Loss of Kif1bβ isoform leads to RoP-like sMN axon pathfinding defects that are reminiscent of those observed in Fignl1 morphants and mutants. (A) Immunolabeling of sMN in 56-hpf control and kif1b^st43 homozygous mutants using the Zn-5 antibody. (B–D) Mean number of defasciculated/split (B), missing (C), or mistargeted (i.e., caudally projecting; D) rostral nerves per embryo. Quantifications were performed in 23 control and 26 kif1b^st43 mutant larvae pooled from three independent experiments. (E) Immunolabeling of sMN in 56-hpf control (fignl1^+/+), fignl1 heterozygous (fignl1^sa22254/+), or homozygous (fignl1^{sa22254/sa22254}) mutant and Fignl1 morphant (Fassier et al., 2018) larvae using the Zn-5 antibody. (F–H) Mean number of defasciculated/split (F), missing (G), or abnormal (H) rostral nerves per embryo. Quantifications were performed in 28 fignl1^+/+, 27 fignl1^sa22254/+, and 20 fignl1^{sa22254/sa22254} larvae pooled from three independent experiments. (I) Immunostaining of sMN in 56-hpf larvae injected with control, Kif1bβ, and Kif1bα morpholinos using the Zn-5 antibody. (J–M) Mean number of defasciculated/split (J and M), missing (K), or mistargeted (i.e., caudally projecting; L) rostral nerves per embryo. Quantifications were performed in 42 control and 37 Kif1bβ morphant larvae (J–L), and 18 control and 16 Kif1bα morphant larvae (M), pooled from three independent experiments. (N) Immunolabeling of sMN in 56-hpf control and kif1b^st43 homozygous mutants injected or not with the Kif1bβ morpholino. (O) Mean number of defasciculated/split rostral nerves per embryo. (A, E, I, and N) Images are lateral views of the trunk, anterior to the left. Empty arrowheads show control rostral nerves. Arrowheads and arrows indicate rostral nerves that are respectively abnormally split or defasciculated. Scale bars, 20 µm. (B–D, F–H, J–M, and O) Quantifications were performed in 12 (F–H) or 24 (B–D, J–M, and O) spinal hemisegments per embryo. *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ns, nonsignificant; unpaired two-tailed t test (B, D, J, and M), Mann–Whitney t test (C and K–L), Kruskal–Wallis ANOVA test with Dunn’s post-test (F, G, and O), or one-way ANOVA test with Bonferroni's multiple comparison post test (H). Error bars are SEM. Nb, number; defasc., defasciculated; Ctl, control.