Photoperiodic schedules during rearing and laying stages modulate photoreceptors transcription and the hypothalamic–pituitary–testicular axis in male White King pigeons

The photoperiod during rearing and laying period is a primary environmental regulator of reproductive performance in poultry, yet its effects on male breeding pigeons have been largely unexplored. Therefore, this study evaluated the impact of four photoperiod regimes on the reproductive performance, testicular morphology, hormone profiles, and gene expression of hypothalamic-pituitary-testicular axis in male White King pigeons. From 101 d of age, the pigeons in the control group were exposed to a natural photoperiod until 160 d, and then to a photoperiod of 16 h (16 light (L): 8 dark (D)) and lasted for 30 weeks. Pigeons in the three experimental groups were exposed to a short photoperiod of 8L: 16D until 160 d, and then to 14L: 10D (S-14L:10D), 16L: 8D (S-16L:8D), and 18L: 6D (S-18L:16D), respectively. The results showed that light restriction (8L:16D) during the rearing period and then 14L:10D or 16L:8D photostimulation produced the highest egg output and fertility rates. Pigeons under S‑14L:10D exhibited larger bilateral testes, increased seminiferous‑tubule diameter and epithelial thickness, elevated numbers of spermatogenic cells, significantly higher plasma FSH, LH and testosterone concentrations, while reduced prolactin. Under the S-14L:10D photoperiod, GnRH, FSH, LH-receptor and steroidogenic enzymes were up‑regulated, alongside down‑regulated GnIH, VIP, PRL. This optimal light program also increased expression of deep‑brain photoreceptors (neuropsin, melanopsin, vertebrate ancient-opsin, pinopsin) and suppressed retinal opsins, suggesting a shift toward non‑visual photoperception that drives reproductive activation. These findings demonstrate that a S-14L:10D light schedule from rearing period optimally stimulates hormonal, hypothalamic-pituitary-testicular (HPT) axis genes expression, and morphological pathways, thereby maximizing reproductive performance in commercial pigeon production.