Synchronization and spatial coherence of noisy circadian clocks in a multicellular1-d organism
The collective behavior of oscillators is a venerable subject in Physics since Huygens’ seminal contributions. Living systems, from simple unicellular bacteria to multicellular plants and mammals also display oscillatory dynamics, the most conspicuous of which are circadian rhythms, coupling the biology of these organisms to day/night cycles on Earth.
While considerable headway has been made in understanding the behavior of individual circadian clocks and their molecular components, the behavior of a large collection of clocks is still poorly understood, constituting a fertile ground of inquiry.
We studied at the single-cell level the collective behavior of one-dimensional arrays of clocks in Anabaena, a cyanobacterial organism of ancient origin, as a model system. Anabaena filaments display remarkable synchrony and spatial coherence at the organismal scale, despite considerable and yet inevitable fluctuations in each cell –demographic noise-, stemming from the stochastic nature of biochemical reactions. Furthermore, we provide experimental evidence supporting the notion that spatio-temporal coherence is largely due to the coupling of clocks by cell-cell communication, and that the clock controls other cellular processes such as cell division. A stochastic, one-dimensional toy model of coupled clocks shows that demographic noise can seed stochastic oscillations outside the region where deterministic limit cycles with circadian periods occur. The model reproduces the observed spatio-temporal coherence along filaments and provides a robust description of coupled circadian clocks in a multicellular organism.