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Seasonality in Zebrafish

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Seasonality in Zebrafish Seasonality in Zebrafish Jessica Olsen Thesis submitted for the degree of Doctor of Philosophy (Ph.D.) Cell and Developmental Biology Division of Biosciences University College London I, Jessica Olsen, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Page 2 ACKNOWLEDGEMENTS I would like to thank a number of supporters who have helped with much needed guidance throughout the life of this project. Academic support has been provided by my primary thesis supervisor, Professor David Whitmore, whose efforts have been instrumental in getting this work over the finish line. I am extremely grateful to Dr. Elodie Peyric for her extended supervision both during the experimental stages of the project and the writing of this thesis. Her skills and help have been invaluable. Thanks also go out to the other members of the Whitmore lab (2006‐2010), including Veronica Ferrer, Lucy Young, Peter Cormie, Helen Moore, Andrew Beale, and Catherine Cox, who all worked with me on the bench and in the office. Additional guidance was provided by our departmental central lab manager, Mary Rahman, and consumables and deliveries organized by Ian Blaney and his team in the CDB stores office. The work done to support this thesis was generously funded by the BBSRC. Finally, thanks to my friends, co‐workers and colleagues, most especially Kepmen Lee, for his incredible patience and optimism over the course of this project. Acknowledgements Page 3 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 Abstract Page 4 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. Abstract Page 5 TABLE OF CONTENTS Acknowledgements ............................................................................................................. 3 Abstract ................................................................................................................................... 4 Table of Contents .................................................................................................................. 6 List of Figures ..................................................................................................................... 11 List of Abbreviations ........................................................................................................ 13 List of Species Names ....................................................................................................... 15 Chapter 1 – General Introduction ................................................................................ 16 1.1 What is Photoperiodism? ................................................................................... 16 1.2 A role for the biological clock in photoperiodism ........................................... 17 1.2.1 What is a circadian clock? .................................................................................. 17 1.2.2 How is photoperiodic time measured? ............................................................. 18 1.2.3 Evidence of a circadian clock underlying photoperiodism ........................... 23 1.3 Circadian and Photoperiodism in Mammals, Birds and Fish .......................... 25 1.3.1 Mammalian circadian systems .......................................................................... 25 1.3.2 Avian circadian systems .................................................................................... 27 1.3.3 Refractory outputs in response to extended photoperiods ............................ 29 1.3.4 Fish circadian system ....................................................................................... 30 1.3.5 Photoperiodism in fish ....................................................................................... 31 1.3.6 Melatonin in fish ............................................................................................... 33 1.4 Photoperiodism reinvented: TH and DIO expression .................................... 35 1.5 Photoperiodism and Reproductive Endocrinology ......................................... 41 1.5.1 GnRH forms in zebrafish ................................................................................... 42 1.5.2 Seasonal expression of Prolactin ...................................................................... 43 1.5.3 Seasonal LH and FSH expression ..................................................................... 44 1.5.4 Seasonal GH expression .................................................................................... 45 1.6 Zebrafish as a circadian model......................................................................... 46 Chapter 2 ­ Photoperiod on Fertility and Growth .................................................. 49 2.1 Introduction ...................................................................................................... 49 2.2 Methods and materials ..................................................................................... 51 2.2.1 Zebrafish breeding .............................................................................................. 51 2.2.2 Lighting conditions .......................................................................................... 52 2.2.3 Measures of body length and weight; gonad weight ...................................... 53 2.2.4 Egg Collection ................................................................................................... 53 2.3 Results ............................................................................................................... 54 2.3.1 Length / Weight of adult zebrafish in different photoperiods ...................... 54 2.3.2 Fecundity of zebrafish on short and long photoperiods ................................ 55 2.3.3 Fertility of zebrafish on short and long photoperiods ................................... 59 2.3.4 Gender‐specific differences in LD and SD entrained gonad tissues .............. 61 2.4 Discussion ......................................................................................................... 62 2.4.1 Growth differences between LD and SD entrained zebrafish ........................ 63 2.4.2 Effects of photoperiod on fecundity in young and mature zebrafish ........... 65 2.4.3 Effects of photoperiod on zebrafish gonadal weight ..................................... 67 Chapter 3 – Photoperiodic Hormone Expression .................................................. 69 3.1 Introduction ..................................................................................................... 69 3.1.1 GnRH and gonadotrophin expression .............................................................. 71 3.1.2 GHRH and GH expression ................................................................................ 71 3.1.3 Melatonin receptor expression (MT1, MT2 and Mel1c) ................................. 72 3.1.4 TH, TSH and Dio expression ........................................................................... 73 3.2 Methods and Materials ....................................................................................
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