Olsen, J; (2013) Seasonality in Zebrafish. Doctoral thesis (PhD), UCL (University College London).

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Abstract

Annual rhythms of seasonal daylength act as powerful cues for seasonal fertility and physiology. As seasonal daylength changes, nocturnal melatonin secretion is altered, providing the organism an internal representation of daylength through melatonin exposure and duration. Melatonin has been linked with various neuroendocrine and gonadal changes. By altering external light/dark phase durations I expected an increase in zebrafish growth and fertility, mediated by changes in the hypothalamic‐pituitary‐gonad axis. These results confirm zebrafish photoperiodic responsively through physiological measures of growth and reproduction, neural gene expression in the hypothalamus and pituitary, and circadian gene expression profiles from clonal cells and tissue explants. Long‐term entrainment of adult zebrafish to long day (16h/8h light dark) photoperiods stimulated growth (length and weight), while short day (8h/16h light dark) groups had delayed and inhibited growth rates throughout life. Long day entrainment increased gonadal weight in females only, while male testis showed no response to photoperiod differences. Zebrafish fecundity and fertility were stimulated by long day entrainment, coupled with dramatic inhibition of spawning immediately after exposure to short day conditions. Neuroendocrine targets showed a number of tissue and subtype specific differences in circadian and photoperiodic expression, including a 3‐fold increase in melatonin receptor expression in the zebrafish pituitary over the hypothalamus, with circadian expression of melatonin receptor 1 and photoperiodic modulation of melatonin receptor 2, a pattern repeated in the circadian expression of hypothalamic diodinases enzyme Dio2 and the seasonal expression of Dio3. Zebrafish cells, tissues contain functional circadian clocks and are directly light responsive. Using transgenic clonal cell lines (Per1:luc and Cry1a:luc) and tissues (Per3 :luc) the effect of light duration vs. pulse entrainment was monitored in vitro using skeleton photoperiod exposure. In all cases, circadian gene expression entrained to the first light pulse after the longest period of darkness, regardless of the time or phase of the exposure, thus selectively oscillating to the shortest daylength. Per3 expression in the hypothalamus showed a direct light responsive profile, not seen in pituitary or pineal tissue explants. The current work presents novel physiological, endocrine and cellular evidence supporting the hypothesis that zebrafish are responsive to changes in seasonal photoperiods.

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