Molecular mechanisms underlying the testicular developmental variations among different lines of Tianfu Nonghua duck

The reproductive and economic performance of male poultry depends on the testicular development and semen quality. However, most previous studies on male duck reproduction have focused on environmental impacts or developmental dynamics within a single breed, so the mechanisms of duck testicular development remain poorly understood. In this study, by taking a total of 240 males from 3 different lines (GF₁, BGF₂, and MC) of Tianfu Nonghua duck during the first 24 post-hatch weeks as the experimental objects, we comprehensively analyzed the age-dependent variations in the testicular histomorphological characteristics, serum testosterone (T) contents, and hypothalamic-pituitary-testicular (HPT) axis-related genes expression profiles, aiming to unravel the molecular mechanisms underlying these observed variations. The histomorphological results showed that the testes of all duck lines developed in an age- and genetic line-dependent manner. Compared to MC and BGF₂, GF₁ exhibited significantly higher bilateral testicular organ index (BTOI) (P < 0.05), which significantly increased since week 8 (P < 0.05). Also, the duck testicular parenchymal-to-interstitial area ratio, the thickness of seminiferous epithelium, and the number of germ cells increased remarkably with age, while the primary spermatocytes (PSC) appeared earlier in GF₁ than in MC and BGF₂. A small number of spermatozoa were only observed in the testes of GF₁ at week 24. Serum T levels of all duck lines exhibited similar developmental patterns of increasing early and decreasing later, reaching the highest during the period from week 8 to 16. However, there observed significantly lower serum T levels in BGF₂ than in GF₁ since week 8 (P < 0.05). In the hypothalamus, GnRH-Ⅰ expression was significantly higher in GF₁ than in MC and BGF₂ at week 24 (P < 0.05), while GnIH expression was significantly higher in GF₁ at week 0 (P < 0.05). The pituitary LHβ expression in GF₁ was significantly higher at weeks 4 and 8, while the testicular CYP17A1 expression in BGF₂ was significantly higher at weeks 4, 8, and 24 (P < 0.05). Moreover, the testicular CYP17A1 expression was positively correlated with serum T levels (P < 0.05), and both the hypothalamic GnRH-Ⅰ and pituitary FSHβ expression showed significant positive correlations with the BTOI (P < 0.01). In conclusion, the testes of GF₁ developed more rapidly than those of MC and BGF₂. These developmental variations were attributed to consistently higher serum T levels in GF₁, which were strongly positively associated with the age-dependent testicular CYP17A1 expression profiles.