The genetics of levamisole resistance in the nematode Caenorhabditis elegans

We have characterized a small group of genes (13 loci) in the nematode Caenorhabditis elegans that, when mutated, confer resistance to the potent anthelmintic levamisole. Mutants at the 7 loci conferring the most extreme resistance generally possess almost identical visible and pharmacological phenotypes: uncoordinated motor behavior, most severe in early larval life, extreme resistance to cholinergic agonists and sensitivity to hypo-osmotic shock. Mutants with exceptional phenotypes suggest possible functions for several of the resistance loci. The most extreme mutants can readily be selected by their drug resistance (211 mutants, as many as 74 alleles of one gene). The more common resistance loci are likely to be unessential genes, while loci identified by only a few alleles may be essential genes or genes conferring resistance only when mutated in a special way. We propose that these mutants represent a favorable system for understanding how a small group of related genes function in a simple animal. The extreme drug resistance of these mutants makes them useful tools for the genetic manipulation of C. elegans. And, as the most resistant class of mutants might lack pharmacologically functional acetyl-choline receptors (LEWIS et al. 1980), these mutants may also be of some neurobiological significance.