Transcriptional breakpoints as predictors of thermal thresholds in a climate-impacted stenothermal fish

Abstract

1. The biogeography of ectotherms is greatly influenced by their thermal tolerance, which is expected to be tightly coupled to aerobic performance. However, preferred temperatures of ectotherms often deviate substantially from aerobic performance optima. This suggests that alternate physiological mechanisms may constrain their thermal niche. Physiological breakpoints can indicate thermal thresholds, which can manifest at the cellular level before impacting organismal performance. Therefore, we tested whether cellular rather than organismal temperature thresholds aligned with thermal habitat use in a stenothermal fish.
2. We reared eggs from a wild, montane population of bull trout (Salvelinus confluentus), a cold-water species vulnerable to climate-induced warming. Juveniles were then acclimated to ecologically relevant temperatures (6, 9, 12, 15, 18, or 21°C) for 3 weeks. We conducted behavioural trials to determine preferred temperatures. We used custom OpenArray qPCR chips to quantify the expression of 56 genes in gill, liver and muscle to estimate tissue-specific transcriptional breakpoint temperatures. Metabolic rates were measured to determine the temperature at which aerobic performance peaks.
3. We found that the interquartile range of the temperature preference of juveniles was 9.3 to 14.3°C, a range that aligned with tissue-specific transcriptional breakpoint temperatures of genes in certain functional categories. When temperatures exceeded transcriptional breakpoints, pathways associated with the cellular stress response were upregulated, whereas genes related to growth and metabolism were downregulated. Aerobic performance increased beyond preferred temperatures and peaked at 20.6°C.
4. Our findings suggest that although juvenile bull trout have the metabolic flexibility to exploit warmer habitats for short periods, they preferred temperatures that avoided the activation of cellular stress responses to potentially invest in the molecular machinery required to promote growth. Our results highlight the connection between thermoregulatory behaviour and cellular thermal thresholds, which has important implications for understanding the mechanistic basis of biogeography in ectotherms as climate change continues to reshape their thermal landscapes.

Citation

Lazaro-Côté, A., Durhack, T. C., Guzzo, M. M., Chapelski, A. J., Kissinger, B. C., Jeffries, K. M., & Mochnacz, N. J. (2026). Transcriptional breakpoints as predictors of thermal thresholds in a climate-impacted stenothermal fish. Functional Ecology, 00, 1–14. https://doi.org/10.1111/1365-2435.70419