C. elegans is grown in the lab typically in the 15 to 25° temperature range. While studying dauer formation we discovered that N2 can grow at a temperature of 27°, though with severely reduced brood size and lifespan. At this temperature, dauer formation is much more strongly induced by pheromone than at 25° and even in the absence of exogenous pheromone, N2 makes some dauers at 27°. This basal level of dauer formation is probably not dependent on endogenous pheromone since
daf-22 mutant animals also form dauers at a similar frequency at 27°. At temperatures slightly above 27° (~28°) N2 begins to exhibit larval arrest and animals that do grow to the adult stage are sterile. At 29°, N2 arrests at the L1 stage. Thus, dauer formation is strongly induced at temperatures that are at the threshold for growth and reproduction. We show that dauers formed at these temperatures have normal fertility if recovered at lower temperatures, suggesting that the strong induction of dauer formation at stressful temperatures was selected for as a protection against sterility or death. If it is indeed the temperature limit that leads to the induction of dauer formation, we reasoned that strains with a different temperature limit for growth should show a parallel shift in the temperature range for dauer formation. To analyze this, we looked at the high temperature growth phenotypes of twenty-one naturally isolated strains of C. elegans, most of which have been described by Hodgkin and Doniach (Genetics 146:149). Amazingly, all the C. elegans strains showed virtually the same temperature limit for growth (within ~0.5°) as N2 even though they have been isolated from all around the world, covering a wide climatic range. Furthermore, all strains except one (CB4852) formed some dauers at 27° but not at 25°, demonstrating that the dauer formation response at these temperatures appears to be a property of C. elegans as a species and not just the N2 isolate. Several strains failed to make dauers at 25° on pheromone isolated from N2 but still made dauers at 27° (without pheromone), suggesting that the temperature induction of dauer formation is more conserved than pheromone induction. Since C. elegans strains isolated from various places have the same temperature threshold for viability, we reasoned that temperature may be a key parameter in defining the ecological niche of the species. To address this question further, we looked at the temperature thresholds for growth in five naturally isolated strains of C. briggsae and in several isolates of other Caenorhabditis species. All five C. briggsae strains could grow at 30°. Strain CB5161, an isolate of a gonochoristic Caenorhabditis species could grow at even higher temperatures. We have also begun to examine growth at temperatures below 15°. All twenty-one C. elegans strains could grow at 12.5° with a generation time of about one week. All five C. briggsae strains grew very poorly at 12.5° with generation times of over two weeks. Thus, temperature may be a key environmental parameter distinguishing these two species, with C. briggsae having a slightly higher optimal temperature. Preliminary data also suggests that C. elegans dauer formation is more induced at 12.5° relative to 15°, suggesting that induction of dauers occurs at both temperature extremes, presumably because both are similarly stressful. We hypothesize that dauer formation is part of a thermal stress response.