Worm Breeder's Gazette 6(1): 25
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.
High population density influences dauer larva formation as demonstrated by growth of nematodes to high density either in liquid medium, on enriched agar plates or on egg white plates. Dauer larvae are produced under these conditions by wild-type animals prior to exhaustion of the food supply. Our recent work, based on the observations of Cassada and Russell (Develop. Biol. 46:326,175), has shown that the medium from such cultures contains a stable, nematode- produced substance of low molecular weight which both promotes dauer larva formation and inhibits exit from the dauer stage. We have named the substance DRIF, for dauer recovery inhibition factor. DRIF can be detected in starved or non-starved liquid culture media, and it can be extracted from worms washed off of non-starved NGM plates. A dauer-recovery bioassay for DRIF has been developed which allows its detection at a 1/120 dilution from starved liquid culture media. DRIF's effect on both dauer larva formation and recovery is in competition with the food supply and can be overridden by an abundance of food. E. coli cells in a concentration greater than 2x10+E9/ml overcome inhibition of dauer recovery by DRIF at starved media concentration. Since the bacteria do not degrade DRIF, we conclude that bacteria and DRIF provide competitive chemosensory signals. DRIF is produced by C. elegans var. Bristol and Bergerac, C. briggsae, and all of the dauer-defective mutants assayed so far. It cannot be found in liquid culture media which has not been inoculated with worms, nor can it be found in starved liquid cultures of Panagrellus redivivus. (Panagrellus does not make dauers.) DRIF has a molecular weight of less than 1000 daltons and has a net negative charge at pH7. It is nonvolatile and cannot be extracted with ether or chloroform:methanol (2:1). It is stable to various treatments including autoclaving, treatment with acid or base, and digestion with protease, DNAase, RNAase, and phosphodiesterase. We are in the process of purifying DRIF so that its structural and biological properties may be more thoroughly studied. Our hypothesis is that DRIF may be the environmental cue which triggers dauer formation in crowded cultures. Our liquid cultures reach a density more than 10-fold greater than that reached by populations on NGM plates. As reported by Peg Swanson in the last Newsletter, dauer-defective mutant phenotypes suggest that the response to starvation is at least partially distinct from the response to high population density. Some mutants, e.g. CB1377 daf-6, do not form dauers in liquid or on plates, while other dauer- defective mutants, e.g. CB1376 (daf-3), form many dauers when grown to starvation in liquid. Thus, CB1377 is blocked in both responses, while CB1376 is blocked in the starvation response, but nearly normal in the response to high population density. It is conceivable that different sensory neurons could mediate the responses to different stimuli. Several laboratories have observed independently that large numbers of dauers form in cultures grown on egg-white plates. Interestingly, worms grown on such plates 'phenocopy' dauer-constitutive mutants in that adults retain eggs, and the dauers formed are large (well-fed). Since chicken egg-white itself does not contain DRIF activity, we presume that dauer formation on egg plates is due to nematode DRIF production, perhaps combined with a declining food supply. It is difficult to determine when an egg white plate is 'starved' because the bacterial food is not visible in all that mess. The worms will not grow on sterile egg white plates.