RNA-seq from Unio pictorum and Unio delphinus individuals exposed to a gradually increasing temperature regimes

Freshwater mussels are among the most endangered animal groups globally, highly sensitive to climate change due to their strict dependence on freshwater habitats and limited dispersal ability. As global temperatures rise, understanding how species respond to thermal stress is crucial for predicting future biodiversity trajectories. While freshwater mussels are often considered ecologically inflexible, their distribution across broad environmental gradients raises the possibility of population-specific adaptations mediated by phenotypic plasticity.This study investigates whether geographically and climatically distinct populations of two congeneric freshwater mussel species (Unio pictorum and Unio delphinus) exhibit different transcriptomic responses to prolonged heat stress. We ask whether these responses reveal signs of local adaptation and how such plasticity might shape conservation priorities under climate change.To address these questions, we exposed northern and southern populations of both species to a gradually increasing temperature regime in controlled laboratory conditions, simulating a prolonged thermal extreme event. Gill tissues were sampled at three timepoints, and RNA-seq was used to quantify differential gene expression and explore functional pathways associated with thermal stress responses.Results showed stark differences between northern and southern populations, both in the magnitude and functional composition of transcriptomic responses. Southern populations exhibited intense expression shifts involving classical stress pathways, heat shock proteins, detoxification (cytochrome P450s), apoptosis, and energy metabolism, while northern populations, particularly U. delphinus, showed a markedly subdued response. Notably, U. pictorum's northern population relied heavily on the cytochrome P450 family even at moderate temperatures, while the southern populations activated broader proteostasis and immune responses at higher stress thresholds.These findings demonstrate clear population-specific phenotypic plasticity, shaped by environmental history rather than phylogenetic proximity. They underscore the need for conservation strategies to move beyond species-level management, embracing intraspecific variation as a buffer against climate impacts. As climate change accelerates, safeguarding the evolutionary potential encoded within populations, not just species, is essential to preserving biodiversity resilience.