Worm Breeder's Gazette 4(1): 22

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.

Egg-laying by C. elegans as a Model Behavioral System

B. Horvitz, J. Sulston, J. White

Egg-laying by C.  elegans has a variety of features that make it an 
attractive system with which to study the development and functioning 
of a simple behavioral network.  First, egg-laying is a quantitative 
behavior, making it easy to assay.  Second, the egg-laying system is 
anatomically simple, involving only 16 muscle cells and about 20 
neurons.  Third, the 'wiring diagram' has been established by 
reconstruction from serial section electron micrographs.  Fourth, the 
cell lineages that generate most of the egg-laying system are rigidly 
determined and precisely known.  Fifth, genetic studies are 
straightforward: egg-laying is a nonessential behavior, and both egg-
laying deficient mutants and revertants of such mutants can be 
obtained by selections.  Sixth, egg-laying is influenced by 
environmental stimuli, such as food and touch, allowing one to examine 
how this behavior is modulated.
Anatomical studies indicate that the vulva, 8 vulval muscles, 8 
uterine muscles, 6 Class C ventral cord neurons, and 2 hermaphrodite-
specific neurons appear likely to be involved in egg-laying.  
Elimination of these elements, either by mutation (see Newsletter Vol. 
1 No.  2 for how egg-laying deficient mutants have been isolated) or 
by using a laser microbeam, confirms the involvement of all of them 
except the Class C neurons.  Destruction of the Class C neurons with a 
laser does not inhibit egg-laying.  Since the Class C neurons 
innervate the vulval muscles, it appears that they have a modulatory, 
possibly inhibitory, role in egg-laying.
Many pharmacological agents have been tested for possible effects on 
egg-laying.  A variety of nicotinic (e.g., nicotine and arecoline) but 
not muscarinic (e.g., methacholine and bethanachol) acetylcholine 
agonists stimulate egg-laying, suggesting that a nicotinic 
acetylcholine receptor may be involved.  Serotonin (5-
hydroxytryptamine) and related compounds (5-hydroxytryptophan and 5-
methoxytryptamine) also stimulate egg-laying.  Octopamine (p-
hydroxyphenylethanolamine) appears to inhibit egg-laying.
Mutants or laser-ablated animals defective in egg-laying have been 
tested for response to 12 drugs that stimulate egg-laying by normal 
hermaphrodites.  Such experiments indicate whether each of these drugs 
acts before or after the egg-laying lesion in the animal tested.  
Using this approach, we hope that it will prove possible to deduce a 
pathway for egg-laying that is consistent with the known neuroanatomy.