Worm Breeder's Gazette 7(2): 60
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.
To be or not to be (a dauer)? We previously reported that the decision between formation of an L3 or a dauer is determined by the relative concentrations of a Caenorhabditis-specific, fatty acid-like pheromone and a competitive 'food-signal.' Dauer pheromone shift experiments were performed by starting with synchronized L1's and shifting into or out of pheromone at various points in the life cycle ( worms are always on a lawn of OP50). Commitment to the dauer occurs a few hours before the L2 molt when removal of the worms from pheromone fails to rescue them from entering the dauer stage. The committed worms enter the L2-dauer molt and progress through at least the dauer- shrinkage stage before recovering. Larvae grown in the absence of added pheromone become committed to the L3 by the end of the L1 molt, after which dauer larva formation cannot be induced by a shift into pheromone. Growth in the presence of pheromone causes a specific 6 hour (at 25 C) elongation of the L2 intermolt which, along with subtle morphological differences, indicates that an L2 destined to become a dauer is undergoing a distinct developmental program. N2 dauer formation is temperature-dependent. Pheromone-induced dauer formation of N2 on a lawn of OP50 is enhanced at higher temperatures in the normal growth range with a transition centered around 22.5 C. Temperature-shift experiments of synchronized populations show transitions of both up-shifts and down-shifts centered at the L1 molt. This suggests that temperatures above 22 C are an environmental cue favoring dauer formation, or that the decision to enter the dauer developmental pathway has a temperature- dependent component. Dauer recovery in the presence of pheromone is also temperature-dependent; 85% recovery was induced by a 24 hour temperature down-pulse (25 C to 15 C ). Putative null alleles with a temperature-sensitive phenotype. The information given above along with other evidence suggested to us that some of our ts dauer-constitutive mutants might be null alleles which uncover and enhance the wild-type ts pheromone response. All of the dauer-constitutives, with the possible exception of daf-2, are hyperresponsive to the dauer pheromone. The ts phenotype of many of the dauer-constitutives closely resembles the ts characteristics of N2 dauer formation. One hypothesis explaining these data is that some of the dauer constitutives are unable to smell 'food' and, therefore, overrespond to pheromone, and this defect could be the result of null alleles. We have tested all of the dauer-constitutive reference alleles and all alleles of daf-4 and daf-7 for suppression by sup-7( st5). We found 2 ts alleles, daf-4(m72) and daf-7(m62), which are suppressed. No non-ts alleles of daf-4 or daf-7 have been found. A mutant which fails to secrete the dauer pheromone. Mutant DR476, daf-22(m130)II produces the pheromone, but fails to secrete it. The mutant was obtained by assaying medium from F2 clones for the absence of pheromone. Pheromone concentration in liquid cultures of the mutant is less than 1% of N2 levels up to and including the day of starvation, but then increases to near wild-type levels a few days after starvation, presumably because of worm Iysis. DR476 obtained from nonstarved plates was homogenized and found to contain an amount of pheromone similar to N2. Pheromone bioassays of single worms were used to show that the mutation is recessive. In both liquid and plate cultures the mutant is dauer-defective, producing only a few dauers which appear later than normal. While the dauer-defective phenotype of DR476 is completely reversed by the addition of exogenous pheromone, no other dauer-defective can be induced to form dauers in this way. The dauer-pheromone is not a single compound. Thin-layer chromatography of pheromone, partially purified by organic extraction of starved liquid medium followed by chromatography of the extract on DEAE-cellulose and silicic acid columns, consistently results in a 'streak' of activity. However when portions of the streak are rerun in the same solvent system they run at their original Rf without restreaking. The activity of different fractions is additive without synergistic effects. Hypotheses which would explain the heterogeneity are (1) that the pheromone may be a mixture of variable length fatty- acids modified in the same way or (2) variably modified forms of a fatty acid may have roughly equivalent biological activities.