Adaptation to heat stress reduces phenotypic and transcriptional plasticity in a marine copepod

Abstract

Summary Organisms may respond to changing environments through phenotypic plasticity or adaptive evolution. These two processes are not mutually exclusive and may either dampen or strengthen each other's effects, depending on the genetic correlation between trait values and the slopes of their norms of reaction. To examine the effect of adaptation to heat stress on the plasticity of heat tolerance, we hybridized populations of the crustacean Tigriopus californicus that show divergent phenotypes for heat tolerance. We then selected for increased heat tolerance in hybrids and measured heat tolerance and the phenotypic plasticity of heat tolerance in both selected lines and unselected controls. To test whether the changes in phenotypic plasticity were associated with changes in the plasticity of gene expression, we also sequenced transcriptomes of selected and unselected lines, both under heat shock and at ambient temperatures. We observed increased heat tolerance in selected lines, but also lower phenotypic and transcriptional plasticity in response to heat stress. The plastic response to heat stress was highly enriched for hydrolytic and catalytic activities, suggesting a prominent role for degradation of misfolded proteins. Our findings have important implications for biological responses to climate change: if adaptation to environmental stress reduces plasticity, then plasticity and adaptive evolution will make overlapping, rather than additive contributions to buffering populations from environmental change.