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 as an assay (12 th International C. elegans Meeting 1999, abstract 392), 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 detection, in the drop test, of quinine, SDS, Cu ++ , 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, 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 evidences that PHA and PHB, the two sensory neurons of the phasmid, the tail sensillum of Secernentea nematodes, function in C. elegans as 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, Cu ++ and SDS. Thus the anatomical rescue correlates with the functional rescue and the results are consistent with those obtained by laser ablation. An EMS mutagenesis screening has been performed to isolated worms unable to avoid quinine hydrochloride. We found 5 strains who showed the quinine non avoiders phenotype and were non-Dyf (abnormal dye filling) indicating that their sensory cilia were intact. One of them, gb 404 , had the most severe phenotype in relation to quinine response although still able to respond to mechanical stimuli and to a subset of other repellents. Complementation analysis revealed that gb 404 and gb 408 were alleles of the same gene we called
qui-1 (QUInine non-avoiders). Mapping data using SNPs placed the mutation on the IV chromosome. Recently, we have discovered the molecular nature of
qui-1 . 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.