Genetic assimilation and accommodation shape adaptation to heat stress in a splash pool copepod.

Understanding organismal responses to dynamic environments is becoming increasingly important in our rapidly changing world. Beyond genetic adaptation, plastic responses to the environment can alter phenotypes and fitness, ultimately driving evolution. However, the interaction between plasticity and adaptation during environmental change is complex and hard to measure in natural systems. Here, we used two populations of Tigriopus californicus copepods, a thermally tolerant southern population and a thermally sensitive northern population, to conduct a fully factorial, split-brood experiment where we exposed animals as larvae and adults to either a sublethal heat stress or control (no heat treatment) before measuring phenotypic plasticity of heat tolerance and gene expression plasticity patterns. We found that increased thermal tolerance across populations came at the expense of physiological plasticity and evolved through both higher baseline expression and increased gene expression plasticity of heat stress-related genes. Importantly, we found evidence for plasticity-led evolution in this system, with the same set of genes contributing to long-term evolutionary changes across populations and to short-term physiological adjustments within populations, providing insight into how these populations will adapt and respond to future environmental change.