Figure 2. The Cul1-based SCF E3 ligase components are required for MB axon pruning.

(A) A schematic representation of axon pruning in the medial and dorsal lobes of MB γ neurons. (B–H) Confocal images of MB neurons expressing UAS-mCD8-GFP driven by 201Y-Gal4 at wL3, 24 h APF, or adulthood. 201Y-Gal4 labels postmitotic γ neurons and a small subset of later-born α/β neurons. The Anti-FasII (1D4) antibody labels α/β neurons strongly and γ neurons weakly. (B) Wild-type MB γ neurons projected their axons into the dorsal and medial lobe of the brain at wL3. (C–F) cul1^EX (C), roc1a^G1 (D), skpA¹ (E), and slimb² (F) neuroblast clones that were labeled with 201Y-Gal4 projected their dorsal and medial axons normally but exhibited proliferation defects at wL3, as the axon branches were less dense than the wild-type control. (B′) Wild-type MB γ neurons pruned their dorsal and medial axon branches by 24 h APF, leaving intact α/β axons (GFP negative and FasII positive). (C′–F′) cul1^EX (C′), roc1a^G1 (D′), skpA¹ (E′), and slimb² (F′) MB neuroblast clones displayed notable axon pruning defects at 24 h APF. The right panels in (C′–F′) show that many unpruned dorsal γ axons persisted outside the main α lobes and were co-labeled by GFP and FasII (white arrowheads) in single confocal sections of the dorsal lobe. (G) At adulthood, MB γ neurons regenerated their medial branches but not the dorsal branches. Blue arrowheads point to α axon branches. (H) cul1^EX MB neuroblast clones retained many GFP-positive unpruned larval γ axons (white arrowheads) outside the FasII-positive main α lobe at the adult stage. However, the GFP-positive homozygous cul1^EX mutant α axon branches were absent in this main α lobe (H), suggesting a proliferation defect. Please note that the FasII-positive α lobe is derived from the other heterozygous MB neuroblasts. Dorsal is up in all images. The scale bar is 50 µm. See genotypes in Text S1.