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.

A Few Notes on Dauers

J.W. Golden, D.L. Riddle

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.