Endogenous circadian clocks are poorly understood within early-diverging animal lineages. We have characterized circadian behavioral patterns and identified potential components of the circadian clock in the starlet sea anemone, Nematostella vectensis: a model cnidarian which lacks algal symbionts. Using automatic video tracking we showed that Nematostella exhibits rhythmic circadian locomotor activity, which is persistent in constant dark, shifted or disrupted by external dark/light cues and maintained the same rate at two different temperatures. This activity was inhibited by a casein kinase 1δ/ε inhibitor, suggesting a role for CK1 homologue(s) in Nematostella clock. Using high-throughput sequencing we profiled Nematostella transcriptomes over 48 hours under a light-dark cycle. We identified 180 Nematostella diurnally-oscillated transcripts and compared them with previously established databases of adult and larvae of the symbiotic coral Acropora millepora, revealing both shared homologues and unique rhythmic genes. Taken together, this study further establishes Nematostella as a non-symbiotic model organism to study circadian rhythms and increases our understanding about the fundamental elements of circadian regulation and their evolution within the Metazoa

Summary The mechanisms that sustain stem cells are fundamental to the maintenance of tissues/organs. Here we identify ‘cell-islands’ (CIs) as a niche for putative germ and somatic stem cells in Botryllus schlosseri, a colonial chordate that undergoes weekly cycles of death and regeneration. Cells within CIs express markers associated with germ and somatic stem cells and gene products that implicate CIs as signaling centers for stem cells. Transplantation of CIs induced long-term germ-line and somatic chimerism, demonstrating self-renewal and pluripotency of CI-cells. Cell labeling and in-vivo time-lapse imaging of CI-cells reveal waves of migrations from degrading CIs, into developing buds, contributing to soma and germ-line development. Knockdown of cadherin, which is highly expressed within CIs, elicited the migration of CI-cells to circulation. Piwi-knockdown resulted in regeneration arrest. We suggest that repeated trafficking of stem cells allow them to escape the constraints imposed by the niche, thereby promoting their self-preservation throughout life.

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