Worm Breeder's Gazette 10(2): 42

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.

Sensory Neurons, Chemotaxis, and Dauer Formation

Cori Bargmann and Bob Horvitz

Figure 1

C.  elegans chemotaxes to Na+ and Cl- ions, and several amino acids (
Ward, PNAS 70:817 (1973)).  We are interested in studying this 
behavior at the cellular and genetic levels.
Single animals are tested in a chemotaxis assay similar to one 
described by Ward.  A gradient of an attractant is established by 
cutting a ~50  I (6 mM) plug out of a 2% agar plate, soaking the plug 
in attractant, replacing it, and allowing the attractant to diffuse 
for 12-24 hours.  A single animal is placed on the plate, removed an 
hour later, and its tracks observed.  An animal that arrives at the 
peak of  the gradient more times than it arrives at a control agar 
plug is scored as positive (see table below for the behavior of 
control animals).  Usually a single animal will be tested for its 
response to six attractants, though chemotaxis-defective animals are 
lost sometimes.  Anyone interested in this assay or similar population 
assays should call to get more details about things that seem to make 
it work well.
To define the cells involved in chemotaxis, we are killing neurons 
using a laser microbeam.  The cells are killed at the L1 stage, and 
the animals tested for their behavior as young adults (0-36 hours 
after the last molt).  The first cells we examined were the putative 
chemosensory neurons, i.e.  those with endings exposed to the 
environment.  There are nine classes of putative chemosensory cells in 
the head (22 cells total) in two types of sensillum, the amphid and 
the inner labial sensillum.  The phasmids, in the tail, were shown by 
Ward to be unnecessary for chemotaxis; we have confirmed his result by 
finding that lin-17 mutants, which have defective phasmids, are wild-
type for chemotaxis.
Preliminary experiments suggested that the amphid contained cells 
necessary for chemotaxis.  Exploratory kills within the amphid 
indicated that the only cell whose death reproducibly leads to (
partially) defective chemotaxis was ASE.  It appears that several 
other chemosensory cells (some or all of ADF, ASG, and ASI) are also 
involved in chemotaxis.  The data are summarized in the table below.
[See Figure 1]
Each entry in this table indicates the number of independent animals 
that gave a positive response (+), the number of animals that gave a 
negative response (-), and the number of animals that gave 
contradictory results in two or more assays with the same attractant (?
).  Thus for N2, 55 animals were tested for their response to cAMP in 
single animal assays, and 53 gave a positive response.  che-2(e1033) 
was used as a negative control.  All chemosensory neurons are grossly 
defective in this strain, so the fraction of positive responses should 
give a good estimate of the fraction of fake positive results that 
will come out of the assay as scored.  Biotin and serotonin (SHT) are 
'new' attractants that we identified in a screen of chemicals.
Given that the range of the assay is 12-29% positive for che-2 and 
83-96% positive for N2, the numbers for the three-cell-kills are too 
small to be conclusive in some cases.  However, the quadruple ASE ADF 
ASG ASI kill is very defective in its response to cAMP, biotin, Cl-, 
and Na+.  The data suggest that these responses are mediated at least 
in part by the same group of redundant cells (we are still testing 
alternative explanations of the data).  The apparent residual 5HT 
response of ASE- ADF- ASG- ASI- animals is being examined.
Ever mindful of the possible problems with laser experiments, we 
have confirmed many kills of subsets of the chemosensory neurons by an 
independent assay.  As described by Hedgecock et al., Dev.  Biol.  
111:158 (1985), six of the cells in each amphid take up FITC 
efficiently through their exposed endings.  This observation allows us 
to confirm that cells whose nuclei have been killed are functionally 
defective, i.e.  fail to take up FITC.
Killing the amphid sheath cell may be a means of reducing the 
function of most amphidial cells (see Thomas and Horvitz, WBG 10:1 p.
89 (1987)).  Amphid sheath-killed animals are partially defective in 
chemotaxis, but not so defective as animals in which ASE, ADF, ASG, 
and ASI are killed; for example, 7/12 amphid sheath-killed animals 
respond to Cl-, as compared with 4/19 E-F-G-I- animals.  FITC fills of 
animals in which the amphid sheath cells have been killed showed that 
about a third of those animals still contain 1-3 (out of 12) dye-
filling cells.  If only one or two of the eight cells identified above 
need be functional for chemotaxis, this observation probably explains 
the residual chemotactic responses present in sheath-killed animals.
In the course of these experiments, we noticed that when most of the 
amphid sensory cells are killed in a single animal (the best guess so 
far is the combination of ADF, ASG, ASI, ASJ, and ADL), the individual 
laser-ablated animals become dauer larvae constitutively on bacterial 
lawns at 20 C.  A few of these dauers have been maintained for a week 
or more; they never recover to form adults.  It may be that some or 
all of these five cells receive the food and pheromone signals that 
regulate dauer formation.  When ADF, ASG, ASI, ASJ, and ADL were 
killed in daf-10(e1387) animals, 2/4 animals developed into dauers on 
bacterial lawns.  This suggests that the dauer-defective lesion in daf-
10 is bypassed by these neuronal kills.
N2 animals in which the combination of ADF, ASG, and ASI were killed 
took about a day longer to grow to adulthood than other animals.  
These animals appear to go through an L2-dauer stage (Golden and 
Riddle, Dev.  Biol.  102:368 (1984)) instead of the normal L2 stage, 
suggesting a partial loss of food-sensing ability in these animals.

Figure 1