Fig. 8. Characterization of the neuroanatomical defects in ttx-3 mutant animals. (A) Axonal defects observed in various mutants. The wild-type AIY axon is shown in black. In wild-type animals, a short process (shown in dark gray) can occasionally be observed in the region where the main axon turns dorsally to enter the nerve ring; those are not scored as a mutant phenotype, unless they are longer than 10 µm (shown as light gray addition to the dark gray line). Schematic examples of aberrant axon sprouts are in light gray. We set the minimum length for scoring aberrant sprouts that emanate from the axon arbitrarily at 2 µm. As cell bodies occasionally narrow into a thin ending, we arbitrarily set the criteria to score sprouts from the cell body more stringently at 4 µm. Occasional ‘splits’ of the axon in the nerve ring region are counted as sprouts. Small enlargements along the axon, rarely observed in wild-type animals, often observed in pathfinding mutants, are not scored as sprouts. Every case in which the main axon does not meet the axon of its contralateral homolog at the dorsal midline is classified as ‘short stop’; the main axon stalls either at the turning point into the nerve ring (but never before that point), or at various points below or above the dorsoventral midline (black arrows). (B) Characteristic examples of axonal defects observed in ttx-3 mutants. AIY is visualized with mgIs18. Quantitative data are shown in Table 3. Similar results have also been obtained with several extrachromosomal ttx-3prom::gfp lines (Hobert et al., 1997). For this analysis we could make use only of the ttx-3(ks5) allele because, owing to the more severe reduction of ttx-3prom::gfp expression in the stronger ttx-3 alleles, the axon of the AIY interneurons cannot be visualized. It is thus possible that in the stronger alleles AIY neuroanatomy is even more strongly affected. (C) Representative examples of AIY neuroanatomy in unc-33(e120), unc-73(e936) and unc-119(e2498) mutant animals, visualized with mgIs18. White triangles point to short stops, white arrows to sprouts. Animals shown in this figure are late larvae and young adults. Notably, besides the short-stop defect of the main axon of the AIY neurons, these axon pathfinding mutants also cause axon sprouting defects. As neural activity defects do not cause axon sprouting in AIY (Table 3), the axon sprouting observed in pathfinding mutants are unlikely to be a secondary consequence of aberrant connectivity caused by aberrant pathfinding. We rather hypothesize that the axon sprouting defect observed in pathfinding mutants as well as in ttx-3 mutants reflects an inability of the neuron to initiate and maintain the proper outgrowth of a single, monopolar axon.