Light-dependent conversion of body color change pattern during zebrafish development

Zebrafish (Danio rerio) undergoes a change in body color in response to light exposure. This color change phenomenon was first observed in our previous study, where it was found that larvae as young as 5 days post-fertilization (dpf) had fully developed this ability, while 2-dpf larvae exhibited the opposite effect - light-induced darkening (Shiraki et al. 2010).

Our latest research has revealed that the change in body color change pattern between 2 dpf and 5 dpf is dependent on light exposure. Larvae reared in darkness initially displayed the light-induced darkening characteristic of 2-dpf larvae. Moreover, we discovered that just a 14-hour light exposure on either 3 or 4 dpf is enough to trigger this conversion.

Interestingly, even after ablating most exorh-expressing pineal photoreceptor cells, the conversion still occurred in larvae that were exposed to light on 4 dpf. Dark-reared larvae had significantly lower levels of transcription of pmch and pmchl, the genes that encode melanin-concentrating hormone precursors, compared to larvae that were reared in light (LD-reared).

However, after the conversion occurred (in 3-L and 4-L larvae), the levels of these genes were not significantly different from those in dark-reared larvae, suggesting that the reduced pmch/pmchl expression alone cannot explain the conversion.

In summary, our findings indicate that light exposure during rearing is crucial for the development of the ability in larval zebrafish to change body color, through a regulatory mechanism that does not rely on exorh-expressing pineal photoreceptors.