Worm Breeder's Gazette 14(5): 57 (February 1, 1997)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Molecular and Cellular Biology Program and Department of Genetics, University of Washington, Seattle, WA 98195
Most previously characterized Daf-c mutants form ~100% dauers at 25C, the nonpermissive temperature for dauer formation. We have discovered a group of mutants that do not form dauers at 25C, but are strongly Daf-c at temperatures 1-2C higher, a phenotype that will be designated here as 27C Daf-c. For example, unc-31(e928) and unc-64(e246) mutants make ~0% dauers at 25C and ~100% dauers at 27C. These two mutants are in the group of Syn-Daf (synthetic dauer formation) mutants, in which certain double mutant combinations are strongly Daf-c, while single mutants are not Daf-c at temperatures 25C or lower. Several other Syn-Daf mutants are also 27C Daf-c, most notably unc-3 and egl-4. Interestingly, two classes of mutants that are Daf-d at lower temperatures, the dyf mutants (which have altered sensory neuron endings) and daf-3 (but not daf-5) also are 27C Daf-c. A survey of behavioral mutants indicates that the 27C Daf-c phenotype is not a general feature of mutations affecting the nervous system, but is rather specific to those known to have effects on dauer formation. Thus, the 27C Daf-c phenotype may be useful for identifying new genes with weaker effects on dauer formation. We have conducted a screen for 27C Daf-c mutants, and have a number of candidate mutations that we are now analyzing. Using several criteria (27C Daf-c phenotype, response to pheromone at 25C, and starvation induced dauer formation), we have shown that unc-3 and egl-4 are suppressed by daf-5 while unc-31 is not. Thus, 27C Daf-c mutants may identify genes acting at more than one point in the dauer genetic pathway. To determine whether the sensitivity of dauer formation to temperature seen in 27C Daf-c mutants reflects a wild-type sensitivity to this temperature range, we examined responses of N2 to exogenous dauer pheromone at 25 and 27C. At very low levels of pheromone that induced no dauers at 25C, nearly 100% dauers formed at 27C. Thus, wild-type worms are extremely sensitive to pheromone over this narrow temperature range. To determine why there is such sensitivity, we examined N2 growth and reproduction at temperatures between 25 and 30C. At 27C, growth is asynchronous, brood sizes are extremely small, and animals are pale and clearly unhealthy. At 28C, there is frequent larval arrest and all animals that do grow to adults are sterile. At 29 or 30C, all animals arrest as L1 larvae. Thus, the high sensitivity to dauer pheromone occurs at temperatures that are nearly lethal. N2 dauers induced by pheromone at ~27.5C had brood sizes of 180 if recovered at 15C, while dauers and nondauers from the same plate were completely sterile if kept at the high temperature. Thus, dauer formation at high temperatures can protect against sterility. While N2 is not strongly 27C Daf-c, it does form a low frequency (~5-10%) of dauers at 27C. These dauers are typically paler than dauers formed at lower temperatures, but are very thin and have several of the dauer features observable by Nomarski, such as the presence of alae and hypodermal bodies, and remodeling of the pharynx (not always complete). Furthermore, such 27C dauers have never been observed in daf-12 mutants, indicating that at least this step in the genetic pathway is shared with dauers formed at lower temperatures. Thus, N2 is weakly Daf-c at 27C without exogenous pheromone. To see if such dauer formation requires endogenously made pheromone, we looked at the daf-22(m130) mutant which does not produce pheromone. Surprisingly, daf-22 animals are as Daf-c at 27C as N2, suggesting that high temperature alone may be sufficient to induce dauer formation independent of pheromone. This contradicts the view that pheromone is both necessary and sufficient for dauer formation.