High temperature induces motility of zebrafish sperm after short-term storage

Short-term storage on ice of zebrafish (Danio rerio) sperm has been successfully applied in diverse basic experimental research. The optimal and economically efficient short-term storage method to date, however, can only maintain the fertilization ability of the sperm for a few hours. In this study, we aimed to improve zebrafish sperm quality after short-term storage and to test whether this affects the nucleotide cargo of spermatozoa. Zebrafish sperm was diluted with E400 extender (130 mM KCl, 50 mM NaCl, 2 mM CaCl2, 1 mM MgSO4, 10 mM D-(+)-Glucose and 30 mM HEPES-KOH, pH 7.9, 400 mOsmol/kg) at a dilution ratio 1:20 (v/v) in tubes (500 mu L) and stored for 24 h at 0-2 degrees C under aerobic conditions. Elevating the temperature of sperm suspension at 28 degrees C for 20 min significantly enhanced sperm motility during activation in both distilled water and Perchec solution with 0.25 % Pluronic F-127. Despite the observed decline in sperm motility during short-term storage, Perchec solution (45 mM NaCl, 5 mM KCl, 30 mM Tris, pH 8.0, 160 mOsmol/kg) enhanced activation of the aged sperm compared to activation of distilled water. Following 24 h of sperm storage, higher fertilization, and hatching rates were observed when sperm suspension was incubated at 28 degrees C for 20 min and activated in Perchec solution at a ratio of approximately 6120 spermatozoa per egg, compared to activation in hatchery water. A tentative RNA-seq study identified a set of 2180 differentially expressed genes after 24 h of sperm storage, significantly enriched in the glycosaminoglycan degradation and galactose metabolism pathways. The validity was discussed due to the limited mRNA content in spermatozoa. No alterations were identified in the DNA methylation level of 24 h stored sperm and neither differentially methylated CpGs (DMCs) nor differentially methylated regions (DMRs) were detected between fresh and aged sperm. Based on these phenotypic and molecular investigations and the successful implementation of high-temperature incubation prior to sperm activation in artificial reproduction, we propose that high-temperature incubation following short-term sperm storage at low temperatures is a safe and significant approach in zebrafish and holds potential benefits for other economically important warm-water species in aquaculture.