Abstract
In response to potentially stressful conditions, for example due to climate change, organisms can move, adjust through phenotypic plasticity and evolve. Evolution can act on mean trait values, or on plasticity itself. Whether phenotypic plasticity, evolution, or evolution of plasticity is the dominant response to environmental change is still subject to debate. Representing a close link between genotype and phenotype, gene expression studies are an effective tool to study these questions, especially combined with environmental clines. Therefore, we used a common garden experiment to quantify gene expression across two populations of Glanville fritillary butterflies originating from different latitudes in Europe, tested at two temperature treatments. We investigated gene expression patterns using differential expression analysis and weighted gene co‐expression network analysis (WGCNA) to disentangle genetic from plastic effects. We found that, regardless of population of origin, most differentially expressed genes and co‐expression modules were responsive to temperature. We also found strong evidence for variation in plasticity between populations (GxE) in both gene‐by‐gene and co‐expression analyses, but only little evidence for genetic assimilation or compensation. While a large proportion of differentially expressed genes showed a reaction norm reversal, this was not the case for co‐expression modules. This indicates the importance of considering coordinated, low‐level changes across many genes. Biological processes overrepresented in genes and gene co‐expression modules differentially expressed with temperature and/or the population by temperature interaction were generally related to cellular maintenance and growth. Our results highlight the importance of intraspecific differences in phenotypic plasticity in response to mild warming.