Insights into variations in thermal tolerance of intertidal clams: Genetic diversity, transcriptomic profiles and physiological response to heat stress.

How organisms adapt to the environment in the context of global climate change is a classic topic in molecular ecology. Heat tolerance polymorphism at multiple organizational levels can enhance species resistance and resilience to global warming, and so clarifying heat tolerance polymorphism can substantially enhance our capability in assessing and predicting organisms' responses to global climate change. In the present study, Asian hard clams were collected along China's coast to explore their genetic diversity, as well as transcriptional and physiological responses to heat stress after common garden acclimation. Our findings indicate that transcriptome-neutral SNPs can differentiate between the two species, Meretrix petechialis and M. lusoria, while M. lusoria and M. petechialis exhibited different molecular and physiological energy-conserved strategies. The ribosome and oxidative phosphorylation pathway can explain the difference in the molecular response pattern between M. petechialis and M. lusoria. BAG3-mediated heat shock response was a shared transcriptional response to heat stress in Meretrix, and the expression levels of heat shock protein genes around the sublethal temperature were positively correlated with the clams' upper thermal limits. High inter-individual variations of cardiac thermal performance curves existed in M. petechialis and M. lusoria, indicating a high degree of thermal physiological polymorphism. Overall, M. petechialis and M. lusoria exhibited genetic polymorphism and differential gene expression profiles in response to heat stress at levels of individual, population, and species. The results highlight the significance of variations in genetic diversity, gene expression profiles, and physiological performance in predicting how organisms will respond to future climate change.