Gene expression and DNA methylation changes in response to hypoxia in toxicant-adapted Atlantic killifish (Fundulus heteroclitus)

Hypoxia in coastal waters is an increasing concern, with the frequency of hypoxic events rising because of climate change and human-driven impacts such as eutrophication. The Atlantic killifish (Fundulus heteroclitus), an estuarine species known for its resilience to environmental stressors, provides a valuable model to investigate the physiological, transcriptional, and epigenetic mechanisms underlying adaptation. Some killifish populations, like those in New Bedford Harbor (NBH), Massachusetts, have evolved resistance to dioxin-like polychlorinated biphenyls (PCBs) due to chronic exposure. These populations offer a unique opportunity to investigate how toxicant resistance might influence responses to secondary stressors like hypoxia. The objective of this study is to characterize the impacts of evolved resistance to toxicants on the ability to cope with acute hypoxia. We compared hepatic gene expression and DNA methylation patterns in response to two levels of hypoxia in killifish from NBH and Scorton Creek (SC), a reference population from a relatively pristine environment. We hypothesized that NBH fish would exhibit altered molecular responses to hypoxia due to trade-offs associated with toxicant resistance. Our results revealed significant differences between the two populations. SC fish demonstrated a dose-dependent increase in gene expression in response to hypoxia, while NBH fish exhibited a muted transcriptional response to severe hypoxia, indicating potential impairment in their ability to cope with hypoxic stress. Interestingly, NBH fish showed significant DNA methylation changes in response to hypoxia, while SC fish did not exhibit notable epigenetic alterations. These findings suggest that toxicant-adapted killifish may face trade-offs in their response to additional environmental stressors, with potential consequences for their ability to survive in increasingly hypoxic coastal habitats. Further research is needed to elucidate the functional implications of these epigenetic modifications and their role in adaptive stress responses. Juvenile fish from two different populations (New Bedford Harbor and Scorton Creek) were exposed to either normoxia (control group) or two different concentrations of hypoxia (5 and 10%) for 6 hours. At the end of the exposure, liver samples were dissected and RNA/DNA was isolated simultaneously.