-R6 project to the lamina (LA), while faint pattern of R8 projections (arrow) is visible in the medulla (ME). (K) In retn-Gal4⁸⁹ homozygous tissue, subsets of R1-R6 cells extend beyond the lamina into the medulla (arrow). A-I, anterior is upwards; J,K, anterior towards the left.

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