Abstract
Animals have internal clocks that generate biological rhythms. In mammals, clock genes such as Period form the circadian clock to generate approximately 24-h biological rhythms. In C. elegans, the clock gene homologs constitute the "developmental clock", which has an 8-h period during larval development to determine the timing of molting. Thus, the ancestral circadian clock has been believed to evolve into the oscillator with a shorter period in C. elegans. However, circadian rhythms have also been observed in adult C. elegans, albeit relatively weak. This prompts the question: if the clock gene homologs drive the developmental rhythm with 8-h period, which genes generate the circadian rhythms in C. elegans? In this study, we discovered that nhr-23, a homolog of the mammalian circadian clock gene Ror, is essential for circadian transcriptional rhythms in adult C. elegans. Interestingly, nhr-23 was also known to be essential for the molting clock. The bilaterian ancestral circadian clock genes might have evolved to function over multiple periods depending on developmental contexts rather than a single 8-h period in C. elegans.