Daily rhythms of reproduction, early development and lipid metabolism in teleost fishinfluence of light and feeding cycles

Abstract

The present PhD thesis reveals the existence of an environmentally synchronized harmonious progression in both sexes of all the neuroendocrine machinery controlling reproduction: from the BPG axis activation, gametogenesis, spawning to conclude with the egg fertilization. On chapter 1-2 we found rhythms in the brain-pituitary-gonadal (BPG) axis and the influence of light and temperature on activating/inhibiting this neuroendocrine axis of reproduction. On the chapter 3, we revealed for the first time in vitro fertilization rhythms which may help improving fertilization protocols by considering the time of day for best fertilization rates according to an up/down gene regulation of the oocyte. On chapter 4, we revealed daily rhythms in the mechanism by which environmental factors transduce into a physiological response in the gonads: epigenetic mechanisms of methylation and demethylation. On chapter 5-6, we reported the light/feeding time importance on lipid metabolism rhythms, which may act as critical factors influencing reproduction. Finally, on chapter 7, we reviewed the influence of light and temperature cycles on growth, development, sexual maturation, reproduction and hatching times. The overall aim of this doctoral thesis was to elucidate the impact of time cues such as light, temperature and meal timing, on the synchronization of daily reproduction rhythms in fish, considering gamete production, fertilization, embryo/larvae development, and lipid metabolism. On this basis we highlighted the importance of biological rhythms in the establishment of the harmonious progression of the reproductive and early development processes, including Brain-Pituitary-Gonad (BPG) axis activation, gamete daily variations, in vitro fertilization rates, gonad epigenetic mechanism rhythms and lipidic gene variations in liver. Specific objectives follow: Reveal the existence of daily rhythms in the expression of key genes in the BPG-liver axis of zebrafish. Investigate the influence of seasonal variations (light and temperature) as favorable or inhibitory reproductive conditions on key genes involved on the BPG axis of medaka. Describe daily rhythms of in vitro fertilization in zebrafish to improve protocols considering daily gamete variations. Search for cross-talk mechanisms transducing environmental factors into physiological responses looking at daily rhythms in epigenetic mechanism of methylation and demethylation in the gonads of zebrafish. Evaluate daily rhythms in lipid metabolic gene expression in response to light-dark and feeding cycles in gilthead seabream and in zebrafish liver. Summarize key environmental cues as light and temperature cycles during early development in Senegal solea. We obtained the the fallowin g conclussions 1. In both female and male zebrafish, nearly all key genes involved in the BPG-liver axis display daily rhythms. Those genes follow a harmonious time-line progression during the reproductive period, which warrants the spawning window and fertilization occur coinciding with favorable conditions for maximum offspring survival. 2. In medaka, seasonal variations (long/short photoperiod and high/low temperature) altered daily rhythms in key reproductive genes in the BPG axis. Most females under long photoperiod/high temperature (summer conditions) exhibited a daily rhythmic gene pattern, while those under short photoperiod/low temperature (winter conditions) failed to present significant rhythms. 3. In vitro fertilization displays a strong daily rhythm in zebrafish determined by the oocyte daily phase. Maximum fertilization rates occur around lights on, and circadian rhythmicity persisted under constant darkness. The transcriptome oocyte analysis at different times of the day revealed differential expressed genes (DEGs) grouped in reproductive-related families that may explain the daily variations in the fertilization rates. 4. Epigenetic mechanisms (DNA methylation and demethylation) show daily rhythms of expression synchronized to the light/dark cycle in zebrafish gonads. In constant darkness such rhythmicity persisted for two consecutive days, suggesting its endogenous control. 5. Lipid metabolism in both zebrafish and gilthead seabream liver shows daily rhythms strongly synchronized to the light/dark cycle, regardless feeding time. Lipogenesis genes presented a diurnal acrophase, while lipolysis genes were nocturnal. 6. In Senegal sole, light and temperature cycles condition growth, yolk sac development, metamorphosis, survival, fin development and jaw malformations.