Chronic stress response in vertebrates: The genetics of constraint and conflict

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IIn biology, stress is often a poorly defined concept, and one that is negatively associated with health in humans and animals. However, stress responses actually play an important positive role in maintaining viability and health. When challenged by a threat in the environment - perhaps a predator, disturbance, or adverse conditions - a set of neuroendocrine pathways trigger physiological and behavioural responses (e.g. fight or flight behaviours) that have evolved under natural selection to counter the threat. Nonetheless, while these acute stress responses are thought to be adaptive, it is also well documented that chronic stress exposure can reduce the health of individuals and sometimes - particularly when mothers are exposed - their offspring. Because chronic stress responses, also called "tertiary stress responses" (TSR) are typically bad for fitness (survival and/or reproduction), natural selection should act over evolutionary time to get rid of them. The fact that the TSR is widespread, being found in vertebrates ranging from fish to humans, therefore poses an important question- what constrains evolution of the stress response towards a state where these harmful, or maladaptive, effects do not occur? The goal of the proposed work is to answer this question by conducting a genetic study. We will use guppies as a model system, experimentally manipulating stressors in the environment, determining how different individuals and genotypes respond through behavioural and hormonal processes, and determining the long term consequences of this variation for fitness. In particular we will test two hypotheses about where the evolutionary constraint comes from that maintains the TSR. The first possibility is a trade-off between the effects of acute and chronic stress on fitness. In simple terms, genes that cause the TSR may persist in a population precisely because they are the ones that lead to the most appropriate acute stress responses. A second possibility is that, where mothers experience chronic stress, a trade-off occurs across the generations. Here, some maternal genotypes are better able than others to maintain the mother's own health, but do so at a cost to offspring (e.g. by reducing the amount of care she provides). Testing these hypotheses will shed light on the evolutionary processes that have shaped vertebrate stress responses in general. However, it is also expected that a better understanding of the genetics of chronic stress could yield tangible benefits for improving animal welfare in captive animal. For instance, if we understand how genes influencing aspects of the acute stress response contribute to the risk of developing disease under chronic stress, we might be able to select these traits so as to reduce health problems in livestock and aquaculture production in future.