STRESS UNDERLIES POPULATION DIFFERENCES IN KILLIFISH

Abstract

Killifish habitats extend from the cold waters of Newfoundland to the relative warmth of Florida, and so individual populations inhabit waters with very different temperatures. However, killifish appear to migrate very little,so individual populations have probably adapted to their local thermal regime. Adapting to a new thermal environment can be stressful. While investigating genetic differences between populations, Patricia Schulte and others discovered an intriguing trend; many genes that differed between these populations in laboratory-acclimated fish were also genes that respond to stress. Yet, the physiological, biochemical and genetic mechanisms by which this species has adapted to its local habitat are unknown. Identifying relationships between genes that vary in expression between populations and the gene's physiological function may aid in understanding how these fish adapt to new environments. Therefore, in their current study, Daniel Picard and Schulte test the hypothesis that stress-responsive genes also differed in their expression between killifish populations.Using molecular techniques, Picard and Schulte first looked at gene expression in the livers of killifish from northern and southern populations,both at rest and after chronic handling stress. Five putative stress-regulated genes were then used to test the relationship between the stress response and differences in gene expression in killifish populations associated with different thermal environments. These were glucokinase, glycogen synthase kinase, phosphoenolpyruvate carboxykinase, cRAF and serine threonine kinase,all of which are important in carbohydrate metabolism and cell signalling. They also measured expression of warm-acclimation-related-protein, which has been implicated in the thermal acclimation of killifish and should therefore vary between cold and warm populations.The team looked at gene expression patterns in response to stress and found that glucokinase, serine threonine kinase and cRAF were stress regulated in the southern population of killifish. But the genes were not affected by stress in northern killifish.The expression patterns of these genes also differed between both populations in unstressed fish, supporting the idea that there is a relationship between the stress response and inter-population differences in liver gene expression. Although only 2–3% of genes had previously been found to vary in expression pattern between populations, by selecting for specific genes that are stress regulated, Picard and Schulte have increased this to 60%. Selecting for stress-regulated genes appears to enhance for genes that differ between populations.Not surprisingly, warm-acclimation-related-protein varied in unstressed fish between both warm and cold populations. Warm-acclimation-related-protein expression was eight fold higher in southern killifish, helping to confirm its role in adaptation of fish to warmer waters.This study demonstrates that differences in liver gene expression between populations of killifish may be related to the stress response, which may indicate a physiological mechanism by which these populations have diverged. Not all of the genes that responded to stress were different between populations, indicating a complex regulatory pattern underlying the link between stress and inter-population gene expression, rather than a simple correlation. Nevertheless, this study clearly highlights the importance of considering the role of stress in population genetic differences.