Sporadic feeding regulates robust food entrainable circadian clocks in blind cavefish.

The circadian clock represents a key timing system entrained by various periodic signals that ensure synchronization with the environment. Many investigations have pointed to the existence of two distinct circadian oscillators: one regulated by the light-dark cycle and the other set by feeding time. Blind cavefish have evolved under extreme conditions where they completely lack light exposure and experience food deprivation. Here, we have investigated feeding regulated clocks in two cavefish species, the Somalian cavefish Phreatichthys andruzzii and the Mexican cavefish Astyanax mexicanus, in comparison with the surface-dwelling zebrafish Danio rerio. Our results reveal that feeding represents an extremely strong synchronizer for circadian locomotor rhythmicity in subterranean cavefish. Indeed, we showed that consuming just one meal every 4 days is sufficient to entrain circadian rhythmicity in both cavefish species, but not in zebrafish. These profound adaptations to an extreme environment provide insight into the connections between feeding and circadian clocks.

Encephalic photoreception and phototactic response in the troglobiont Somalian blind cavefish Phreatichthys andruzzii.

Many physiological and behavioural responses to changes in environmental lighting conditions are mediated by extraocular photoreceptors. Here we investigate encephalic photoreception in Phreatichthys andruzzii, a typical cave-dwelling fish showing an extreme phenotype with complete anophthalmy and a reduction in size of associated brain structures. We firstly identified two P. andruzzii photopigments, orthologues of rod opsin and exo-rod opsin. In vitro, both opsins serve as light-absorbing photopigments with λ(max) around 500 nm when reconstituted with an A(1) chromophore. When corrected for the summed absorption from the skin and skull, the spectral sensitivity profiles shifted to longer wavelengths (rod opsin: 521 nm; exo-rod opsin: 520 nm). We next explored the involvement of both opsins in the negative phototaxis reported for this species. A comparison of the spectral sensitivity of the photophobic response with the putative A(2) absorbance spectra corrected for skin/skull absorbance indicates that the A(2) versions of either or both of these pigments could explain the observed behavioural spectral sensitivity.

Daily Rhythms of the Expression of Key Genes Involved in Steroidogenesis and Gonadal Function in Zebrafish.

Fish present daily and seasonal rhythms in spawning and plasmatic levels of steroids that control reproduction. However, the existence of the rhythms of expression of the genes that underlie the endocrine mechanisms responsible for processes such as steroidogenesis and reproduction in fish have still been poorly explored to date. Here we investigated the daily pattern of the expression of key genes involved in sex steroid production that ultimately set the sex ratio in fish. Adult zebrafish were maintained under a 12:12 h light-dark cycle at a constant temperature of 27°C and were sampled every 4 h during a 24-hour cycle. The expression of key genes in the gonads and brains of female and male individuals were analyzed. In gonads, the expression of aromatase (cyp19a1a, ovarian aromatase) and the antimüllerian hormone (amh, testis) was rhythmic, with almost opposite acrophases: ZT 5:13 h (in the light phase) and ZT 15:39 h (at night), respectively. The expression of foxl2 (forkhead box L2) was also rhythmic in the ovary (acrophase located at ZT 5:02 h) and the expression of dmrt1 (doublesex and mab-3-related transcription factor 1) was rhythmic in testes (acrophase at ZT 18:36 h). In the brain, cyp19a1b (brain aromatase) and cyp11b (11beta-hydroxylase) presented daily differences, especially in males, where the expression peaked at night. These results provide the first evidence for marked time-of-the-day-dependent differences in the expression of the genes involved in sex ratio control, which should be considered when investigating processes such as reproduction, sex differentiation and steroidogenesis in fish.

The Light Wavelength Affects the Ontogeny of Clock Gene Expression and Activity Rhythms in Zebrafish Larvae.

Light plays a key role in synchronizing rhythms and setting the phase of early development. However, to date, little is known about the impact of light wavelengths during the ontogeny of the molecular clock and the behavioural rhythmicity. The aim of this research was to determine the effect of light of different wavelengths (white, blue and red) on the onset of locomotor activity and clock gene (per1b, per2, clock1, bmal1 and dbp) expression rhythms. For this purpose, 4 groups of zebrafish embryo/larvae were raised from 0 to 7 days post-fertilization (dpf) under the following lighting conditions: three groups maintained under light:dark (LD) cycles with white (full visible spectrum, LDW), blue (LDB), or red light (LDR), and one group raised under constant darkness (DD). The results showed that lighting conditions influenced activity rhythms. Larvae were arrhythmic under DD, while under LD cycles they developed wavelength-dependent daily activity rhythms which appeared earlier under LDB (4 dpf) than under LDW or LDR (5 dpf). The results also revealed that development and lighting conditions influenced clock gene expression. While clock1 rhythmic expression appeared in all lighting conditions at 7 dpf, per1b, per2 and dbp showed daily variations already at 3 dpf. Curiously, bmal1 showed consistent rhythmic expression from embryonic stage (0 dpf). Summarizing, the data revealed that daily rhythms appeared earlier in the larvae reared under LDB than in those reared under LDW and LDR. These results emphasize the importance of lighting conditions and wavelengths during early development for the ontogeny of daily rhythms of gene expression and how these rhythms are reflected on the behavioural rhythmicity of zebrafish larvae.

Circadian rhythms of embryonic development and hatching in fish: a comparative study of zebrafish (diurnal), Senegalese sole (nocturnal), and Somalian cavefish (blind).

During early development, most organisms display rhythmic physiological processes that are shaped by daily changes in their surrounding environment (i.e., light and temperature cycles). In fish, the effects of daily photocycles and their interaction with temperature during early developmental stages remain largely unexplored. We investigated the existence of circadian rhythms in embryonic development and hatching of three teleost species with different daily patterns of behavior: diurnal (zebrafish), nocturnal (Senegalese sole), and blind, not entrained by light (Somalian cavefish). To this end, fertilized eggs were exposed to three light regimes: 12 h of light: 12 h of darkness cycle (LD), continuous light (LL), or continuous darkness (DD); and three species-appropriate temperature treatments: 24°C, 28°C, or 32°C for zebrafish and cavefish and 18°C, 21°C, or 24°C for sole. The results pointed to the existence of daily rhythms of embryonic development and hatching synchronized to the LD cycle, with different acrophases, depending on the species: zebrafish embryos advanced their developmental stage during the light phase, whereas sole did so during the dark phase. In cavefish, embryogenesis occurred within 24 h post fertilization (hpf) at the same pace during day or night. The hatching rhythms appeared to be controlled by a clock mechanism that restricted or "gated" hatching to a particular time of day/night (window), so that embryos that reached a certain developmental state by that time hatch, whereas those that have not wait until the next available window. Under LL and DD conditions, hatching rhythms and the gating phenomenon persisted in cavefish, in zebrafish they split into ultradian bouts of hatching occurring at 12-18-h intervals, whereas in sole DD and LL produced a 24-h delay and advance, respectively. Hatching rates were best under the LD cycle and the reported optimal temperature for each species (95.2±2.7% of the zebrafish and 83.3±0.1% of the cavefish embryos hatched at 28°C, and 93.1±2.9% of the sole embryos hatched at 21°C). In summary, these results revealed that hatching rhythms in fish are endogenously driven by a time-keeping mechanism, so that the day and time of hatching are determined by the interplay between the developmental state (temperature-sensitive) and the circadian clock (temperature-compensated), with the particular phasing being determined by the diurnal/nocturnal behavior of the species.