C. elegans can sense a range of temperatures and migrates toward the temperature at which it was cultivated with food for several hours on a temperature gradient. To date, several genes required for this thermotaxis behavior have been identified. The
tax-4 and
tax-2 genes encoding alpha and beta subunits of cyclic nucleotide-gated channel, respectively, are expressed in the AFD thermosensory neuron. The
ttx-3 gene encodes a LIM homeodomain protein that is expressed in the AIY interneuron. Like
ttx-3 mutants,
ttx-1 and
ttx-2 mutants show cryophilic phenotype. These cryophilic phenotypes are mimicked by killing AFD or AIY neurons, which is consistent with the possibility that
ttx-1 and
ttx-2 genes function in AFD or AIY. To observe any defects in AFD or AIY neurons, we introduced AFD-specific
gcy-8::GFP and AIY-specific
ttx-3::GFP markers into
ttx-1 and
ttx-2 mutants as well as into
ttx-3 ,
tax-2,
tax-4 and
tax-6 mutants. We found that the expression of
gcy-8::GFP in AFD was strongly down-regulated in the
ttx-1 mutant. Since
gcy-8 encodes a membrane-bound type guanylyl cyclase, we next investigated whether an AFD-specific, GFP-tagged H13 transcription factor is affected in
ttx-1 mutants. The expression of H13::GFP was almost completely undetectable in
ttx-1 mutants. These results are consistent with the hypothesis that
ttx-1 gene encodes a transcriptional regulator. In addition, this study enabled us to detect abnormal microvillus-like structures in the AFD sensory endings of
ttx-1 mutants, as previously reported through the EM analysis. The
unc-101 gene encodes a clathrin adaptor, a molecule that is thought to be important for intracellular transport. We observed structural abnormalities in the AFD sensory endings of
unc-101 mutants, which is quite similar to those of
ttx-1 mutants. Altogether, our results suggest that the
ttx-1 gene functions in AFD. Besides
ttx-1 mutants, our study also revealed that the expression of
gcy-8::GFP was significantly down-regulated in
tax-2 and
tax-4 mutants. Our current model is that abnormal signaling accompanied by
tax-2 or
tax-4 mutation leads to the reduced overall activity of the AFD neuron, which in turn down-regulates the transcriptional level of
gcy-8 . We thank T. Ishihara for H13::GFP prior to publication.