| dc.description.abstract | Thousands of chemical pollutants enter the environment continuously, each with the potential to cause adverse effects in both terrestrial and aquatic organisms. As a result, organisms are often exposed to a mixture of stressors within their habitat. Populations of fish inhabiting most aquatic environments are exposed to time-varying or repeated pulses of exposure, driven by run-off events or spills, or due to their mobility between polluted and clean waters. Therefore, the sustainability of fish populations is critically dependent on their ability to adapt to frequent changes in their local environment. Despite this, legislation to protect the environment from chemical contamination are generally based on toxicological measurements following exposures to single stressors, conducted under optimal laboratory conditions, and that do not take into account the variation in susceptibility of wild populations, or the potential consequences of exposure for the susceptibility of the population during future exposures, including across generations.
Increasing evidence is suggesting that a number of chemicals may interact with the epigenome, and that differential responses to pollutants may be modulated, at least in part, via epigenetic mechanisms. However, our understanding of the role of epigenetic mechanisms in normal development in fish models or its susceptibility to exposure to environmental stressors is currently very limited.
This thesis aimed to document the mechanisms of genetic and epigenetic responses to industrial pollutants in fish, and to explore the extent to which differential responses can be induced in the lab following exposure during the critical window of embryonic development or in adults. To address these objectives, I performed a series of experiments using both the zebrafish (Danio rerio) and the three-spined stickleback (Gasterosteus aculeatus) as fish models.
I first used the zebrafish (Danio rerio) model to investigate the sex-specific transcription and DNA methylation profiles for genes involved in the regulation of reproduction and in epigenetic signalling in the livers and gonads. I provide evidence of the sex-specific transcription of genes involved in reproduction and their regulation by epigenetic signalling in this commonly used vertebrate model and highlight important considerations regarding the use of whole tissues comprised of multiple cell types in epigenetic and transcriptomic studies. I then investigated the potential for exposure to Bisphenol A (BPA) to cause adverse effects on reproduction and to disrupt the expression profiles and promotor DNA methylation of target genes important for reproductive function and epigenetic signalling in the zebrafish. To do this, I exposed breeding zebrafish to a range of BPA concentrations over 15 days and found that BPA disrupted reproductive processes in zebrafish, likely via estrogenic mechanisms, but only at high concentrations. Importantly, exposure to environmentally relevant concentrations of BPA resulted in altered transcription of key enzymes involved in DNA methylation maintenance, and caused changes in promoter DNA methylation.
I also conducted a series of repeated exposures to copper in the three-spined stickleback to investigate the extent to which differential susceptibility can be induced in the lab. This work provides evidence that pre-exposure to copper results in differential responses in future exposure scenarios both when the initial exposure occurred in adults and during embryogenesis. For adults, fish appeared to recover completely from the initial exposure following a period of depuration of 30 days, but displayed decreased susceptibility upon re-exposure. In contrast, for fish exposed during the critical windows of embryonic development when epigenetic reprogramming are hypothesised to occur, differential copper accumulation was maintained throughout life. Importantly, the initial exposure caused increased tolerance in the offspring, which was inherited up to the F2 generation. This work provides valuable information regarding potential critical windows of development which may be more susceptible to effects associated with pre-exposure, highlighting that early life exposure to a low concentration of copper can induce differential responses to copper across generations.
These data highlight the extent of differential responses to chemical stressors likely to be present in wild populations, and point towards the possibility that effective population management will likely require an in-depth understanding of the exposure history of a given population in order to manage restocking initiatives, and to inform conclusions drawn from toxicity testing studies conducted using individuals originating from wild populations. In addition, these data suggest that it is likely that both epigenetic and genetic changes can contribute to the adaptation of individual populations to their local environment. Finally, other vertebrates including humans have been shown to be exposed to the chemicals tested in this thesis. Therefore, this highlights the potential for these chemicals to also cause toxic effects in humans, potentially via (epi) genetic mechanisms, and advocate the testing of the potential for inheritable phenotypes, such as those described in this thesis, to occur in mammalian models.
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