Fig. 1. Babo inactivation results in axon overextension. Babo regulates axon growth through Babo_a and Babo_b isoforms. (A) Schematic of the adult Drosophila brain. The boxed region shows mushroom body (MB) neurons in the left hemisphere of the central brain (cb). Arrows show the MB axon trajectory extending from posterior dorsal cell bodies, projecting anteroventrally and then turning towards the midline. The MB images shown are either from the left hemisphere in this orientation, or of the central brain, showing both hemispheres. Dashed white lines indicate the midline. ol, optic lobe; D, dorsal; V, ventral; P, posterior; A, anterior; L, lateral; M, medial. (B) A wild-type MB neuroblast clone. Typical adult wild-type clones generated from newly hatched larvae have axonal projections that terminate either in the dorsal anterior cortex or just prior to the midline. Only , and β projections are indicated. (C-E) Representative images of babo⁵² (C), tkv⁴ (D) and sax⁴ (E) neuroblast clones. Note the β lobe overextensions (open red arrowhead) across the midline in babo clones. In these and subsequent figures, open white arrowheads indicate axon pruning defects. (F,G) Representative images of babo⁵² neuroblast clones expressing either UAS-babo_a (F), or UAS-babo_b (G). Many of the axons in the UAS-babo_a rescue exhibited small protrusions that were not characteristic of any lobe (thin white arrow in F). These represent ectopic projections of a subclass of MB axons induced in the OK107>babo_a genetic background. In these and subsequent figures, solid red or white arrowheads indicate normal and β or lobe termination points, as indicated. All images in this and subsequent figures are z-projections of confocal sections. Green, expression of the marker mCD8::GFP on all MB, neuroblast or single-cell MARCM clones (sometimes multiple single-cell clones); magenta, Fas2 staining of all MB (weakly stained) and β (strongly stained) axons (appearing white when overlapping with mCD8::GFP). Dashed white line, midline. Scale bar: 20 µm. (H) Quantification of axon overextension defects in the indicated genotypes. n, number of neuroblast clones examined.