Acute thermal stress alters sperm swimming activity, viability, and gene expression in blue catfish, Ictalurus...

Fish sperm are vulnerable to changes in external environments, particularly temperature fluctuations. Such changes can directly impact sperm of externally fertilizing species, including blue catfish, Ictalurus furcatus, which in turn can affect physiological and molecular mechanisms. The objective of this study was to investigate how thermal stressors impact sperm kinematics, viability, and gene expression. Sperm from nine mature catfish were stored at 4-, 32- (representing pond temperature at end of spawning season), and 36°C (representing a pond heatwave) for 30-, 60-, and 90 min. Fresh sperm were also evaluated immediately after processing (control group). Sperm kinematics and viability were assessed, and sperm samples were flash-frozen for transcriptomic analysis. Sperm quality declined with increasing temperature and exposure time. Transcriptomic comparisons among fresh sperm and those held at 4-, 32-, and 36°C for 90 min revealed 29 differentially expressed genes. Upregulated genes included members of heat shock protein (HSP) 70 and 40 families. Gene ontology (GO) analysis identified 30 enriched terms between the control group and cells exposed to 36°C for 90 min, including protein refolding, cellular response to unfolded protein, and heat-shock protein binding. Similar GO terms were enriched between sperm held at 4°C for 90 min and sperm exposed to 36°C for 90 min. KEGG pathway analysis revealed significant enrichment of spliceosome, protein processing in endoplasmic reticulum, endocytosis, and MAPK signaling pathway. Together, these results demonstrate the sensitivity of blue catfish sperm to an abiotic stressor (temperature), identify biomarkers related to thermal stress, and provide novel insights into the molecular mechanisms underlying sperm function and quality.