Worm Breeder's Gazette 12(5): 64 (February 1, 1993)
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.
Pharyngeal muscle relaxation is normally very fast: in the corpus, it is probably complete in less than 20 msec. The speed of relaxation is regulated by the pharyngeal nervous system, and if relaxation isn't fast enough, bacteria aren't efficiently trapped and transported. Muscarinic acetylcholine agonists and mutations in the genes eat-4 , eat-6 ,and eat-12 slow and inhibit pharyngeal relaxation. In the hope of identifying more genes involved in the control of relaxation, we tested 14 feeding-defective mutants for hypersensitivity to the agonists arecoline. One of them, eat-11 ( ads4 )I, was hypersensitive: it was killed by 1 mM arecoline, a concentration that barely affects wild type. In the absence of drugs, eat-11 produces only a mild phenotype: the corpus is weakly defective in transporting bacteria. (Under some conditions, however, some eat-11 larvae arrest at L1 ,an observation whose significance we are unsure of.) eat-11 worms are also slightly longer than normal, and they have slightly abnormal movement: bends are deeper than normal.
To identify more genes that affect the response of muscle to acetylcholine, we isolated about 20 eat-11 suppressors by selecting for growth of F2 self-progeny of mutagenized hermaphrodites on 5 mM arecoline. Our analysis has concentrated on the seven strongest, all of which are dominant suppressors of eat-11 arecoline hypersensitivity and semi-dominant suppressors of the visible phenotypes. In addition, they produce phenotypes of their own. All turn out to be gain-of-function mutations in the gene egl-30 . egl-30 (gf)mutations arose at frequencies of one in a few thousand. The following table summarizes our observations and those of Trent et al (Genetics 104: 619) and Park and Horvitz (Genetics 113: 821):
The phenotypes of the gain-of-function alleles are consistent with a defect in egg-laying, pharyngeal, and body wall muscles, weakest in ad803 and ad806 ,and strongest in ad810 .The flaccid paralysis phenotype of ad805 , ad809 , ad814 ,and surviving n715 homozygotes is very similar to that of unc-54 or unc-15 mutants. ad810 and ad813 arrest growth at hatching, and have no ( ad810 )or very feeble ( ad813 )body wall, defecation, and pharyngeal muscle movements. They are flaccid, unlike lethals such as unc-104 or cha-1 that lack nervous system function.
Two observations suggest that the gain-of-function phenotype is not caused by a defect in muscle contractile structures per se. First, no other muscle-defective mutations were isolated as eat-11 suppressors. Since such mutations are common, egl-30 (gf)mutations must have some effect other than the reduction of muscle contraction. Second, the early, nervous system independent embryonic movement that occurs during elongation is normal in ad810 embryos, unlike mutants such as myo-3 that lack all body wall muscle contraction.
A model that reconciles all these observations is that egl-30 (gf)mutations block a pathway by which muscles respond to acetylcholine, e.g. by preventing membrane excitation. our guesses as to eat-11 'srole are even vaguer: one possibility is that it is necessary for desensitization of the response to acetylcholine. We may be able to test these ideas for pharyngeal muscle by measuring EPGs in egl-30 and eat-11 mutants (see Raizen and Avery, this WBG).