Worm Breeder's Gazette 14(3): 52 (June 1, 1996)

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.

A screen for mutations eliminating posterior body contractions of the defecation cycle reveals a complex locus on LG V.

Paola Dal Santo, Erik M. Jorgensen

University of Utah Department of Biology Salt Lake City, UT 84112

The C. elegans defecation cycle is characterized by a series of three muscle contractions which occur roughly every 50 seconds. Each cycle is initiated by a contraction of the posterior body muscles (pBoc) which begins at the tip of the tail and is propagated anteriorly toward the vulva. Relaxation of these muscles is followed by contraction of the anterior body muscles (aBoc) in the head, and finally, by contraction of the enteric muscles and expulsion of intestinal contents (Exp). Neuronal input to muscles contracting during the aBoc and Exp steps have been identified by laser ablation experiments, while neurons regulating the pBoc remain unidentified. Ablations of neurons which synapse on posterior body muscles have no significant effect on this step of the cycle. Furthermore, mutants defective in neurotransmission or axon outgrowth also have a normal pBoc while the aBoc and Exp may be affected. Taken together, these data suggest that the posterior body con! tractions at the start of each defecation cycle are likely to be under control of a non-neuronal signal to the muscles.

Previous screens for general defecation defects identified three loci that could be mutated to specifically eliminate the pBoc. We screened for additional pBoc defects by mutagenizing a strain carrying the dominant allele, n2331, which confers a cramped phenotype. In this mutant, posterior body muscles contract, but fail to relax, and expulsion occurs while posterior muscles are still contracted. We expected that mutations in genes required for transmitting, receiving, or propagating the signal would result in absence of posterior muscle contractions altogether (a Pbo phenotype) and therefore, suppress the cramp phenotype. Approximately 9000 genomes were screened, and 20 independent, recessive mutants were selected in the F2 generation. Surprisingly, all 20 mutants isolated from this screen, as well as n2331, are tightly linked and appear to be alleles of a single locus on chromosome V. We did not recover any wild type revertants of n2331, no! r did we isolate any additional alleles of the other two loci.

The mutants can be divided into four phenotypic classes. The largest class exhibits a strong Pbo phenotype as a homozygote, but expresses the dominant cramp phenotype as a heterozygote. Homozygous mutants for the second class also have a strong Pbo phenotype, however, these mutations eliminate the cramp defect and have a wild type phenotype as heterozygotes. The third class of mutants have an incomplete pBoc, that is, only the tip of the tail contracts, while the fourth class has a complete, but weak pBoc which may be made more evident due to the n2331 mutation in the background. Results from complementation experiments between the various mutations indicate that this region can be separated into four functional domains which are not correlated with the four phenotypic classes. Individual mutations can abolish either one or two of the functions, but not more. The complex complementation pattern suggests that this locus may consist of several open reading ! frames comprising a polycistronic locus, a gene with complex splicing, or a single open reading frame that encodes a multifunctional protein.