Worm Breeder's Gazette 10(3): 167

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.

Osmotic Avoidance Mutants with Normal Sensory Endings

Jim Thomas and Bob Horvitz

Figure 1

The amphid cell ASH is the primary sensory cell for the avoidance 
response to high osmotic strength and other avoidance stimuli (WBG 10-
1, p.89 and WBG 10-2, p.41).  Many mutants defective in avoiding high 
osmotic strength (Osm) have been identified in the past.  However, in 
all of these mutants many different sensory neurons (including ASH) 
have structurally abnormal sensory endings (Perkins et al., Dev.  Biol.
117: 456).  Nearly all of these sensory ending mutants share three 
other phenotypes: they are dauer defective, chemotaxis defective, and 
their amphid neurons fail to fill with the dye FITC.  No mutants 
specifically defective in the avoidance response were known.
I set out to find such avoidance-specific mutants.  To counterscreen 
against general sensory ending mutants, I took advantage of the dauer 
defective and FITC filling defective phenotypes shared by the sensory 
ending mutants.  I reasoned that mutations causing defective Osm 
avoidance would not necessarily cause these other defects.  The F2 
animals from an EMS mutagenesis were screened for Osm mutants by 
picking animals that crossed a ring of high osmotic strength.  The 
nineteen candidates whose progeny retested Osm were scored for filling 
with FITC and for dauer formation.  Any mutant that was normal for 
either phenotype was analyzed further.  Twelve of the nineteen Osm 
mutants proved to be normal for FITC filling or dauer formation (with 
one exception, see below, all twelve were normal for both).  Of these 
twelve, half were rather weak, showing reduced reliability or slow 
responsiveness to the Osm ring.  Only the six strong Osm mutants, plus 
two others that seem interesting because of their mutant pleiotropies, 
are discussed here.  The phenotypes of each of these mutants is listed 
in the table, along with those of the sensory ending defective osm-3 
mutant for comparison.
There are some interesting general points.  1) The amphidial sensory 
endings in these mutants must be present and are probably normal, 
because it is through these endings that the neurons fill with FITC.  
2) Chemotaxis, which requires amphid neurons other than ASH, is normal 
or mostly normal in all of these mutants except osm-7(n1515) (C.  
Bargmann, personal communication; Bargmann and Horvitz, WBG 10-2, p.42)
.  3) The mutants can be classified into those that affect only 
osmotic avoidance and those that affect additional avoidance responses.
We propose that the response to various avoidance stimuli involves 
some upstream steps that are specific to each stimulus (eg.  
chemoreceptors), and some downstream steps that are shared by the 
various avoidance responses (eg.  synapse formation by ASH).  4) Many 
of the Osm mutations have tightly linked nervous system defective 
pleiotropies, supporting the notion that these mutants are affected in 
nervous functions.
Two of the mutations deserve special comment.  n1600 is unusual in a 
number of ways.  It is the only mutation that causes a defect in 
osmotic, avoidance factor, and garlic avoidance.  It also causes a 
slightly kinky jerky backward Unc phenotype.  The backward Unc looks 
similar regardless of the stimulus that activates backward movement - 
avoidance response, head touch, heat, or spontaneous.  These facts 
suggest that the defect caused by n1600 is in the activation of 
backward movement, rather than in the Osm sensory cells.  n1600 maps 
on chromosome III covered by sDp3, so it's focus of action can be 
assessed by mosaic analysis using ncl-1.  
ftc-1(n1540) (FITC filling defective, pronounced fitssy) is a very 
weak Osm defective mutant, but has a unique FITC-filling pattern.  Of 
the 8 classes of neurons that normally FITC fill in the wild-type, 
only one, ADF, fills in n1540 animals.  ADF fills nearly all of the 
time, and to the normal staining intensity, while the other neurons (
including ASH) are invariably fully defective.  This result indicates 
that the machinery necessary for filling with FITC is still intact, 
and the seven classes of FITC-filling cells other than ADF are 
specifically altered by n1540.  Not all of the n1540 FITC-filling 
defective cells can be functionally defective, since chemotaxis to 
most compounds is normal in n1540 and requires more than ADF (C.  
Bargmann, personal communication).
[See Figure 1]

Figure 1