Worm Breeder's Gazette 12(3): 10 (June 15, 1992)
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.
In intact worms, about one out of every three pumps is followed by a peristaltic contraction of the posterior isthmus muscles. If the pharyngeal motor neuron M4 is killed the muscles of the posterior isthmus don't contract, and as a result no bacteria are swallowed, although pharyngeal pumping continues (Avery and Horvitz, 1987, Cell 51 1071). I suspect that two signals are normally required for posterior isthmus contraction: membrane depolarization, which happens during every pump, and an unknown second messenger, which is activated by the M4 neurotransmitter. If this model is true, it might be possible to overcome the lack of M4 by increasing the time during which the muscle membrane remains depolarized.We have five relaxation-defective mutations, which cause pumps to last longer than normal, perhaps because the muscle membrane is unable to return to resting potential. In the strongest of these, eat-6 ( ad467 ),pharyngeal muscles may stay contracted for over a minute, instead of the normal 170 msec. I therefore killed M4 in 17 eat-6 worms. Although they grew more slowly than unoperated eat-6 worms, 2/17 M4 - eat-6 worms became fertile adults.
This result suggests that it might be possible to identify genes involved in the control of pharyngeal muscle contraction by looking for mutants able to survive without M4 .I call this the Lib screen, short for "Liberated isthmus" (thanks to Cori Bargmann for the name). To see how feasible such a screen would be, I did a pilot Lib screen. I killed M4 in 679 newly hatched F2 progeny of EMS-mutagenized hermaphrodites, and looked for worms that became fertile adults. The viability of unoperated larvae was 62% (31/50), so the effective number of F2 sscreened was about 400. Strains established by six worms that reached adulthood were retested for survival without M4 .The overall survival rate was 4% (4/103), and none of the mutants had a survival rate of greater than 10%. While this is clearly better than wild-type (<1%), it's still not very impressive. A seventh mutant, although too infertile to test for Lib, proved to carry a new eat-6 mutation, ad 792, that produces a phenotype very similar to that of ad467 .
How well did the Lib screen work? On the positive side, it was not difficult. I spent only four mornings killing M4 s.I believe a screen ten times as large would be entirely practical. Furthermore, I isolated a new relaxation-defective mutation; this is only the second strong one we have. On the negative side, it seems likely that there are many mutations that produce a low-penetrance Lib phenotype. Wlthout a secondary screen to eliminate these, the Lib screen would not be useful.