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.

Dauer formation and life at temperatures above 25C

Michael Ailion, James H. Thomas

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