Dietary selenium nanoparticles alleviate heat stress-induced reproductive impairment in Onychostoma macrolepis, with MeSeCys as a key bioactive selenium species.

This study aimed to determine whether increased water temperature during the reproductive period impairs male reproductive function in Onychostoma macrolepis and to evaluate whether dietary selenium nanoparticles (SeNPs) alleviate heat stress-induced reproductive damage. As water temperature increased during the reproductive period, sperm total motility, progressive motility, beat cross frequency (BCF), and plasma membrane integrity decreased by 42.3%, 66.5%, 41.7%, and approximately 55%, respectively, from May 9 to July 2. Increased water temperature also impaired testicular antioxidant status, as shown by an approximately 60% decrease in T-AOC and an approximately 110% increase in MDA content on July 2, together with structural damage, mitochondrial abnormalities, and increased protein levels of HSP70 and NF-κB p65. Dietary supplementation with SeNPs at 0.1, 0.3, and 0.9 mg/kg alleviated heat stress induced reproductive damage, with 0.3 mg/kg showing the most pronounced effects. At this level, SeNPs improved sperm quality, antioxidant status, and fertilization capacity, increasing fertilization rate by approximately 20% on June 15 and by more than 100% on July 2. Further analysis showed that SeNPs significantly increased the levels of selenate, SeCys2, MeSeCys, and SeMet in the testis. In vitro experiments further revealed that MeSeCys was the most effective selenium species in maintaining sperm motility, stabilizing ROS levels, and improving fertilization capacity. Overall, these findings indicate that heat stress is a key factor leading to reduced reproductive performance in O. macrolepis, whereas SeNPs mitigate reproductive damage by reducing oxidative stress, with MeSeCys likely serving as a key bioactive selenium species.