In zebrafish, the circadian clock, which is the internal timekeeper that coordinates multiple cellular, physiological and behavioural processes with the external rhythmic environment, begins cycling very early in development. However, the functional relevance for embryonic and larval development of these early circadian oscillations is unclear. Here (p. ), Ricardo Laranjeiro and David Whitmore find that a number of important developmental regulators show rhythmic expression in a manner consistent with circadian regulation. In particular, they uncover strong circadian expression of the neural transcription factor Neurod, whose levels oscillate specifically in the photoreceptor layer of the retina. They further show that a number of other key regulators of retinal photoreceptor differentiation oscillate, but only after differentiation itself is essentially complete, implying that this rhythmic expression is unrelated to the known functions of these factors in cell fate specification. Instead, the authors propose that certain components of the phototransduction pathway – which also show cyclic expression – may be controlled by these developmental transcriptional regulators, suggesting an intriguing interplay between the circadian clock and key regulators of retinal differentiation and function.
