Transplanted Drosophila excretory tubules maintain circadian clock cyclingout of phase with the host

Circadian rhythms in behaviors and physiological processes are driven by co nserved molecular mechanisms involving the rhythmic expression of clock gen es in the brains of animals [I], The persistence of Similar molecular rhyth ms in peripheral tissues in vitro [2,3] suggests that these tissues contain self-sustained circadian clocks that may be linked to rhythmic physiologic al functions. It is not known how brain and peripheral clocks are organized into a synchronized timing system; however, it has been assumed that perip heral clocks submit to a master clock in the brain. To address this matter we examined the expression of two clock genes, period (per) and timeless (t im), in host and transplanted abdominal organs of Drosophila. We found that excretory organs in tissue culture display free-running, light-sensitive o scillations in per and tim gene activity indicating that they house self-su stained circadian clocks. To test for humoral factors, we monitored cycling of the TIM protein in excretory tubules transplanted into host flies entra ined to an opposite light-dark cycle. We show that the clock protein in the donor tubules cycled out of phase with that in the host tubules, indicatin g that different organs may cycle independently, despite sharing the same h ormonal milieu. We suggest that one way to achieve circadian coordination o f physiological sub-systems in higher animals may be through the direct ent rainment of light sensitive clocks by environmental signals.