Heat-waves during teleost's embryogenesis induce epigenetic and gill remodeling responses in rainbow trout

The increasing frequency of heat-waves within the current climate change scenarios represents a major emerging stressor for aquatic organisms, yet the adaptive responses at molecular level during early life stages remain poorly understood. Here, we investigated the impact of an experimentally induced heat-wave (H-W) during early development of rainbow trout (Oncorhynchus mykiss) on survival, incidence of deformities, gill histomorphology, and epigenetic regulation. Fertilized eggs were reared under control conditions (11-12 °C) until hatching. A subset of hatched embryos was exposed to a 15 days H-W (+6.8 °C maximum anomaly) while others remained at 13 ± 1 °C. Fish exposed to the H-W showed lower survival and a higher incidence of deformities, particularly craniofacial abnormalities, while juvenile growth was similar between groups. Exposure to the H-W did not induce the expression of classical heat shock genes, whereas antioxidant defense genes were differentially regulated, suggesting a redox imbalance response. Using enzymatic methyl-sequencing (EM Seq) at single-base resolution, we identified widespread and context-dependent DNA methylation differences in juvenile gills, with hundreds of differentially methylated regions predominantly in HCG and exhaustive contexts. Functional enrichment analyses revealed significant alterations in pathways related to ion transport and osmoregulation, including IgSF cell adhesion signaling. At the juvenile stage, histological analyses showed increased chloride cell area and filament thickness in exposed fish, consistent with a potential enhancement of osmoregulatory capacity. These findings suggest that early-life thermal stress may result in epigenetic alterations linked to physiological traits, highlighting DNA methylation as a potential mechanism underlying developmental plasticity to climate-related stressors in fish.