The mechanisms of amniote photoperiodism: from deep-brain photoreceptors to rhythmic epigenetics

Tolla, Elisabetta (2021) The mechanisms of amniote photoperiodism: from deep-brain photoreceptors to rhythmic epigenetics. PhD thesis, University of Glasgow.

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Abstract

Seasonal reproduction is a strategy conserved across nature. The duration of light (photoperiod) regulates the reproductive molecular control within the Hypothalamic-Pituitary-Gonadal Axis of seasonal species, and supplementary cues fine-tune the exact timing of breeding. In recent years, epigenetic mechanisms have been shown to be involved in an array of circannual rhythms, including reproduction. The aim of this thesis was to explore the molecular reproductive neuroendocrine processes that underlie the onset of reproduction in two summer-breeding animal models, the Japanese quail (Coturnix japonica) and the Siberian hamster (Phodopus sungorus). In birds, light penetrates the skull and is detected by deep-brain photoreceptors (DBPs) within the hypothalamus, stimulating the photoperiodic reproductive response. However, the identity of these DBPs is unclear to this day. In the present thesis, the expression of two photoreceptors, Vertebrate-Ancient Opsin (VA Opsin) and Neuropsin (OPN5), was repressed through adeno-associated viral injection, and the breeding response was monitored in control and treated birds maintained under short-days (SD), or long-days (LD) for 2, 7 or 28 days. The data revealed that both opsins may be involved in seasonal reproduction in the Japanese quail, and that OPN5’s role includes modulating gonadal sensitivity to gonadotropins during breeding. It was also found that hypothalamic OPN5, GNRH and DNAmethyltransferase (Dnmt) expression increases at embryonic day 14 in this species. In addition, higher global methylation levels were found in the pituitary gland of adult LD quail, compared to SD.
In the Siberian hamster, two studies were conducted to investigate the effect of triiodothyronine (T3) on the photoperiod-dependent regulation of reproductive physiology and hypothalamic DNA methyltransferase enzyme expression in both males and females.
Two weeks of daily T3 injections induced gonadal growth in SD males, but not in females. Female SD hamsters, but not males, were found to express lower levels of de novo Dnmts compared to LD individuals. However, exogenous T3 did not affect hypothalamic Dnmt
expression in neither males or females. The data indicated sex differences in the gonadal response to T3, as well as in the regulation of hypothalamic DNA methyltransferase expression. It is likely that female Siberian hamsters require additional cues to initiate reproductive processes. The studies presented allowed for an exploration of reproductive mechanisms in both an avian and a mammalian model, including the role of epigenetic processes in seasonal breeding. Future studies are required to elucidate the precise mechanisms of DBPs, as well as identify downstream targets of maintenance and de novo Dnmts.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: rhythmic epigenetics, seasonal reproduction, seasonality, avian light detection, deep-brain photoreceptors, DNA methyltransferases, VA opsin, OPN5.
Subjects: Q Science > QL Zoology
Q Science > QP Physiology
Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
Funder's Name: Leverhulme Trust (LEVERHUL)
Supervisor's Name: Stevenson, Dr. Tyler
Date of Award: 2021
Depositing User: Ms Elisabetta Tolla
Unique ID: glathesis:2021-82238
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 03 Jun 2021 07:26
Last Modified: 03 Jun 2021 07:33
Thesis DOI: 10.5525/gla.thesis.82238
URI: http://theses.gla.ac.uk/id/eprint/82238

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