Evaluating the use of telomere length for the assessment of cumulative stress in zebrafish (Danio rerio) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Physiology at Massey University, Manawatū, New Zealand

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Zebrafish (Danio rerio) are used in their millions worldwide for scientific research and testing. Despite their popularity as an animal model, their welfare has not been well-considered until recently. In particular, the role of environmental enrichment for improving zebrafish welfare is increasingly being examined. However, the welfare assessment indices available to researchers are limited, particularly for studies of long-term conditions such as housing. Telomeres are protective caps on the end of chromosomes that shorten during cell division and thus provide an indication of biological ageing. Recent research has demonstrated that telomeres shorten faster during stress. Thus, telomere length may be a useful marker of cumulative and chronic stress and thus serve as an indicator of an animal’s longer-term state of welfare. The aim of this study was to assess the response of telomeres to cumulative stress in zebrafish, to establish whether this marker could be used for future welfare assessment in this species. 57 fish were exposed to an Unpredictable Chronic Stress protocol for four weeks, while 57 non-stressed controls were maintained under identical, industry standard conditions. After this time, the telomere length of a mixed-tissue sample taken from fish in each group was compared. Whole-body cortisol concentration was also measured to evaluate whether any change in telomere length was correlated with a physiological stress response. I hypothesized that telomere length would be shorter and cortisol concentration higher in the stressed group. Contrary to expectations, there was no difference in telomere length between stressed and control fish. Nor was there a difference in cortisol concentration, suggesting that either the fish were not sufficiently stressed, or that a ceiling effect had been reached. The most likely reason for this is that the stress treatment selected was not stressful enough to induce a measurable response. However, an alternative explanation is that the rate of telomere shortening was masked by the activity of telomerase, an enzyme that maintains telomere length in this species. Future explorations of the effect of stress in telomere dynamics should include evaluation of both length and telomerase activity. An effect of fish sex on telomere length was found, with females having shorter telomeres than males. Although these results cannot be used to confirm the utility of telomere length as a welfare indicator, they raise an interesting and thus far unexplored question of the role that sex plays in telomere maintenance.