Mechanisms of Thermal Adaptation: Transcriptional Response to Heat Stress Among Desert and Montane Populations of Redband Trout

As ectothermic organisms have evolved to differing aquatic climates, the molecular basis of thermal adaptation is a key area of research.  In this study, we tested for intraspecific transcriptional response of natural populations of redband trout (Oncorhynchus mykiss gairdneri) that have evolved to warm and cool climates.  Fish were reared in a common garden environment and exposed over four weeks to diel water temperatures that were similar to those experienced in desert climates within the species’ range.  Tissues were collected from gill arches of three strains of fish (desert, montane, F1 crosses) at the peak of heat stress and tested for mRNA expression differences across the transcriptome with RNA-seq methods.  Strong differences in transcriptional response to heat stress was observed across strains confirming that fish from desert environments have evolved diverse mechanisms to cope with stressful environments.  Each strain had both shared and unique contigs that were differentially expressed, but the desert population had more than double the number of unique differentially expressed genes than the montane population (4339 versus 2112 genes respectively), with the F1 strain intermediate (2490 unique differently expressed genes).  Key stress response genes such as molecular chaperones (i.e., heat shock proteins) had adaptive patterns of gene expression among strains, but also a much higher number of metabolic and cellular process genes were differentially expressed in the desert strain demonstrating these biological pathways are critical for thermal adaptation to warm aquatic climates.  The results of this study further elucidate the molecular basis for thermal adaptation in aquatic ecosystems and extend the potential for identifying regions in the transcriptome and genome that may be critical for adaptation to changing climates.