C. elegans defecation is a rhythmic behavior. With abundant food a defecation motor program is activated every 45 seconds. This motor program consists of a stereotyped series of three muscle contractions, called pBoc, aBoc, and Exp. Various evidence indicates that this rhythm is controlled by a biological clock. Intriguingly, the rhythm is nearly constant at temperatures from 19C to 30C (temperature-compensated). In order to investigate how the defecation cycle is genetically controlled, we screened approximately 3,000 mutagenized haploid genomes and reexamined existing mutants. We identified 12 genes that can mutate to cause abnormal defecation cycle periods. Mutations in these genes fall into two major groups, short cycle (Dec-s) and long cycle (Dec-l). Detailed characterization of these mutants revealed the following points. 1) Most Dec mutations affected the cycle period quite differently at 20C and 25C. Because of their high frequency of isolation, we think most of the mutations affect temperature compensation rather than causing thermolabile gene products. 2) Mutations in the
flr-1,
flr-3, and
flr-4 genes (Katsura, et. al.Genetics 136 145) showed a very short mean cycle period. Short cycle period correlated strongly with weakened pBoc and Exp motor steps, suggesting depletion of a factor by a high frequency of motor program activation. 3) Dec-s mutations in
dec-9 and
dec-10 were dominant. These mutations were gain-of-function based on heterozygous deficiency phenotypes. 4) All but 2 Dec mutations did not affect internal timing of the motor program. Mutations in two Dec-l genes,
dec-2 and
dec-4, lengthened the interval between pBoc and Exp steps. We are currently building double mutants among the Dec mutations in order to construct a genetic regulatory pathway for the clock.