Effect of Season on Testicular Development and Spermatogenesis in Hu Sheep: Insights from Antioxidant Indices, Oxylipins, and Transcriptomics

Numerous factors, including genetic, environmental, and nutritional, are involved in testicular development and spermatogenesis. However, little is known about the effects of seasonal factors on pre-sexual maturity testicular development in Hu rams, which are famous for their high fertility and year-round estrus onset. This study explored the effect of the birth season on testicular development and spermatogenesis in Hu sheep. Thirty-six 6-month-old male lambs born in summer (n = 18) and winter (n = 18) were selected for analysis. Results showed that summer-born lambs exhibited significantly higher cauda sperm density (102.65 ± 9.56 vs. 16.86 ± 2.02 × 10⁷/g), antioxidant indices such as superoxide dismutase (SOD: 6.29 ± 1.01 vs. 4.09 ± 0.25 U/mgprot), and higher expression levels of glutathione peroxidase 3 (<i>GPX3</i>), glutathione peroxidase 4 (<i>GPX4</i>), and copper/zinc superoxide dismutase (<i>Cu</i>/<i>Zn-SOD</i>) than winter-born lambs. Conversely, the malondialdehyde content (1.08 ± 0.32 vs. 2.13 ± 0.34 nmol/mgprot) was significantly lower in the summer-born group (<i>p</i> < 0.05) than in the winter-born group. A total of 44 differential oxylipins and 326 differentially expressed genes (DEGs) were screened by ultra-performance liquid chromatography-tandem mass spectrometry and transcriptomics, respectively. An integrated analysis of oxylipins and transcriptomics revealed that these differential molecules were enriched in metabolic pathways. Notably, downregulated DEGs (e.g., UAP1L1 and NAT8L) were significantly correlated with upregulated differential oxylipins (e.g., epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acids). These results indicate that compared to the winter-born group, the testicular tissues of summer-born rams showed stronger testicular antioxidant capacity and lower lipid peroxidation at the sexual maturity stage, which contributes to spermatogenesis.