Allele-Specific Expression and Evolution of Gene Regulation Underlying Acute Heat Stress Response and Local Adaptation in the Copepod Tigriopus californicus.

Geographic variation in environmental temperature can select for local adaptation among conspecific populations. Divergence in gene expression across the transcriptome is a key mechanism for evolution of local thermal adaptation in many systems, yet the genetic mechanisms underlying this regulatory evolution remain poorly understood. Here we examine gene expression in 2 locally adapted Tigriopus californicus populations (heat tolerant San Diego, SD, and less tolerant Santa Cruz, SC) and their F1 hybrids during acute heat stress response. Allele-specific expression (ASE) in F1 hybrids was used to determine cis-regulatory divergence. We found that the number of genes showing significant allelic imbalance increased under heat stress compared to unstressed controls. This suggests that there is significant population divergence in cis-regulatory elements underlying heat stress response. Specifically, the number of genes showing an excess of transcripts from the more thermal tolerant (SD) population increased with heat stress while that number of genes with an SC excess was similar in both treatments. Inheritance patterns of gene expression also revealed that genes displaying SD-dominant expression phenotypes increase in number in response to heat stress; that is, across loci, gene expression in F1's following heat stress showed more similarity to SD than SC, a pattern that was absent in the control treatment. The observed patterns of ASE and inheritance of gene expression provide insight into the complex processes underlying local adaptation and thermal stress response.