The ability to recognize, avoid and reject toxic compounds in the environment is crucial for survival. In humans, oral exposure to poisonous compounds often results in the association of sets of aversive responses with the perception of bitter taste. Using the drop test (12th International C. elegans Meeting 1999, abstract 392) as an assay, we have observed that C. elegans senses as repellent many water soluble substances that are toxic or related to toxic compounds and that also taste very bitter to humans, suggesting a conserved response to toxicity and an evolutionary relationship between avoidance and bitter taste. Through laser ablation of specific neurons we have identified sensory neurons responsible for the detection, with the drop test, of quinine, SDS, Cu++, shikimic acid, high osmotic strength and low pH. Our results suggest that the polymodal sensory neuron ASH, which has already been shown to mediate avoidance responses to high osmotic strength -using different avoidance assays- and to some kind of mechanical stimuli (nose touch), plays also a central role in the avoidance of all water soluble repellents tested. We have found that ASK, described as responsible for chemotaxis toward the attractant lysine, is also involved in water soluble avoidance, suggesting a new double-opposite role for this cell. We will also present evidence that PHA and PHB, the two sensory neurons of the phasmid, the tail sensillum of Secernentea nematodes, function in C. elegansas modulators of the avoidance response to SDS. To complement the laser ablation data, we developed a genetic cell specific rescue using the mutants strain
osm-6. These mutants fail to respond to many repellent tested because of defects in sensory cilia formation.
osm-6 has been shown to act cell autonomously (1). We used different neuronal specific promoters to drive the expression of the wild type
osm-6 gene in a subset of neuronal cells, in order to see if it was possible to recover their cilia defect and their function. We observed anatomical rescue of the cilia structure in those cells where the w.t.
osm-6 gene expression was directed. Behavioral assay, using the drop test, have shown that worms in which ASH, ADL and ASK where structurally recovered, rescued also their ability to respond to high osmotic strength, quinine and other repellents. Thus the anatomical rescue correlates with the functional rescue and the results are consistent with those obtained by laser ablation. Finally, we used the drop test to isolate behavioral mutants that fail to avoid quinine hydrochloride. The gene
qui-1, defined by two alleles gb 404 and
gb408, has been mapped to chromosome IV; the molecular cloning, using SNPs strategy is in progress. So far we have confined the mutation between two snip-SNPs that define an interval of less than 1 map unit, which correspond to about 250 kb. 1. Collet, J., Spike, C.A., Lundquist, E.A., Shaw, J.E. and Robert K. Herman. Analysis of
osm-6, a gene that affects sensory neuron function in Caenorhabditis elegans. 1998. Genetics 148:187-200.