Phenotypic plasticity, the ability of an organism to express different phenotypes depending on the environment, provides an important mechanism by which an animal population can persist under rapid climate change. We experimentally tested both life-history and transcriptional responses of an ecological model species, the three-spined stickleback, to warm acclimation at the southern edge of its European range. We explored cross-environment genetic correlations of key life-history traits in male sticklebacks exposed to long-term temperature changes to examine whether the plasticity pattern was variable among genotypes by using a character-state approach. We also studied gene expression plasticity by analysing both whole-transcriptome and candidate gene expression in brain and liver. Male sticklebacks that developed under warmer conditions during winter were smaller in size and invested less in nuptial coloration at the beginning of the breeding season, showing similar responses across different genotypes. The lack of genetic variation in life-history responses may limit any future evolution of the thermal reaction norm in the study population. After long-term exposure to increased winter temperatures, genes responsible for several metabolic and oxidation-reduction processes were upregulated, and some hormone genes involved in growth and reproduction were downregulated in the brain. In the liver, there was no significantly represented gene ontology by the differentially expressed genes. Since a higher temperature leads to a higher resting metabolic rate, living in warmer environments may incur higher energetic costs for ectotherms to maintain cellular homoeostasis, resulting in negative consequences for life-history traits. The expression of genes related to metabolism, cellular homoeostasis and regulatory signalling may underlie temperature-induced changes in life history.

This study was supported by the Spanish Ministerio de Economía y Competitividad (CGL2014‐60291‐JIN and CGL2015‐69338‐C2‐1‐P) and the Xunta de Galicia (2012/305). CNAG‐CRG laboratory is a member of the Spanish National Bioinformatics Institute, PRB2‐ISCIII, supported by grant PT13/0001 of the PE I+D+I 2013‐2016, funded by ISCIII and FEDER.