Biological rhythms are widespread in nature and regulate processes such as vertebrate and invertebrate heartbeats, and invertebrate swimming and gut peristalsis. Such rhythms are called ultradian since they have a period of less than 24 hours. The defecation cycle of C. elegans is an ultradian rhythm which is amenable to genetic analysis. Every 45 to 50 seconds, wild type C. elegans initiates a series of muscle contractions known as the defecation motor program. The first muscle contraction is the pBoc, or posterior body-wall muscle contraction. About three seconds later, the anterior body-wall muscles contract (aBoc), immediately followed by the enteric muscle contraction, which expels gut contents. Mutants have been identified that are lacking one or more of these steps; however, none of these have a large effect on cycle period. This indicates that the mechanism controlling the timing of the contractions is distinct from the motor program itself. The defecation rhythm shares several characteristics with other defined molecular clocks. First, it exhibits temperature compensation over a range of temperatures between 19-30 degrees Celsius. Second, it can be reset by an external stimulus. Specifically, light touch with an eyelash to the head region resets the phase of the clock. Finally, the clock can keep time in the absence of the defecation motor program. When wild type worms spontaneously leave a lawn of food, the contractions of the motor program cease, and on return to food the next defecation tends to be in phase with the original cycle (Liu and Thomas, 1994). Thirteen genes have been identified that can mutate to alter the length of the cycle. One of these Dec genes,
itr-1 , has been shown to encode the only inositol trisphosphate receptor in C. elegans . Loss-of-function mutations in
itr-1 cause the cycle period to be lengthened or eliminated, while overexpression of
itr-1 produces a short period. In addition, mosaic analysis has shown that ITR-1 function in intestinal cells is necessary and sufficient for the defecation rhythm (DalSanto et al , 1999). Four other Dec genes have been cloned:
unc-43 encodes the only CaMKII in C. elegans (Reiner et al , 1999),
flr-1 encodes a DEG/ENaC sodium channel, and
flr-3 and
flr-4 share homology with protein kinases (Take-Uchi et al , 1998). The molecular identification of other Dec genes may provide more information regarding the signaling pathways that control the timing of the clock. In addition, better characterization of the phenotypes and gene interactions among the Dec genes may allow inference of formal genetic pathways that regulate the clock. I have begun work towards identifying
dec-2.
dec-2 (
sa89) causes a defecation cycle period of ~100 seconds. I am also analyzing Dec mutant responses to light touch and reduced concentrations of food and constructing
dec-2 double mutants for gene interaction studies. Current progress on the cloning of
dec-2 and the phenotypic analysis will be presented. Dal Santo P, Logan MA, Chisholm AD, Jorgensen EM. (1999) The inositol trisphosphate receptor regulates a 50-second behavioral rhythm in C. elegans . Cell 98: 757-767. Liu DW, Thomas JH. (1994) Regulation of a periodic motor program in C. elegans . J Neurosci 14: 1953-62. Reiner DJ, Newton EM, Tian H, Thomas JH. (1999) Diverse behavioural defects caused by mutations in C. elegans
unc-43 CaM kinase II. Nature 402: 199-203. Take-Uchi M, Kawakami M, Ishihara T, Amano T, Kondo K, Katsura I. (1998) An ion channel of the degenerin/epithelial sodium channel superfamily controls the defecation rhythm in C. elegans . Proc Natl Acad Sci USA 95: 11775-80.