ABSTRACT
Photoperiodic responses are observed in many organisms living in the temperate zones. The circadian clock is involved in photoperiodic time measurement; however, the underlying molecular mechanism for detection of the day length remains unknown. We previously compared the expression profiles of the Cryptochrome(Cry) genes in the zebrafish eye and reported that Cry1ab has a double peak with variable expression duration depending on the photoperiod. In this study, to understand commonalities and differences in the photoperiodic responses of ocular Cry genes, we identified Cryptochrome genes in two other teleost species, goldfish and medaka, living in temperate zones, and measured ocular Cry mRNA levels in all of the three species, under different photoperiods (long-day [14 h light: 10 h dark] and short-day [10 h light: 14 h dark] and in constant darkness. Cry1ab mRNA levels did not show dual peaks in goldfish or medaka under the examined conditions; however, the mRNA expression profiles of many Crys were altered in all three species, depending on the day length and light condition. Based on their expression profiles, Cry mRNA peaks were classified into three groups that better synchronize to sunrise (light-on), midnight/midday (middle points of the dark/light periods), or sunset (light-off). These results suggest the presence of multiple oscillators that oscillate independently or a complex oscillator in which Cry expression cycles change in a photoperiod-dependent manner in the eye.
ABSTRACT
The zebrafish (Danio rerio) is a model species that is used to study the circadian clock. It possesses light-entrainable circadian clocks in both central and peripheral tissues, and its core circadian factor cryptochromes (CRYs) have diverged significantly during evolution. In order to elucidate the functional diversity and involvement of CRYs in photoperiodic mechanisms, we investigated the daily expression profiles of six Cry transcripts in central (brain and eye) and peripheral (fin, skin and muscle) tissues. The zCry genes exhibited gene-specific diurnal conserved variations, and were divided into morning and evening groups. Notably, zCry1ab exhibited biphasic expression profiles in the eye, with peaks in the morning and evening. Comparing ocular zCry1ab expression in different photoperiods (18L:6D, 14L:10D, 10L:14D and 6L:18D) revealed that zCry1ab expression duration changed depending on the photoperiod: it increased at midnight and peaked before lights off. zCry1ab expression in constant light or dark after entrainment under long- or short-day conditions suggested that the evening clock and photic input pathway are involved in photoperiod-dependent zCry1ab expression. Laser microdissection followed by qRT-PCR analysis showed that the evening peak of zCry1ab was likely ascribed to visual photoreceptors. These results suggest the presence of an eye-specific photoperiodic time measurement served by zCry1ab.
