Figure 4.
Liprin-β contributes to the termination of R7 axon growth in the correct target layer. A–F show head sections of adult flies. R7 and R8 axons are stained with anti-Chaoptin (red) and R7 axons are stained with anti-β-galactosidase to reveal the expression of panR7-lacZ (green). A′–F′ show enlargements of the medulla neuropil. The M3 and M6 layers are marked by dashed lines (M3, left; M6, right). A, A′, In wild-type flies, R7 photoreceptors project their axons to the M6 layer in the medulla and form round termini (arrows). B, B′, In Liprin-α mutants, R7 axons often terminate prematurely in M3, the R8 target layer (asterisks). Mutant R7 axons that reach the M6 layer usually do not expand their termini (arrows). C, C′, In Liprin-β mutants, some R7 axons extend beyond the M6 layer (arrows), even when the axons have normally expanded termini (arrowheads). D, D′, Many R7 axons in Liprin-α Liprin-β double mutants either do not reach their normal target layer (asterisks) or fail to expand their termini (arrows). Occasional R7 axons project much deeper into the brain (arrowhead), where they can form round termini (empty arrowhead). E, E′, Lar mutants show a stronger phenotype than Liprin-α; the majority of R7 axons stop in the M3 layer (asterisks). F, F′, In Lar Liprin-β double mutants, most R7 axons stop at the M3 layer (asterisks), but some project much further into the brain (F, F′, arrowhead). G, Quantification of the number of R7 axons that project beyond the M6 layer. Liprin-β mutants [Liprin-βΔ83: 14.08 ± 1.18%, 19 hemispheres (hm); Df(3L)BSC614/Liprin-βΔ83: 15.13 ± 1.07%, 15 hm; and Liprin-βΔ51/Liprin-βΔ83: 17.01 ± 1.16%, 36 hm] have more protrusions than the controls (wild type: 4.10 ± 0.63%, 14 hm; precise excision Liprin-βΔ45: 7.66 ± 0.83%, 24 hm). This phenotype is rescued by the expression of a UAS-Liprin-β transgene with the pan-neuronal driver elav-GAL4 (9.91 ± 1.58%, 13 hm). Liprin-γ mutants (6.88 ± 1.36%, 10 hm) are comparable with the control, and Liprin-β Liprin-γ double mutants (17.58 ± 1.57%, 7 hm) are similar to Liprin-β. Error bars represent SEM. *p < 0.05; ***p < 0.0001.