With ~100,000 sequence polymorphisms to N2, the C. elegans wild isolate CB4856 (Hawaiian) is widely used to map mutations in the N2 background. This technology, however, is limited for phenotypes in which CB4856 is different than N2. Here we show that CB4856 exhibits a thermotaxis-defective phenotype relative to N2 that limits its use as a mapping strain for thermotaxis mutants. In a population-based thermotaxis assay on a linear thermal gradient, well-fed N2 worms exhibit robust and highly reproducible negative thermotaxis at 3 C above their cultivation temperature (Tc+3 C). CB4856 worms, however, show variable responses at Tc+3 C, exhibiting reduced negative thermotaxis, athermotactic behavior, or positive thermotaxis. Chromosome substitution strains, in which each N2 chromosome pair is replaced by the corresponding CB4856 chromosome pair, can facilitate mapping in situations where the CB4856 polymorphism that confers the mutant phenotype is on a different chromosome than the mutation being mapped in N2 (Chen W.C. et al., 2006 European Worm Meeting). We have determined that the polymorphic gene(s) responsible for the thermotaxis differences between CB4856 and N2 are located on the X chromosome, as the X chromosome substitution strain behaved like CB4856, while all other chromosome substitution strains were indistinguishable from N2. We found that allelic variation in
npr-1, which is polymorphic between N2 and CB4856 and located on the X chromosome, may be at least partially responsible for the CB4856 thermotaxis phenotype:
npr-1 mutants in the N2 background and other wild isolates containing the CB4856
npr-1 isoform exhibited defects in thermotaxis similar to CB4856. These findings indicate that chromosome substitution strains will be valuable tools in our ongoing effort to map new thermotaxis mutants, and further suggest that
npr-1 plays a role in thermotaxis by N2 worms.