Adaptation to local environments involves the evolution of ecologically important traits and underlying physiological processes. Here, we used low coverage whole-genome resequencing (lcWGR) on individuals to identify genome regions involved in thermal adaptation in wild redband trout Oncorhynchus mykiss gairdneri, a subspecies of rainbow trout that inhabits ecosystems ranging from cold montane forests to high elevation deserts. This study includes allele frequency-based analyses for selective sweeps among populations, followed by multiple association tests for specific sets of phenotypes measured under thermal stress (acute and chronic survival/mortality; high or low cardiac performance groups). Depending on the groups in each set of analyses, sequencing reads covered between 43-75% of the genome at ≥15X and each analysis included millions of SNPs across the genome. In tests for selective sweeps among populations, a total of six chromosomal regions were significant. However, association tests for specific phenotypes revealed that the region on chromosome 4 was consistently the most significant and contains the cerk gene (ceramide kinase). This study provides insight into a potential genetic mechanism of local thermal adaptation and suggests cerk may be an important candidate gene. However, further validation of this cerk gene is necessary to determine if the association with cardiac performance results in a functional role to influence thermal performance when exposed to high water temperatures and hypoxic conditions. 

Acute and chronic thermal tolerance was measured by rearing fish under daily temperature cycled between 17.5°C (evening) to 28.5°C (afternoon) that mimic the 24-h water temperature fluctuation in natural desert streams during the hottest summer days

Cardiac phenotypes was measured using a custom-built electrocardiogram (ECG) system, where its heart rate were pharmacologically stimulated to maximum (maximum heart rate or f_H,max) by atropine sulphate and isoproterenol.