Time of day dependent changes in embryonic heart rate are detectable after maturation of rhythmic circadian gene expression in the eye, but before the heart in tadpoles cultured in LD.

We systematically characterized onset of expression of circadian genes in the embryonic eye and heart of tadpoles. We found that and ( were the first circadian genes to display significant 24-hour rhythms in the developing eye and heart in a 12-hour light-dark cycle (LD). Rhythmic expression of both oscillator and output genes were present in the eye by 2.75 days post fertilization (dpf), but not in 15 dpf hearts. Surprisingly, rhythmic oscillation of heart rate occurred after 3.2 dpf suggesting that heart rate may be controlled directly by light or indirectly by the pineal in LD.