Early life thermal plasticity and adaptive divergence among populations of Arctic charr (Salvelinus alpinus)
Climate warming alters the thermal environment experienced by ectotherms, whose physiological performance and fitness are constrained by temperature. Early life stages are often the temperature-sensitive phases of the life cycle, particularly in freshwater stenotherms such as the Arctic charr (Salvelinus alpinus). In this study, we used a common garden approach to compare the response and phenotypic plasticity of four charr populations to warmer conditions. These populations inhabit thermally contrasted lakes, including both native and introduced populations. We reared embryos at either an optimal (5{degrees}C) or a warmer (8.5{degrees}C) temperature. We tested adaptive divergence among populations in four traits (survival, incubation duration, body length and yolk sac volume) using Qst - Fst comparisons. We found that warmer temperature reduced body size, survival and incubation time, while thermal reaction norms differed among populations. Unexpectedly, populations originating from warmer environments did not consistently exhibit higher trait values under elevated temperatures. In contrast, the colder high-altitude population exhibited higher survival rates and lower yolk reserves for a given size under heat stress than the other populations. Our results highlight substantial variation among populations in responses to heat stress, emphasizing the complexity of predicting population responses to climate warming.