Transcriptomic responses to environmental temperature in eurythermal and stenothermal fishes

Ectothermic species like fishes differ greatly in the thermal ranges they tolerate; some eurythermal species may encounter temperature ranges in excess of 25°C, whereas stenothermal species in polar and tropical waters live at essentially constant temperatures. Thermal specialization comes with fitness trade-offs and as temperature increases due to global warming, the physiological basis of specialization and thermal plasticity has become of great interest. Over the past 50 years, comparative physiologists have studied the physiological and molecular differences between stenothermal and eurythermal fishes. It is now well known that many stenothermal fishes have lost an inducible heat shock response (HSR). Recent advances in transcriptomics have now made it possible to examine genome-wide changes in gene expression (GE) in non-model ecologically important fish, broadening our view beyond the HSR to regulation of genes involved in hundreds of other cellular processes. Here, we review the major findings from transcriptomic studies of extreme eurythermal and stenothermal fishes in response to acute and long-term exposure to temperature, both time scales being critically important for predicting climate change responses. We consider possible molecular adaptations that underlie eurythermy and stenothermy in teleosts. Furthermore, we highlight the challenges that still face the field of comparative environmental genomics and suggest fruitful paths of future investigation.