Genes that control ray sensory neuron axon development in the C. elegans male

We have studied how a set of male-specific sensory neurons in C. elegans establish axonal connections during postembryonic development. In the adult male, eighteen bilateral pairs of ray sensory neurons innervate an acellular fan that serves as a presumptive tactile and olfactory organ during copulation. We visualized ray axon commissures with a ray neuron-specific reporter gene and studied both known and new mutations that affect the establishment of connections to the pre-anal ganglion. We found that the UNC-6/netrin-UNC-40/DCC pathway provides the primary dorsoventral guidance cue to ray axon growth cones. Some axon growth cones also respond to an anteroposterior cue, following a segmented pathway, and most or all also have a tendency to fasciculate. Two newly-identified genes, rax-1 and rax-4, are highly specific to the ray neurons and appear to be required for ray axon growth cones to respond to the dorsoventral cue. Among other genes we identified, rax-2 and rax-3 affect anteroposterior signaling or fate specification, and rax-5 and rax-6 affect ray identities. We identified a mutation in sax-2 and show that the sax-2/Furry and sax-1/Tricornered pathway affects ectopic neurite outgrowth and establishment of normal axon synapses. Finally, we identified mutations in genes for muscle proteins that affect axon pathways by distorting the conformation of the body wall. Thus ray axon pathfinding relies on a variety of general and more ray neuron-specific genes and provides a potentially fruitful system for further studies of how migrating axon growth cones locate their targets. This system is applicable to the study of mechanisms underlying topographic mapping of sensory neurons into target circuitry where the next stage of information processing is carried out.

Key Words: axon guidance, male behavior, netrin, sexual development

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