Life history & environmental effects on telomere dynamics in Atlantic salmon

While much of the study of molecular biology inevitably focuses on the parts of the genome that contain active genes, there are also non-coding regions that nonetheless play an essential role in maintaining genome integrity. One such region are telomeres, which cap the ends of all eukaryotic chromosomes and play an important role in chromosome protection. Telomere loss occurs at each cell division as a result of the ‘end replication problem’ and a relatively short telomere length is indicative of poor biological state. Thus far, the majority of studies on the dynamics and role of telomeres have been biased towards certain taxa. Research to date has mostly focussed on humans, other mammals and birds. There has been far less research on the telomere dynamics of ectotherms. It is important that we do so, especially since ectothermic vertebrates do not seem to down-regulate telomerase expression in the same way as endotherms, suggesting that their telomere dynamics may be less predictable in the later life stages. The main objective of this thesis was therefore to investigate how life history and environmental effects may influence telomere dynamics in Atlantic salmon Salmo salar. I carried out carefully designed experiments, both in the laboratory and in the wild, using a longitudinal approach where possible, in order to address a number of specific questions that are connected to this central theme. In chapter 2, I demonstrate that there can be significant links between parental life history and offspring telomere dynamics. Maternal life history traits, in particular egg size, were most strongly related to offspring telomere length at the embryonic stages. Paternal life history traits, such as early life growth rate, had a greater association with offspring telomere dynamics in the later stages of development. In chapter 3, using a wild Atlantic salmon population, I found that most individuals experienced a reduction in telomere length during the migratory phase of their life cycle; however the relative rate of telomere loss was dependent on sex, with males experiencing a relatively greater loss. Unexpectedly, I also found that juvenile salmon that had the shortest telomeres at the time of outward migration, had the greatest probability of surviving through to the return migration. In chapter 4, again using a wild system involving experimental manipulations of juvenile Atlantic salmon in Scottish streams, I found that telomere length in juvenile fish was influenced by parental traits and by direct environmental effects. Faster-growing fish had shorter telomeres and there was a greater cost (in terms of reduced telomere length) if the growth occurred in a harsher environment. I also found a positive association between offspring telomere length and the growth history of their fathers (but not mothers), represented by the number of years that fathers had spent at sea. Chapter 5 explored the hypotheses that oxidative DNA damage, catalase (CAT) antioxidant activity and cell proliferation rate are underlying mechanisms linking incubation temperature and telomere dynamics in salmon embryos. No evidence was found for any such effects, but telomere lengths in salmon embryos were found to be significantly affected by the temperature of the water in which they were living. There is also evidence that telomere length significantly increases during embryonic development. In summary, this thesis has shown that a complex mix of environmental and parental effects appear to influence telomere dynamics in Atlantic salmon, with parental effects especially evident during early life stages. It also demonstrated that telomeres lengthen through the embryo stages of development before reducing once the fry begin feeding, indicating that the patterns of telomere loss commonly found in endotherms may differ in ectotherms. Reasons for this variation in telomere dynamics are presented in the final Discussion chapter of the thesis.