(A) In wild-type males, the gpa-1::lacZ marker is expressed in one cell per spicule. Cell ablation experiment identifies this cell being SPD neuron. Both spicules are shown on this focal plane. (B–D) gpa-1::lacZ is expressed in two cells per spicule in lin-44 and lin-17 mutant males. (B) lin-44(n1792); (C) lin-17(n671); (D) lin-17(n698), early B lineage defect is rare in this weak allele and ∼80% males show a two cells per spicule marker expression pattern. Lateral view, only one spicule is in focus. (E–H) The gpa-1::lacZ marker is expressed in two cells per spicule in son-1(sy549) mutants. Cell ablation experiment indicated that son-1(sy549) confers a SPD sheath to neuron fate transformation. (E) sy549 intact animal, lateral view showing the left spicule. (F) When the precursor of the SPD neuron and the SPD sheath is ablated, no expression is observed in the spicule region. (G) When the SPD neuron is ablated, the SPD marker is expressed in one cell per spicule. (H) When the SPD sheath is ablated, the SPD marker is expressed in one cell per spicule. Scale bar, 20 μm.