Photoperiodic regulation of reproduction and hypothalamic-pituitary-gonadal axis in Phoxinus lagowskii adults

The photoperiod, an essential environmental factor influencing the growth, reproduction, and physiological functions of teleosts, exhibits effects that are highly species-specific. This study investigates the impact of five distinct photoperiod treatments on the growth performance, gonadal development, and hypothalamic-pituitary-gonadal (HPG) axis regulation in Phoxinus lagowskii, a high-latitude cold-water fish with significant aquaculture potential. Through comprehensive analyses of growth indices, serum sex hormone levels, gonadal histology, and gene expression, we demonstrate that photoperiod significantly affects the physiology of P. lagowskii. Females exhibited optimal growth and gonadal development under a 16L:8D photoperiod, with increased expression of key genes (gnrh2, gnrh3, kiss1, kiss2, cyp19a1, and foxl2) and elevated levels of gonadotropins (Fsh, Lh) and estradiol (E₂). In contrast, males showed enhanced reproductive performance under an 8L:16D photoperiod, characterized by higher testosterone (T) levels and upregulated expression of dmrt1 and sox9a. The inverse relationship between the hepatosomatic index (HSI) and gonadosomatic index (GSI) suggests an energy trade-off between hepatic energy reserves and reproductive investment. Moreover, melatonin further regulates the HPG axis by affecting the synthesis and release of GnRH and kisspeptins, highlighting its role in mediating photoperiodic effects on reproduction. These findings underscore the sex-specific responses of P. lagowskii to photoperiod and provide critical insights into the physiological mechanisms underlying photoperiod regulation in cold-water fish. This study offers practical strategies for optimizing aquaculture practices to enhance growth and reproductive efficiency.