Circadian clocks display remarkable reliability despite significant stochasticity in biomolecular reactions. We study the dynamics of a circadian clock-controlled gene at the individual cell level in Anabaena sp. PCC 7120, a multicellular filamentous cyanobacterium. We found significant synchronization and spatial coherence along filaments, clock coupling due to cell-cell communication, and gating of the cell cycle. Furthermore, we observed low-amplitude circadian oscillatory transcription of kai genes comprising the post-transcriptional core oscillatory circuit, and high-amplitude oscillations of rpaA coding for the master regulator transducing the core clock output. Transcriptional oscillations of rpaA suggest an additional level of regulation. A stochastic, one-dimensional toy model of coupled clock cores and their phosphorylation states 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.

Tooth replacement is a conserved process in vertebrates extending back to the ancesters of modern fish, amphibians, reptiles and mammals. The patterns in dinosaurs and reptiles have been described by scientists as regular waves going from posterior to anterior of the jaws where every other tooth is replaced. The patterns suggest that local and possibly jaw-wide communication betwen tooth families is involved. The tooth replacement process is not well understood since there are few animals with acessible dentitions. In this study we study leopard geckos in which tooth succession occurs repeatedly throughout life thus giving multiple opportunities to track the process. In addition we performed 2 types of experiments that unlaterally interfere with tooth replacement. Animals were followed using serial wax bites for 2-3 months prior to unilateral, selective tooth removal and then for 7-9 months after surgery. The data on tooth presence or absence was transformed mathematically to capture the time between eruption events of the successional tooth at the same position. Relationships between neighbouring tooth families and across the midline were measured to look for relative phase symmetry or asymmetry. We then tested 5 alternative models of tooth replacement to explain the observed patterns of recovery. The selective removal of unerupted second generation teeth showed that there were no signals passing back to the tooth forming field. The pattern of tooth eruption was not only re-established but it was in phase with the surrounding unperturbed tooth locations, as reflected in the nearest-neighbour diagonal lines. In animals that had permanent ablation of the dental lamina, the cycling of teeth anterior and posterior to the gap was unaffected refuting the presence of directional factors that pass from one tooth family to the next. Finally we compared the patterns of replacement between geckos and alligators using Edmund, 1962 data. The general nearest neighbour staggered patterns were conserved but the alligator had very slower rates of replacement in the posterior jaws. The geckos maintain similar cycling frequences across the jaws. In conclusion, we reject the Osborn model of zones of local inhibition and Edmunds wave-stimulus theory.Instead propose that teeth themselves are cycling and that these cycles regulate the staggered timing of tooth initiation. We also showed that once established, the local control of tooth replacement is very resilient to environmental perturbations as long as the dental epithelium is retained.

Circadian clock arrays in multicellular filaments of the heterocyst-forming cyanobacteriumAnabaenasp. strain PCC 7120 display remarkable spatio-temporal coherence under nitrogen-replete conditions. To shed light on the interplay between circadian clocks and the formation of developmental patterns, we followed the expression of a clock-controlled gene under nitrogen deprivation, at the level of individual cells. Our experiments showed that differentiation into heterocysts took place preferentially within a limited interval of the circadian clock cycle; that gene expression in different vegetative intervals along a developed filament was discoordinated; and that the circadian clock was active in individual heterocysts. Furthermore,Anabaenamutants lacking thekaiABCgenes encoding the circadian clock core components produced heterocysts but failed in diazotrophy. Therefore, genes related to some aspect of nitrogen fixation, rather than early or mid-heterocyst differentiation genes, are likely affected by the absence of the clock. A bioinformatics analysis supports the notion that RpaA may play a role as master regulator of clock outputs inAnabaena,the gating of differentiation by the circadian clock and the involvement of the clock in proper diazotrophic growth. Together, these results suggest that under nitrogen deficient conditions, the functional unit inAnabaenais reduced from a full filament under nitrogen-rich conditions, to the vegetative cell interval between heterocysts.

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