Analysis of photoentrainment in the diurnal zebrafish suggests a profound divergence with nocturnal rodents

Abstract The eye is instrumental for controlling circadian rhythms in nocturnal mammals. Here, we address the conservation of this function in the zebrafish, a diurnal vertebrate. Using lakritz (lak) mutant larvae, which lack retinal ganglion cells (RGCs), we show that while a functional eye is required for a phenomenon known as masking, it is largely dispensable for the establishment of circadian rhythms of locomotor activity. Furthermore, the eye is dispensable for the induction of a phase delay following a pulse of white light at CT 16 but contributes to the induction of a phase advance upon a pulse of white light at CT21. Melanopsin photopigments are important mediators of photoentrainment in nocturnal mammals. One of the zebrafish melanopsin genes, opn4xa, is expressed in RGCs but also in photosensitive projection neurons in the pineal gland. To address the role of this photopigment, we generated an opn4xa mutant. Abrogating opn4xa has no effect on masking and circadian rhythms of locomotor activity, or for the induction of phase shifts, but is required for period length control when larvae are subjected to constant light. Finally, analysis of opn4xa;lak double mutant larvae did not reveal redundancy between the function of the eye and Opn4xa in the pineal for photoentrainment or phase shifts after light pulses. Our results challenge the dogma that the eye as the sole mediator of light influences on circadian rhythms and highlight profound differences in the circadian system and photoentrainment between different animal models. Significance statement The eye in general and melanopsin expressing cells in particular are crucial for circadian rhythms in nocturnal mammals, most notably during photoentrainment, by which circadian rhythms adapt to a changing light environment. In marked contrast to this, we show that in the diurnal zebrafish the eye and photosensitivity dependent on the melanopsin gene opn4xa, which expressed in both the eye and the pineal gland, are largely dispensable for correct circadian rhythms. These results provide the first insight that the light sensors orchestrating circadian rhythms are different between animal models raising the intriguing possibility that vertebrates might employ different molecular/cellular circuits for photoentrainment depending on their phylogeny and/or temporal niche