Mechanisms of heat and hypoxia defense in hard clam: Insights from transcriptome analysis

With global changes and anthropogenic activities, heat and hypoxia have become frequent stressors in aquatic environments. However, the molecular defense mechanisms of aquatic organisms, especially bivalves, against the synergistic effects of heat and hypoxia have not been fully elucidated. In this study, we analyzed transcriptomic data of the gills of Mercenaria mercenaria, an economically and ecologically important bivalve, under several stress challenges: high temperature (heat), low dissolved oxygen (hypoxia), and heat plus hypoxia. GO and KEGG enrichment analyses were performed on the differentially expressed gene sets of heat-induced genes under various dissolved oxygen (DO) solubilities, hypoxia-induced genes under different (normal and high) temperatures and common heat plus hypoxia stress-induced genes. We found that protein-folding item was significantly enriched in common differentially expressed heat-induced genes comparisons, with a high expression of molecular chaperones HSP90 and TCP1. Moreover, microtubule-related GO terms were significantly enriched in differentially expressed hypoxia-induced genes under different temperatures, and kinesin and dynein-related genes were commonly upregulated in the hypoxia challenge groups. Protein processing in endoplasmic reticulum and ubiquitin-mediated proteolysis were the most significantly enriched pathways in common heat plus hypoxia stress-induced genes. Furthermore, correctly-folding related genes, luminal chaperones, and ER-associated degradation related genes were upregulated under heat plus hypoxia stresses. Altogether, our results provided new insights on understanding the molecular defense mechanisms against heat and hypoxia stress in hard clams.