Ovarian Development in Channa argus var.: Joint Mediation by Melatonin, the cAMP Signaling Pathway, and the PI3K-Akt Signaling Pathway in Response to Photoperiod.

Photoperiod can influence the gonads of fish. This study examined the effects of different light cycles on the gonads of Channa argus var.(C. argus var.). Over a 14-day rearing period, C. argus var. were subjected to five photoperiods: 0 L:24D, 6 L:18D, 12 L:12D, 18 L:6D, and 24 L:0D. On day 14, samples of blood and gonad tissue were collected at 12:00 (noon) to measure enzyme activities and perform transcriptomic analyses. The results showed that prolonged light exposure generally increased ovarian levels of estradiol (E2), progesterone (P), follicle-stimulating hormone (FSH), and vitellogenin (VTG). Melatonin(MT) concentrations in both plasma and ovaries also rose with increasing light duration. Notably, at 18 h of light, both melatonin and sex hormone levels were high. Additionally, transcriptional analysis indicates that the cyclic adenosine monophosphate (cAMP) signaling pathway may be activated in concert by the key factor hrt4 and E2 to stimulate phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt) signaling pathway, thereby promoting FSH secretion and facilitating the accumulation and transport of yolk substances, thereby promoting ovarian development and oocyte maturation. In summary, photoperiod regulates melatonin synthesis, signals related to the cAMP pathway, and the PI3K-Akt signaling pathway, as well as the expression of the key factor hrt4, which in turn influences sex hormone secretion, lipid metabolism, and yolk deposition. Integrated analysis suggests that a day length of 18 h is most favorable for ovary development in C. argus var.