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Background  Ultradian rhythms, rhythms with a period of less than 24 hours, are a widespread and fundamental aspect of life. The mechanisms underlying the control of such rhythms remain only partially understood. Defecation in  C. elegans  is a very tightly controlled rhythmic process. Underlying the defecation motor programme is an oscillator which functions in the intestinal cells of the animal. This mechanism includes periodic calcium release and subsequent intercellular calcium waves which in turn regulate the muscle contractions that make up the defecation motor programme. Here we investigate the role of TRPM cation channels in this process.    Results  We use RNA interference (RNAi) to perturb TRPM channel gene expression. We show that combined knock down of two of the TRPM encoding genes,  gon-2  and  gtl-1 , results in an increase in the variability of the cycle but no change in the mean, in normal culture conditions. By altering the mean using environmental (temperature) and genetic approaches we show that this increase in variability is separable from changes in the mean. We show that  gon-2  and  gtl-1  interact with components of the calcium signalling machinery ( itr-1  the  C. elegans  inositol 1,4,5-trisphosphate receptor) and with plasma membrane ion channels ( flr-1  and  kqt-3 ) which are known to regulate the defecation oscillator. Interactions with these genes result in changes to the mean period and variability. We also show that knocking down a putative transcription factor can suppress the increased variability caused by reduction of  gon-2  and  gtl-1  function. We also identify a previously unrecognised tendency of the defecation cycle to compensate for cycles with aberrant length by adjusting the length of the following cycle.    Conclusion  Thus TRPM channels regulate the variability of the defecation oscillator in  C. elegans . We conclude that the mean and the variability of the defecation oscillator are separable. Our results support the notion that there is a strong underlying pacemaker which is able to function independently of the observable defecation rhythm and is not perturbed by increases in the variability of the cycle.  The interaction of  gon-2  and  gtl-1  with other components of the oscillator shows that TRPM channels play an important role in the oscillator machinery. Such a role may be through either regulation of cation levels or membrane properties or both. Specifically our results support previous proposals that  gon-2  and  gtl-1  regulate IP 3  signalling and that  kqt-3  may act by altering calcium influx.  Our results provide novel insights into the properties of the defecation oscillator and thus to our understanding of ultradian rhythms.

TRPM channels are required for rhythmicity in the ultradian defecation rhythm of C. elegans