How distinct killifish populations respond differently to stress

Down the east coast of North America, populations of Atlantic killifish, Fundulus heteroclitus, experience wildly different thermal environments. Southern populations experience water that is, on average, 12°C hotter than northern populations annually. These northern and southern killifish populations cope with stress differently. When fish are stressed, such as when predators are nearby, they activate a suite of mechanisms that regulate the release of the stress hormone cortisol, allowing the fish to remain in good condition. Researchers found previously that killifish from southern populations had higher plasma cortisol levels than killifish from northern populations after they experienced the same stress. To figure out why some fish respond strongly to stress while others do not, Madison Earhart and co-workers from the University of British Columbia, Canada, with colleagues from the University of Manitoba, Canada, and the University of Glasgow, UK, set out to determine what genes are responsible for the differences in killifish stress responses by investigating different mechanisms that trigger cortisol release after experiencing stress.Earhart and colleagues travelled to the USA and collected adult killifish from northern New Hampshire and southern Georgia populations. Then, they brought the fish back to the lab in British Columbia. First, the team wanted to see whether the differences in population stress response changed when the fish experienced stress briefly or were exposed to the same stress repeatedly over the period of a week by either putting the fish in buckets and shaking them for half an hour or repeatedly shaking them every day for a week. After stressing the fish, the team collected samples of the fish's blood to measure their cortisol plasma levels, in addition to collecting samples of the fish's brain, head kidney – the organ that produces cortisol – and liver – the organ that produces and breaks down glucose – to measure the expression of genes important for the production and release of cortisol due to stress.After experiencing the single stressful situation, the southern killifish population had higher cortisol levels than the northern killifish population. However, when the fish were repeatedly stressed over the period of a week, there was no difference in cortisol levels between the northern and southern populations. The southern population of killifish, which had a stronger response to the individual stressful situation, expressed more of the genes involved in cortisol production in the brain and head kidney. In addition, the livers of fish from the southern population were more responsive to the cortisol stress hormone as they expressed larger amounts of the gene for the protein that triggers the protective mechanisms that are activated by cortisol when a fish is stressed.The team also measured the condition of the fish after repeated stress. They found that the southern killifish, which were more stress responsive than the northern killifish, were in better condition after experiencing repeated stress than were northern populations. This showed that responding to stress by increasing cortisol was beneficial for the southern population of fish.Earhart and colleagues noted that the answer as to why some fish respond more strongly to stress than others is more complex than simply increasing cortisol levels in the body. They showed that there are differences in the expression of genes all along the pathways that make cortisol, respond to cortisol and cease cortisol production in these two different populations of fish. Therefore, it is important to continue to study these differences in populations of the same species adapted to different environments.

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