Epigenetic Mechanisms as Drivers of Environmental Responses in Stony Corals

The current pace of anthropogenic global change is imposing unprecedented conditions to biological systems. Coral reef ecosystems are particularly sensitive to the rapid increase in thermal anomalies and the changes in water chemistry caused by global change. However, although their decline has been documented worldwide, there are signs suggesting that stony corals harbor greater phenotypic plasticity than previously expected, sparking the interest in the study acquired non-genetic modifications (e.g., epigenome, microbiome) potentially increasing their resilience to global change, and constituting one of the main targets for intervention. Epigenetics constitutes an exciting frontier to understand how the environment influences the regulation of the expression of genetic information and modulates phenotypic variation. This has the potential to change the way we understand short-term acclimation and adaptation to a changing environment, aiding to improve predictive models of ecosystemic persistence under current and future climatic scenarios. However, while there is evidence supporting the idea of epigenetic mechanisms participating in rapid-response acclimatization, specific details about how this process is influenced by specific environmental conditions are lacking. In non-model organisms, we often lack information about the presence and functionality of some of these mechanisms, limiting the application of epigenetics in the study of ecosystem resilience in response to global change. This dissertation aims to elucidate how epigenetic mechanisms contribute to coral phenotypic responses to the effects of global change in the oceans. For that purpose, hypotheses about the presence and responsiveness of different epigenetic mechanisms in corals, its interaction with the genome and microbial communities, as well as its role modulating gene expression and phenotypic responses to diverse stressors were explored. Histone repertoires and/or full methylomes were characterized for the first time in the corals Acropora cervicornis and Montastraea cavernosa. The participation of these epigenetic mechanisms modulating responses to nutrient contamination, seasonal environmental change, thermal stress and acidification was demonstrated, providing evidence supporting its participation in intragenerational plasticity. A conserved seasonal methylation program was observed in A. cervicornis. This together with the strong influence of the genome over DNA methylation evidence its heritability and its potential to participate in intergenerational plasticit

Tissue explants as tools for studying the epigenetic modulation of the GH-IGF-I axis in farmed fish

Somatic growth in vertebrates is mainly controlled by the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis. The role of epigenetic mechanisms in regulating this axis in fish is far from being understood. This work aimed to optimize and evaluate the use of short-term culture of pituitary and liver explants from a farmed fish, the gilthead seabream Sparus aurata, for studying epigenetic mechanisms involved in GH/IGF-I axis regulation. Our results on viability, structure, proliferation, and functionality of explants support their use in short-term assays. Pituitary explants showed no variation in gh expression after exposure to the DNA methylation inhibitor decitabine (5-Aza-2′-deoxycytidine; DAC), despite responding to DAC by changing dnmt3bb and tet1 expression, and TET activity, producing an increase in overall DNA hydroxymethylation. Conversely, in liver explants, DAC had no effects on dnmts and tets expression or activity, but modified the expression of genes from the GH-IGF-I axis. In particular, the expression of igfbp2a was increased and that of igfbp4, ghri and ghrii was decreased by DAC as well as by genistein, which is suggestive of impaired growth. While incubation of liver explants with S-adenosylmethionine (SAM) produced no clear effects, it is proposed that nutrients must ensure the methylation milieu within the liver in the fish to sustain proper growth, which need further in vivo verification. Pituitary and liver explants from S. aurata can be further used as described herein for the screening of inhibitors or activators of epigenetic regulators, as well as for assessing epigenetic mechanisms behind GH-IGF-I variation in farmed fish