Comparative Thermal Tolerance and Tissue-Specific Responses Patterns to Gradual Heat Stress in Reciprocal Cross Hybrids of Acipenser baerii and A. schrenckii

The increasing frequency and intensity of heatwaves due to climate change pose significant challenges to sturgeon aquaculture. This study investigated the effects of gradual heat stress (1 °C every 8 h) on two reciprocal hybrid sturgeon strains (Acipenser baerii ♀ × A. schrenckii ♂, (BS hybrid); A. schrenckii ♀ × A. baerii ♂, (SB hybrid)), focusing on their antioxidant defense mechanisms, heat shock protein (HSP) expression, and liver and gill tissue histology. When water temperature raised to 34.3 °C (about 104 h), LOE (loss of equilibrium) individuals appeared. Twenty-four hours after sampling, fifteen BS hybrid sturgeon remained alive, whereas no SB hybrid sturgeon survived. In this study, the slow heat stress significantly elevated the expression of HSP-related genes (hsc70, hsp70, hsp90) in both the liver of BS hybrid sturgeon and the gills of SB hybrid sturgeon. However, in the gills of BS hybrid sturgeon and the liver of SB hybrid sturgeon, the expression of hsp family genes in the experimental groups was either lower than or comparable to the control group. Significant liver damage, including cellular vacuolization and necrosis, was observed in BS hybrids, while SB hybrid sturgeon exhibited more pronounced gill tissue damage. Among the four antioxidant enzymes-superoxide dismutase (SOD), lactate dehydrogenase (LDH), catalase (CAT) glutathione peroxidase (GPx)-only LDH activity was elevated in the hepatic tissue of BS hybrid sturgeon, corresponding to increased serum lactate levels, while gill LDH activity was higher in SB hybrid sturgeon. In both hybrids, LDH activity exhibited an increasing trend in the kidney. However, total antioxidant capacity (T-AOC) remained unchanged across all three tissues. Both plasma cortisol and lactate were substantially affected by thermal stress. MDA remained at a relatively stable level after heat stress and recovery. These results demonstrate differential tissue-specific responses to heat stress in the reciprocal hybrids. More importantly, the BS hybrid sturgeon exhibited significantly higher thermal tolerance and post-stress survival compared to the SB hybrid sturgeon. These findings reveal that the choice of maternal parent is a critical factor influencing heat resistance in these hybrids, providing a key basis for selective breeding programs and optimizing aquaculture management.