The AFD sensory neurons play a critical role in thermosensation. Like other sensory neurons, AFD neurons extend dendrites to the tip of the head and project axons to the nerve ring where synaptic connections with other neurons occur. The sensory ending structure in AFD has a characteristic shape. All the head sensory neurons except AFD have conspicuous ciliated structures, whereas AFD neurons have a degenerated cilium and elaborated microvillus-like structure at the sensory ending. How are these AFD neurons with complex sensory ending structure generated durning development? To answer this question, we have screened mutants that are defective in AFD development. We co-introduced AFD specific marker H13::GFPand AIY specific marker
ttx-3::GFPto NL917(
mut-7)strain. AIY neurons are downstream interneurons that are also critical for thermotactic behavior. We have screened 450,000
mut-7animals and found over a dozen mutants that are categorized into the following groups. Group1-
nj11: In
nj11,ectopic GFP expression is observed in ASI chemosensory neurons. Neuronal cell fate may be changed in this mutant. Group2-
nj12and other two mutants: Axon extensions are defective in these mutants. After entering the nerve ring, pre-mature termination of axons occurs within the ring. Movement of
nj12is normal, but other two mutants show UNC phenotype. Group3-
nj13and other eight mutants: Cell body position of AFD is abnormal in these mutants. Although the normal position of AFD cell body is posterior to the nerve ring, AFD cell bodies are mislocalized anterior to the nerve ring. We are now characterizing phenotype of these mutants and cloning genes by transposon display method. In this screening, we additionally isolated two alleles of
tax-4and one allele of
tax-2that were detected through down-regulation of AFD specific marker H13::GFP.
tax-2and tax-4encode subunits of cyclic nucleotide-gated channel, and are essential for thermosensation in AFD. These results are consistent with our previous observation that
tax-2and tax-4activities are required for proper expression of H13::GFP(Sasakura et al,
l999, INWM). We thank T. Ishihara for H13::GFP, O. Hobert for
ttx-3::GFP, L. Henri, S. Wicks and R. Plasterk for
mut-7strain and advice on transposon display method.